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RESEARCH ARTICLE
Cite as: Laurin M, Lapauze O, and
Marjanović D (2019). What do ossification
sequences tell us about the origin of extant
amphibians? bioRxiv 352609, ver. 4
peer-reviewed by PCIPaleo. DOI:
10.1101/352609
Published: 15 October 2019
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What do ossification sequences tell us about
the origin of extant amphibians?
Michel Laurin1, Océane Lapauze1& David Marjanović2
1CR2P (Centre de Recherche sur la Paléodiversité et les Paléoenvironnements; UMR 7207), CNRS/MNHN/Sorbonne Université,
Muséum National d’Histoire Naturelle, Département Histoire de la Terre –Paris, France
2Museum für Naturkunde (Leibniz Institute for Evolutionary and Biodiversity Research) –Berlin, Germany
THIS ARTICLE HAS BEEN PEER-REVIEWED BY PCIPALEO
Read the Editor’s recommendation and Referees’reports at DOI:10.24072/pci.paleo.100002
ABSTRACT
The origin of extant amphibians has been studied using several sources of data and methods, including
phylogenetic analyses of morphological data, molecular dating, stratigraphic data, and integration of ossification
sequence data, but a consensus about their affinities with other Paleozoic tetrapods has failed to emerge. We
have compiled five datasets to assess the relative support for six competing hypotheses about the origin of
extant amphibians: a monophyletic origin among temnospondyls, a monophyletic origin among lepospondyls,
a diphyletic origin among both temnospondyls and lepospondyls, a diphyletic origin among temnospondyls
alone, and two variants of a triphyletic origin, in which anurans and urodeles come from different temnospondyl
taxa while caecilians come from lepospondyls and are either closer to anurans and urodeles or to amniotes.
Our datasets comprise ossification sequences of up to 107 terminal taxa and up to eight cranial bones, and up
to 65 terminal taxa and up to seven appendicular bones, respectively. Among extinct taxa, only two or three
temnospondyl can be analyzed simultaneously for cranial data, but this is not an insuperable problem because
each of the six tested hypotheses implies a different position of temnospondyls and caecilians relative to other
sampled taxa. For appendicular data, more extinct taxa can be analyzed, including some lepospondyls and the
finned tetrapodomorph Eusthenopteron, in addition to temnospondyls. The data are analyzed through maximum
likelihood, and the AICc (corrected Akaike Information Criterion) weights of the six hypotheses allow us to assess
their relative support. By an unexpectedly large margin, our analyses of the cranial data support a monophyletic
origin among lepospondyls; a monophyletic origin among temnospondyls, the current near-consensus, is a
distant second. All other hypotheses are exceedingly unlikely according to our data. Surprisingly, analysis of
the appendicular data supports triphyly of extant amphibians within a clade that unites lepospondyls and
temnospondyls, contrary to all phylogenies based on molecular data and recent trees based on paleontological
data, but this conclusion is not very robust.
Keywords: macroevolution; paleontology; evo-devo; ossification sequences; Lissamphibia; Tetrapoda; phylogeny
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Lissamphibian origin and ossification sequences
Introduction
Paleontologists have been studying the origin of the extant amphibian clades for more than a century.
Early studies generally proposed an origin of at least some extant amphibians from temnospondyls.
Cope
(1888)
initially suggested that batrachians (anurans and urodeles) derived from temnospondyls (a large
clade of limbed vertebrates known from the Early Carboniferous to the Early Cretaceous) because he
believed that the batrachian vertebral centrum was an intercentrum, the dominant central element of
temnospondyls. Later,
Watson (1940)
argued that anurans were derived from temnospondyls because of
similarities (mostly in the palate) between the temnospondyl “Miobatrachus”(now considered a junior
synonym of Amphibamus) and anurans. Monophyly of extant amphibians (Lissamphibia) was proposed
by
Parsons and Williams (1962, 1963)
, an idea that was accepted more quickly by herpetologists than
by paleontologists. Lissamphibian monophyly was supported by (among a few other character states)
the widespread occurrence of pedicellate, bicuspid teeth. The subsequent discovery of such teeth in
the amphibamid temnospondyl Doleserpeton (
Bolt, 1969
) reinforced the widespread acceptance of an
origin of Lissamphibia from within temnospondyls (e.g.,
Schoch and Milner, 2004
). Recently, this hy-
pothesis, referred to as the temnospondyl hypothesis or TH for short (
Fig. 1c
), has been supported by
several phylogenetic analyses based on phenotypic data matrices (e.g.,
Ruta and Coates, 2007
;
Sigurdsen
and Green, 2011
;
Maddin et al., 2012
;
Pardo et al., 2017a, fig. S6
;
Pardo et al., 2017b
;
Mann et al., 2019
).
Other hypotheses about the origin of extant amphibians have been available in the literature for nearly
as long a time (see
Schoch and Milner, 2004
for a historical review). These were initially formulated espe-
cially for the urodeles and caecilians, which are less similar to temnospondyls and lack a tympanic middle
ear (which is present in most anurans and often inferred for at least some temnospondyls but absent in
lepospondyls). Thus,
Steen (1938)
highlighted similarities in the palate (broad cultriform process of the
parasphenoid) and cheek (loss of several bones) between lysorophian lepospondyls and urodeles.
Carroll
and Currie (1975)
and
Carroll and Holmes (1980)
argued that the extant amphibians had three distinct
origins among early stegocephalians; while they accepted an origin of anurans among temnospondyls,
they suggested that urodeles and caecilians originated from two distinct groups of lepospondyls (Rhyn-
chonkos for caecilians, Hapsidopareiidae for urodeles). Later, based mostly on developmental similarities
between the temnospondyl Apateon and urodeles,
Carroll (2001, 2007)
and
Fröbisch et al. (2007)
pro-
posed another hypothesis involving a triphyletic origin of lissamphibians, with an origin of anurans and
urodeles from two distinct temnospondyl groups, while the caecilians would remain in the lepospondyl
clade. This is what we call the polyphyly hypothesis (PH). We have tested two versions. One (here called
PH1;
Fig. 1e
) was cautiously suggested by
Fröbisch et al. (2007)
; it agrees with the paleontological consen-
sus in placing all or most lepospondyls closer to Amniota than to Temnospondyli (
Fig. 1b
;
Sigurdsen and
Green, 2011
;
Pardo et al., 2017a, fig. S6
;
Pardo et al., 2017b
;
Marjanović and Laurin, 2019
;
Clack et al.,
2019
;
Mann et al., 2019
). The other (PH2;
Fig. 1f
) is modified to make Lissamphibia monophyletic with
respect to Amniota, a fact we consider demonstrated beyond reasonable doubt by multiple phylogenetic
analyses of molecular data (
Fig. 1a
;
Irisarri et al., 2017
;
Feng et al., 2017
; and references cited therein);
this comes at the expense of contradicting the paleontological consensus, which was not yet established
when
Milner (1993, 16–18, fig. 5B)
argued for something like the PH2 as one of two more or less equal
possibilities.
Anderson (2007)
and
Anderson et al. (2008)
found lissamphibian diphyly, specifically a
monophyletic, exclusive Batrachia among the temnospondyls while keeping the caecilians among the
lepospondyls (DH1;
Fig. 1g
).
Pardo et al. (2017a, fig. 2, S7)
presented a similar hypothesis, with batrachi-
ans and caecilians having separate origins within the temnospondyls (DH2;
Fig. 1h
); we should point out,
however, that their dataset contained only temnospondyls and lissamphibians, and while they found the
DH2 using Bayesian inference, it was only one of four equally parsimonious results (see
Marjanović and
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Lissamphibian origin and ossification sequences
Apateon
Sclerocephalus
Anura
Urodela
Gymnophiona
Amniota
Apateon
Hyloplesion
Sclerocephalus
Anura
Urodela
Gymnophiona
Amniota
Apateon
Anura
Urodela
Gymnophiona
Amniota
TH
Compatibility:
Lissamphibia
Lepospondyli + Amniota
Lissamphibia
LH
Compatibility:
molecular consensus paleontological consensus
PH1
Compatibility:
PH2
Compatibility:
DH1
Compatibility:
DH2
Compatibility:
Lissamphibia
Lepospondyli + Amniota
Lepospondyli + Amniota
Lissamphibia
not Lepospondyli + Amniota
Lissamphibia
Lepospondyli + Amniota
Latimeria
Dipnoi
Anura
Urodela
Gymnophiona
Amniota
Lissamphibia
Lissamphibia
Lissamphibia
(Lissamphibia)
Batrachia
Batrachia
Batrachia
Batrachia
Lepospondyli
Dipnomorpha
Tetrapoda
Tetrapoda
Tetrapoda
Tetrapoda
Sarco-
pterygii
Sarco-
pterygii
Tetrapodo-
morpha
Tetrapodo-
morpha
Actinistia
Amphibia
Amphibia
Amphibia
Actinistia
Dipnomorpha
Apateon
Sclerocephalus
Temno-
spondyli
Sclerocephalus
Temnospondyli
Amphibia
(Temnospondyli)
Amphibia
(Temnospondyli)
Apateon
Sclerocephalus
Hyloplesion
Anura
Urodela
Gymnophiona
Amniota
(Lissamphibia)
(Batrachia)
Tetrapoda
Temnospondyli
Lepospondyli
Hyloplesion
Micromelerpeton
Hyloplesion
Micromelerpeton
Lepospondyli
S.
D.
S.
Stereospondylomorpha D.
S.
Dissorophoidea
S.
D.
S.
D.
D.
S.
Dissorophoidea
(Lepospondyli)
Ichthyostega
Eusthenopteron
Amniota
a) b)
c) d)
e) f)
g) h)
Apateon
Hyloplesion
Sclerocephalus
Anura
Urodela
Gymnophiona
Amniota
not Lissamphibia
Lepospondyli + Amniota
Batrachia
Tetrapoda
Temnospondyli
Lepospondyli
Apateon
Sclerocephalus
Hyloplesion
Anura
Urodela
Gymnophiona
Amniota
not Lissamphibia
Lepospondyli + Amniota
(Batrachia)
Tetrapoda
Temnospondyli
Lepospondyli
Lepospondyli Hyloplesion
Archegosaurus
Archegosaurus
Archegosaurus
Archegosaurus Archegosaurus
Archegosaurus
Archegosaurus
Figure 1. (Caption next page)
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Lissamphibian origin and ossification sequences
Figure 1. (Previous page) Hypotheses on the relationships of the extant amphibian clades since the late 20th century.
The names of terminal taxa
sampled here for cranial characters are in boldface, those sampled for appendicular characters are underlined; the names of larger clades are placed toward
the right end of a branch if they have minimum-clade (node-based) definitions, to the left if they have maximum-clade (branch-based) definitions. Names in
parentheses would, given that phylogenetic hypothesis, not be used, but replaced by synonyms. Among terminal taxa, “Melanerpeton”humbergense, sampled
for appendicular characters, is not shown, but is always the sister-group of Apateon;Microbrachis, likewise sampled for appendicular characters, is not shown
either, but is always the sister-group of Hyloplesion;Eusthenopteron is not shown in c)–h), where it forms the outgroup (b)). See text for Micromelerpeton and
for references. The first two trees (a, b) show the current consensus; the other trees (c–h) show the various tested paleontological hypotheses. Abbreviations:
D., Dissorophoidea; S., Stereospondylomorpha.
a)
Consensus of the latest phylogenetic analyses of molecular data ; all named clades are therefore extant.
Note the monophyly of the extant amphibians (Lissamphibia, marked with a light gray dot) with respect to Amniota.
b)
Consensus of all recent analyses of
Paleozoic limbed vertebrates, omitting the extant amphibian clades. Note the monophyly of “lepospondyls”+ amniotes (marked with a dark gray dot).
c)
TH:
“temnospondyl hypothesis”. Lissamphibia nested among dissorophoid temnospondyls. Compatible with both a) and b) (gray dots).
d)
LH: “lepospondyl
hypothesis”. Lissamphibia nested among “lepospondyls”; consequently, temnospondyls are not crown-group tetrapods. Compatible with both a) and b)
(gray dots).
e)
PH1: “polyphyly hypothesis”, first variant. Urodela as dissorophoid temnospondyls close to Apateon, Anura as a separate clade of dissorophoid
temnospondyls, Gymnophiona as “lepospondyls”. Compatible with b) (dark gray dot) but not with a) (light gray circle).
f)
PH2: “polyphyly hypothesis”, second
variant. Like PH1, but with restored monophyly of extant amphibians with respect to amniotes (light gray dot; see a)) at the expense of compatibility with the
paleontological consensus concerning the position of temnospondyls, lepospondyls, and amniotes (dark gray circle; see b)).
g)
DH1: “diphyly hypothesis”,
first variant. Batrachia as dissorophoid temnospondyls, Gymnophiona as “lepospondyls”. Compatible with b) (dark gray dot) but not with a) (light gray
circle).
h)
DH2: “diphyly hypothesis”, second variant. Batrachia as dissorophoid temnospondyls, Gymnophiona as stereospondylomorph temnospondyls.
Compatible with both a) and b).
Laurin, 2019
for this fact and a discussion of Bayesian analysis of paleontological datasets). Further, a
monophyletic origin of all extant amphibians among lepospondyls has also been proposed (
Laurin, 1998
;
Pawley, 2006, appendix 16
;
Marjanović and Laurin, 2009, 2013, 2019
). This will be referred to below as
the lepospondyl hypothesis (LH; Fig. 1d).
Phylogenetic analyses of molecular data cannot distinguish the TH, the PH2, the DH2 or the LH from
each other by topology (
Fig. 1
) because all of these imply lissamphibian monophyly with respect to
amniotes, and molecular data are not available from any other tetrapodomorphs. Several other types of
data and methods have, however, been used to try to discriminate between the various hypotheses on the
origin of extant amphibians. In addition to classical phylogenetic analyses of morphological data matrices,
these include the use of molecular dating (
Zhang et al., 2005
;
Marjanović and Laurin, 2007
;
Pardo et al.,
2017a
) and stratigraphic data (
Marjanović and Laurin, 2008
) to compare the inferred divergence dates
between the three main extant amphibian clades on the basis of molecular data with predictions based
on the fossil record under the TH and the LH on one side and the PH and the DH on the other. However,
developmental data, in the form of ossification sequences, have been the second-most frequently used
(after classic morphological data) to argue for particular phylogenetic hypotheses. These data include
mainly cranial (e.g.,
Schoch, 2002, 2006
;
Schoch and Carroll, 2003
;
Schoch and Milner, 2004
;
Anderson,
2007
;
Carroll, 2007
;
Germain and Laurin, 2009
) and autopodial ossification sequences (e.g.,
Fröbisch et
al., 2007, 2015
). Ossification sequences of other parts of the skeleton, like the vertebrae, shoulder girdle
and scales, are also documented in a few Paleozoic stegocephalians (e.g.,
Carroll et al., 1999
;
Witzmann,
2006
;
Anderson, 2007
;
Carroll, 2007
;
Olori, 2013
), not to mention finned tetrapodomorphs (
Cloutier,
2010
), but these have played a minor role in the controversy about the origin of extant amphibians.
Recently,
Danto et al. (2019)
concluded that vertebral ossification sequences varied too quickly and could
not be used to assess the origin of lissamphibians. This study relies on both cranial and appendicular
ossification sequences and compares their implications for tetrapod phylogeny.
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Lissamphibian origin and ossification sequences
Material and methods
Ossification sequence data
From all the literature we could access, we compiled the most extensive database on ossification se-
quences for osteichthyans that exists to date. The most useful sources for extant taxa included compila-
tions:
Harrington et al. (2013)
for amphibians,
Weisbecker and Mitgutsch (2010)
for anurans,
Hugi et al.
(2012)
for squamates,
Maxwell et al. (2010)
for birds, and
Koyabu et al. (2014)
and
Weisbecker (2011)
for mammals. The cranial and appendicular sequences of Permian temnospondyls (the stereospondy-
lomorphs Sclerocephalus and Archegosaurus, the non-branchiosaurid “branchiosaur”Micromelerpeton
and the branchiosaurids “Melanerpeton”humbergense,Apateon caducus and A. pedestris) were assembled
from several references cited in the
Appendix
; note that the two Apateon species are each represented
by two different sequences scored after populations from two separate paleo-lakes (Erdesbach and
Obermoschel) in which both species occur. Appendicular ossification sequences of the lepospondyls
Microbrachis and Hyloplesion are incorporated from
Olori (2013)
, that for the finned tetrapodomorph
Eusthenopteron was combined from Cote et al. (2002) and Leblanc and Cloutier (2005).
All sources of our sequence data can be found in the
Appendix
. The sequences themselves and the
phylogenetic trees corresponding to the tested hypotheses are included in the
Supplementary informa-
tion
. The sequences were not used to generate the tree topology or the branch lengths (which represent
evolutionary time); the tree is compiled from published sources (provided below) which did not use any
ossification sequences in their phylogenetic analyses.
The software we used to compute AICc weights, the CoMET module (
Lee et al., 2006
) for Mesquite
3.6 (
Maddison and Maddison, 2018
), cannot handle missing data. This unfortunately meant we had to
discard much information. In order to keep as many taxa as possible in the analysis, we first compiled
a matrix (not shown) of 244 taxa and 213 characters. All of these characters are positions of skeletal
elements (cranial, appendicular, axial and others) in ossification sequences, standardized between 0
and 1 following
Germain and Laurin (2009)
, as explained below. Of these, we kept characters that were
scored in the Paleozoic taxa in our initial database, and extant taxa that were scored for the same sets of
characters. This resulted in two initial datasets, one of cranial and one of appendicular sequences (it was
not possible to include both sets of sequences together because this would have left too few taxa in the
matrix).
In the end, however, we were left with three overlapping cranial datasets. The largest cranial dataset
we could make, dataset 2 of
Table 1
, has 105 taxa (103 extant, plus the two species of Apateon scored
from Erdesbach) and seven characters: the appearance times of the premaxilla, maxilla, nasal, parietal,
pterygoid, exoccipital and squamosal bones. It lacks Sclerocephalus, which cannot be scored for the
appearance time of the squamosal. This is unfortunate because Sclerocephalus is one of only three extinct
taxa for which a usable cranial ossification sequence is known at all, and further because it occupies a
special place in the DH2, according to which it lies on the caecilian stem. We attempted to compensate for
this deficiency by assembling two more cranial datasets: dataset 1, which contains 107 taxa (104 extant,
Apateon spp. from Erdesbach, and Sclerocephalus) but only six characters by lacking the squamosal, and
dataset 5, which includes 84 taxa (81 extant, Apateon spp. from Erdesbach, and Sclerocephalus) and eight
cranial characters (the vomer and the frontal bone are added to the six of dataset 1).
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Lissamphibian origin and ossification sequences
Table 1. List of datasets used in this paper.
All are subsets of our global compilation that were selected to meet the requirement of the method used
(missing data cannot be handled). The temnospondyl species Apateon caducus and A. pedestris are included in all datasets, but scored after populations from
two different paleo-lakes in which both species occur.
Dataset number 1 2 3 4 5
Type of characters cranial cranial appendicular appendicular cranial
Number of characters 6 7 7 4 8
Number of taxa 107 105 62 65 84
Sclerocephalus yes no yes yes yes
Source of data for Apateon Erdesbach Erdesbach Obermoschel Erdesbach and Obermoschel Erdesbach
Additional Paleozoic taxa None None Archegosaurus,Micromelerpeton,Hyloplesion,
Microbrachis,Eusthenopteron
Archegosaurus,Micromelerpeton, "Melanerpeton"
humbergense,Hyloplesion,Microbrachis,
Eusthenopteron
None
Tables in which it is used 2, 5 3, 6 4, 8 4, 9 7
For the appendicular characters, in addition to dataset 3 which contains seven characters (humerus,
radius, ulna, ilium, femur, tibia and fibula) and 62 taxa (54 extant, Apateon spp. from Obermoschel,
Sclerocephalus,Archegosaurus,Micromelerpeton,Hyloplesion,Microbrachis and Eusthenopteron), another
(dataset 4) includes only four characters (radius, ulna, ilium, and femur), but it features 65 sequences, the
additional data being Apateon spp. from Erdesbach and “Melanerpeton”humbergense. See
Table 1
for a list
of these datasets and the Supplementary information for the datasets themselves.
The data loss in these various datasets is not as severe as it may first seem, because most of the
characters that have been excluded from these analyses had less than 10% scored cells (sometimes less
than 1%), and most of them could not be scored for any temnospondyl or lepospondyl, so they could not
have helped resolve the main question examined in this study.
The order in which the sampled cranial bones ossify varies substantially in our sample of taxa, but
based on simple (not phylogenetically-weighted) average position, the frontal appears first, followed
closely by the premaxilla, parietal, and maxilla (in close succession), and then by the squamosal, exoccipi-
tal, pterygoid, and last by the nasal. However, each of these bones ossifies first (among these bones; not
necessarily in the whole skeleton) in at least one of the included taxa. Among the appendicular bones,
there is more variability; each ossifies first in at least one of the 62 sampled taxa, and three (radius, ulna
and ilium) ossify last in at least one taxon.
Due to the homology problems between the skull bones of tetrapods and actinopterygians and missing
data, we had to omit all actinopterygians from our analyses. As cranial ossification sequences remain
poorly documented for extant finned sarcopterygians, except perhaps lungfish, whose skull bones seem
mostly impossible to homologize (
Criswell, 2015
), our analyses of those data are restricted to limbed
vertebrates. However, for appendicular data, we were able to include the Devonian tristichopterid Eu-
sthenopteron foordi.
Unfortunately, the only cranial ossification sequence available for any supposed lepospondyl, that of
the aïstopod Phlegethontia longissima, is documented from only three ossification stages (
Anderson et al.,
2003
;
Anderson, 2007
). This poses a problem for our analysis method, which assumes that character
evolution can be modeled as Brownian motion; this assumption is decreasingly realistic as the number of
character states (sequence positions) decreases, because the resulting distribution deviates increasingly
from that of a continuous character. Furthermore, some recent anatomical restudies and phylogenetic
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Lissamphibian origin and ossification sequences
analyses suggest that aïstopods are not lepospondyls, but early-branching stem-stegocephalians (
Pardo
et al., 2017b;Pardo et al., 2018;Clack et al., 2019;Mann et al., 2019).
The low taxon sample is more limiting for this analysis than the low character sample. However,
as explained below, the absence of lepospondyl sequences in our cranial dataset does not preclude
testing the six hypotheses (TH, PH1, PH2, DH1, DH2, LH; see above or
Figure 1
for the explanation of
these abbreviations) because each of these six hypotheses makes different predictions about where
temnospondyls and caecilians fit relative to other taxa. Thus, in the absence of lepospondyls in our
dataset, the tests of these hypotheses are somewhat indirect and inference-based, but they remain
possible. Our tests based on appendicular data include two lepospondyls (Hyloplesion longicostatum and
Microbrachis pelikani), but the absence of caecilians in that dataset proves more limiting than the absence
of lepospondyls in the cranial dataset because the TH, DH1 and DH2 become indistinguishable (
Fig. 1c,
g, h
). However, the presence of the temnospondyl Micromelerpeton allows us to test two variants of the
TH/DH distinguished by the monophyly (e.g.,
Ruta and Coates, 2007
) or polyphyly (e.g.,
Schoch, 2019
) of
“branchiosaurs”(the temnospondyls Apateon,“Melanerpeton”humbergense and Micromelerpeton).
Sensitivity analysis for sequence polymorphism
Given the potential impact of intraspecific variability in ossification sequence on inferred nodal sequences
and heterochrony (
Olori, 2013
;
Sheil et al., 2014
), we compiled two consensus sequences for Apateon
caducus and A. pedestris each, representing two localities where both species occur, the paleo-lakes
of Erdesbach (
Schoch, 2004
) and Obermoschel (
Werneburg, 2018
). Based on dataset 4 (see
Table 1
),
we incorporated these into a global and two separate analyses (one analysis per locality) to determine
the impact of the observed variability. As detailed above, incorporating the sequences from Erdesbach
reduced the number of characters from seven to only four because the software used cannot handle
missing data (see above and below), but this information loss is compensated by the great increase in
number of sequences from extinct taxa (eleven instead of two, when counting the sequences of Apateon
from both localities separately) and the fact that this includes some lepospondyls (see above and below).
It would have been even better to perform a sensitivity analysis incorporating variability for all taxa for
which such information was available, but given the scope and nature of our study, this would have been
exceedingly time-consuming and is best left for the future.
Standardization of the data
Given that various taxa differ in their numbers of bones and that the resolution of the sequences is also
variable between taxa, these data needed to be standardized to make comparisons and computations
meaningful, as suggested by
Germain and Laurin (2009)
. Note that we performed this standardization
on the complete dataset of characters, before filtering for data completeness. This complete dataset (not
shown) includes 213 cranial, appendicular and other characters, but no taxon is scored for all characters,
because that matrix has much missing data. For instance, the most completely scored taxon, Amia calva,
still has 57.4% missing data (more than half), which indicates that 92 characters were scored for this taxon,
including several ties (the resolution was 41 positions, so they varied by increments of 0.025 or 2.5% of the
recorded ontogeny). We did not re-standardize after filtering characters out because we believe that the
initial standardization better reflects the relative position of events in development than a standardization
based on only seven events in ontogeny. Because of this, some characters in the reduced datasets lack
states 0 or 1 for some taxa. This is simply because the first or last events in the ontogenetic sequence
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Lissamphibian origin and ossification sequences
were filtered out. Thus, we used the position in the sequence (from first to last, in the complete dataset)
and standardized this relative sequence position between 0 and 1 using the formula given by
Germain
and Laurin (2009). The standardized sequence position (Xs) is:
Xs= (Xi−Xmin)/(Xmax −Xmin )
where:
Xiis the position of a given bone in the sequence
Xmin is the lowest position in the sequence (generally 0 or 1)
Xmax
is the highest position in the sequence (for instance, if there are 20 bones,
Xmin
is 1 and the
sequence is completely resolved, Xmax = 20).
This yields a standardized scale that varies between 0 and 1 for each taxon, in which 0 and 1 are the
positions of the first and last events in the sequence, respectively. For instance, for Ambystoma maculatum
(an extant urodele), in the original dataset, the first events (tied) were the ossification of premaxilla, vomer,
dentary and coronoid (standardized position: 0); the last event was the articular (standardized position:
1), and there is a resolution of 12 positions (hence, increments of 0.0909 or 1/11). However, in the final
dataset of 7 characters, the articular is absent; hence, the first bone in the sequence is the premaxilla, at
a standardized position of 0, and the last is the nasal, as a standardized position of 0.8181 because all
events in position 1 (articular) and 0.9091 (stapes) have been filtered out.
We also experimented with using size (skull length) or developmental stage as standards, but this led
to lower sequence resolution because body size is not available for all sequence positions and for all taxa
(results not shown), so we worked only with sequences standardized by position. Given that our data
filtering procedure retains few data (only six, seven or eight characters for the cranial dataset, and four or
seven characters for the postcranial dataset), it is important to use the method that discards the least
amount of data, and this was achieved by using sequence position. We do not imply that standardizing by
size is not recommended in general. On the contrary, if good body size data were available for all taxa and
all developmental stages, this should be a better strategy, and only having access to absolute time should
be even better. However, practical limitations of data availability prevent us from using these methods
now.
Our ossification sequence data (reduced dataset of four to eight characters) of extant and extinct taxa,
and the phylogenetic trees we used, are available in the Supplementary information.
Analysis methods
To discriminate between the six hypotheses about the origin of extant amphibians, two methods are
available: direct phylogenetic analysis of the sequence data, and comparisons of the tree length (number
of steps in regular parsimony, squared length in squared-change parsimony, likelihood, or similar mea-
sures) of various trees selected a priori to represent these hypotheses (in these trees, only the position of
caecilians and extinct taxa, here temnospondyls and lepospondyls, varies). We used both approaches
but expected the second to perform much better because relatively few data are available, and thus,
phylogenetic analysis of such data is unlikely to provide a well-resolved tree.
For the first approach, we first transformed the standardized sequence positions back into discrete
characters using formulae in a spreadsheet and scaled the characters so that the highest state in all
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Lissamphibian origin and ossification sequences
would be 9. This ensures that each character has equal weight in the analysis, regardless of its variability
in the ossification sequence. The characters were ordered to reflect the assumed evolutionary model
(ontogenetic timing is a quantitative character that was discretized) and because for such characters,
ordering yields better results (
Rineau et al., 2015, 2018
; see discussion in
Marjanović and Laurin, 2019
).
The resulting data matrices (one for cranial and another for appendicular characters, both with seven
characters each) were analysed using parsimony in PAUP* 4.0a165 (
Swofford, 2019
). We used the TBR
(tree bisection-reconnection) branch swapping algorithm and performed a search with 50 random addi-
tion replicates (or several such searches, for the cranial data) while holding two trees at each step and with
a maximum number of trees set at one million. For cranial data, the main search lasted about 100 hours
on a MacBook Pro Retina with a 2.5 GHz iCore 7 quadri-core processor and 16 GB RAM. The exact search
time cannot be reported because PAUP* crashed after saving the trees to a file for one of the longest
runs (several analyses were made, over several days), but before the log could be saved. The analysis of
the seven appendicular characters was much faster (27 minutes and a half), presumably because that
matrix has fewer taxa (62 instead of 105).
For the second approach (comparison of fit of various trees selected a priori to reflect previously
published hypotheses), we used the CoMET module (
Lee et al., 2006
) for Mesquite 3.6 (
Maddison and
Maddison, 2018
) to test the relative fit of the data on trees representing the six hypotheses. CoMET
calculates the likelihood and the AIC (Akaike Information Criterion) of nine evolutionary models given
continuous data and a tree. Note that our data only represent an approximation of continuous data; if
standardization had been performed on developmental time or body size, the data would actually have
been continuous. Standardization was carried out using sequence position because of data limitation
problems, so the data actually follow a decimalized meristic scale. However, the difference between
these situations decreases as the number of sequence positions increases, and our global scale includes
up to 41 positions (and an average of 10.9 positions), so our data should approximate a continuous
distribution sufficiently well for our analyses to be valid. This consideration prevents us from adding
the highly apomorphic aïstopod Phlegethontia, for which only three cranial ossification stages are known
(
Anderson et al., 2003
;
Anderson, 2007
); moreover, five of the eight bones included in our analyses
appear in the last two of these stages, one of the relevant bones (vomer) is absent and two (parietal
and exoccipital) are not present as separate ossifications, which would create additional missing data.
In that case, the very low number of stages would create strong departures from the assumption of
continuous data. This would probably create statistical artifacts, and the uncertainty about the position
of Phlegethontia (
Pardo et al., 2017b
;
Pardo et al., 2018
;
Clack et al., 2019
;
Marjanović and Laurin, 2019
)
would complicate interpretation of the results.
The nine models evaluated by CoMET are obtained by modifying the branch lengths of the reference
tree. Thus, branches can be set to 0 (for internal branches only, to yield a non-phylogenetic model),
to 1 (equal or speciational model), left unchanged from their original length (gradual evolution in our
case, where the original lengths represent geologic time), or set free and evaluated from the data (free
model). This can be applied to internal and/or external branches, and various combinations of these yield
nine models (
Lee et al., 2006, fig. 1
). Among these nine models two have been frequently discussed in
the literature and are especially relevant. The first is gradual evolution, in which branch lengths (here
representing evolutionary time) have not been changed. The second is the speciational model, in which all
branches are set to the same length because changes are thought to occur at speciation events, which are
typically equated with cladogeneses in evolutionary models (
Bokma et al., 2016
). This model has some
similarities with Eldredge and Gould’s
(1972)
punctuated equilibria (though a model with one internal
branch stemming from each node set to 0 and the other set to 1 would be even closer to the original
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Lissamphibian origin and ossification sequences
formulation of that model). In this study, we assessed the fit of six of the nine models covered by CoMET;
the other three (the punctuated versions of distance [original branch length], equal and free), in which
one of each pair of daughter-lineages has a branch length of zero, could not be assessed due to problems
in the current version of CoMET and possibly the size of our dataset.
Provided that the same evolutionary model is optimal for all compared phylogenetic hypotheses (this
condition is met, as shown below), the AIC weights of the various trees under that model can be used to
assess the support for each tree. In such comparisons, the topology is part of the evolutionary model,
and the data are the sequences. These comparisons can show not only which tree is best supported,
but how many times more probable the best tree is compared to the alternatives. This quantification
is another reason to prefer this approach over a phylogenetic analysis (performed below, but with the
poor results that we anticipated), which can at best yield a set of trees showing where the extinct taxa
most parsimoniously fit (if we had dozens of characters, this might be feasible). Comparisons with other
hypotheses through direct phylogenetic analysis are not possible. Given the small sample size (which
here is the number of characters), we computed the corrected AIC (AICc) and the AICc weights using the
formulae given by Anderson and Burnham (2002) and Wagenmakers and Farrell (2004).
Our tests make sense only in the presence of a phylogenetic signal in the data. In addition to the test
of evolutionary model in CoMET mentioned above (which tests non-phylogenetic as well as phylogenetic
models), we performed a test based on squared-change parsimony (
Maddison, 1991
) and random taxon
reshuffling (
Laurin, 2014
). For this test, we compared the length of the LH (lepospondyl hypothesis;
Fig. 1d
) reference tree (with and without Sclerocephalus) to a population of 10,000 random trees produced
by taxon reshuffling.
It could be argued that using other methods (in addition to the method outlined above) would have
facilitated comparisons with previous studies. However, the two main alternative methods, event-pair
cracking with Parsimov (
Jeffery et al., 2005
) and Parsimov-based genetic inference (PGI;
Harrison and
Larsson, 2008
), have drawbacks that led us to not using them. Our objections against event-pair cracking
with Parsimov were detailed by
Germain and Laurin (2009)
.In short, that method requires an unneces-
sary decomposition of sequences into event pairs, and it cannot incorporate absolute timing information
(in the form of time, developmental stage or body size, for instance) or branch length information. More
importantly, the simulations performed by
Germain and Laurin (2009)
showed that event-pair cracking
with Parsimov yields more artefactual change and has lower power to detect real sequence shifts. That
method is also problematic when trying to infer ancestral sequences and can lead to impossible ancestral
reconstructions (e.g. A occurs before B, B occurs before C, and C occurs before A), as had been docu-
mented previously (
Schulmeister and Wheeler, 2004, p. 55
). This would create problems when trying to
compare the fit of the data on various phylogenetic hypotheses. The performance of Parsimov-based
genetic inference (PGI;
Harrison and Larsson, 2008
) has not been assessed by simulations, but it rests on
an edit cost function that is contrary to our working hypothesis (that the timing of developmental events
can be modeled with a bounded Brownian motion model, which is assumed by continuous analysis).
More specifically,
Harrison and Larsson (2008, p. 380)
stated that their function attempts to minimize the
number of sequence changes, regardless of the magnitude of these changes. We believe that disregarding
the size of changes is unrealistic, as shown by the fact that Poe’s
(2006)
analyses of thirteen empirical
datasets rejected that model (which he called UC, for unconstrained change) in favor of the model we
accept (AJ for adjacent states, which favors small changes over large ones). Furthermore, analyses of
ossification sequence data using techniques for continuous data as done here (see above) have been
performed by an increasingly large number of studies (e.g.,
Skawiński and Borczyk, 2017
;
Spiekman and
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Lissamphibian origin and ossification sequences
Figure 2. Reference phylogeny used for some of the analyses, illustrating the LH (lepospondyl hypothesis) of lissamphibian origins.
The tree was
time-calibrated, but analyses showed that branch lengths are irrelevant, given that the best model is speciational (
Tables 2–4
). Main sources for topology
and divergence times:
Reeder (2003)
;
Brandley et al. (2005)
;
Pons et al. (2005)
;
Lecompte et al. (2008)
;
Bossuyt and Roelants (2009)
;
Germain and Laurin
(2009)
;
Hugall et al. (2007)
;
Gonzalez et al. (2009)
;
Meredith et al. (2011)
;
Sterli et al. (2013)
;
Wang et al. (2013)
;
Marjanović and Laurin (2014, 2019)
);
Pyron (2014)
;
Rabosky et al. (2014)
;
Schoch (2014b)
;
Prum et al. (2015)
;
Zhuang et al. (2015)
;
Tarver et al. (2016)
;
Feng et al. (2017)
;
Irisarri et al. (2017)
;
Lu et al. (2017)
;
Pardo et al. (2017a)
;
Jetz and Pyron (2018)
. The colored bands represent geological stages from the international geological timescale (
Ogg
et al., 2016).
Werneburg, 2017
;
Werneburg and Geiger, 2017
; just to mention papers published in 2017), so the issue
of ease of comparisons of our results with other studies is not as serious as it would have been only a few
years ago, and it should be decreasingly so in the future.
Reference phylogenies
We built a reference timetree that attempts to capture the established consensus (
Fig. 2
; see the next
paragraphs for the sources). The tree was compiled in Mesquite versions up to 3.6 (
Maddison and Mad-
dison, 2018
) and time-calibrated using the Stratigraphic Tools module for Mesquite (
Josse et al., 2006
).
For consistency and to avoid the effects of gaps in the fossil record, we used molecular divergence dates
whenever possible. The tree had to be time-scaled because many of the evolutionary models that we fit
on the tree in the first series of tests (to determine which evolutionary model can be used to compare the
fit of the hypotheses) use branch lengths to assess model fit. Note that our procedure requires estimating
divergence times between all taxa (geological ages of all nodes). When taxa are pruned, branch lengths
are adjusted automatically. The main sources we used for topology and divergence times (and hence
branch lengths) are as follows.
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Lissamphibian origin and ossification sequences
The phylogeny of lissamphibians follows the work of
Jetz and Pyron (2018)
. However, several other
sources have been used for the temporal calibration of the tree:
Germain and Laurin (2009)
was used
for the urodeles, whereas
Feng et al. (2017)
, supplemented by
Bossuyt and Roelants (2009)
and
Pyron
(2014)
, was used for the anurans as well as more rootward nodes (Batrachia, Lissamphibia, Tetrapoda;
also Amniota). Marjanović and Laurin (2014) was used for the Ranidae, Ceratophryidae and Hylidae.
The sediments that have preserved the temnospondyls Apateon and Sclerocephalus are not easy to
correlate with each other or with the global chronostratigraphic scale. Combining stratigraphic infor-
mation from
Schoch (2014b)
,
Schneider et al. (2015)
and
Werneburg (2018)
, we have placed all three
sampled species (A. pedestris,A. caducus,S. haeuseri) at the Sakmarian/Artinskian stage boundary (Permian;
290.1 Ma ago); combining stratigraphic information from
Schneider et al. (2015)
with the phylogeny in
Schoch (2014b)
, we have tentatively placed the divergence between the two Apateon species (which are
not sister-groups:
Schoch, 2014b
) at the Kasimovian/Gzhelian stage boundary (Carboniferous; 303.7 Ma
ago). The age of the last common ancestor of Apateon and Sclerocephalus depends strongly on tem-
nospondyl phylogeny, which remains unresolved (
Pardo et al., 2017a
;
Marjanović and Laurin, 2019
; and
numerous references in both); as a compromise between the various options, we have provisionally
placed it at the boundary between the Early and the Late Carboniferous (Serpukhovian/ Bashkirian,
323.2 Ma ago) where applicable.
We sampled many extant amniotes to achieve broad coverage of Tetrapoda. For the birds,
Pons et al.
(2005)
was used for the Laridae,
Wang et al. (2013)
for the Phasianidae and
Gonzalez et al. (2009)
for
the Anatidae. The temporal calibration was taken from
Prum et al. (2015)
as recommended by
Berv and
Field (2018); gaps were filled in using the database www.birdtree.org.
Several papers, mainly
Tarver et al. (2016)
, were used for the phylogeny and divergence times of
mammals. For the Muridae, three references were used:
Lecompte et al. (2008)
,
Zhuang et al. (2015)
,
and
Lu et al. (2017)
for the position of two taxa: Mesocricetus auratus and Peromyscus melanophrys. Other
species were placed following the work of
Meredith et al. (2011)
, which also gives divergence times.
We caution, however, that all available molecular dates for Paleogene and earlier mammal nodes are
controversial and may be overestimates (Berv and Field, 2018;Phillips and Fruciano, 2018).
Three references were also used to integrate squamates in the phylogenetic tree and for the calibration
of divergence times:
Brandley et al. (2005)
,
Rabosky et al. (2014)
,
Reeder (2003)
.
Sterli et al. (2013)
was
used for turtles.
For turtles, there is now a near-consensus that they are diapsids, a hypothesis that is not necessarily
incompatible with an origin among “parareptiles”(
Laurin and Piñeiro, 2017
). Thus, following most recent
phylogenetic analyses of molecular data (e.g.,
Hugall et al., 2007
;
Irisarri et al., 2017
), we have inserted
them as the sister-group of Archosauria.
We disagree with several of the calibration dates in
Irisarri et al. (2017)
, which often appear unreason-
ably old. For instance, they place the divergence between caecilians and batrachians and the divergence
between anurans and urodeles in the Early Carboniferous, around 330 and 320 Ma, respectively, but our
thorough analyses of the fossil record, with due consideration of its incompleteness, suggest significantly
more recent dates, in the Permian (
Marjanović and Laurin, 2007, 2008, 2014
). This is not surprising
because some of the dating constraints used by
Irisarri et al. (2017, table S8)
are wrong. For instance,
they enforced a minimal divergence age between cryptodiran and pleurodiran turtles of 210 Ma (Late
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Lissamphibian origin and ossification sequences
Triassic), but all analyses of the last fifteen years (e.g.,
Sterli et al., 2013
,
2018
) strongly suggest that the
oldest known turtles that fit within this dichotomy date from the Late Jurassic, less than 165 Ma. The
divergence between humans and armadillos (boreotherian and xenarthran placentals) was constrained
to the middle of the Cretaceous (95.3–113 Ma), based on outdated literature that assigned a wide variety
of stem-eutherians to highly nested positions in the placental crown; there are currently no clear pla-
centals known from any Cretaceous sediments even as young as 66 Ma (e.g.,
Halliday et al., 2016
,
2017
;
Davies et al., 2017
;
Phillips and Fruciano, 2018
), barely half the age of the older end of the constraint
range. Conversely, the divergence between diapsids (hence sauropsids) and synapsids had a minimal
age constraint of 288 Ma (Early Permian), which is much too young given the presence of sauropsids
(and presumed synapsids) in Joggins, in sediments that have recently been dated (
Carpenter et al., 2015
)
around 317–319 Ma (early Late Carboniferous). Thus, we have not used divergence dates from that source.
To discriminate among the hypotheses on lissamphibian origins, we inserted the temnospondyl
Apateon in the tree where each predicts that it should be (
Fig. 1c–h
). Thus, according to the TH (tem-
nospondyl hypothesis;
Fig. 1c
), Apateon lies on the lissamphibian stem. Under the LH (lepospondyl
hypothesis;
Fig. 1d
), Apateon lies on the tetrapod stem. Under both versions of the DH (diphyly hypothesis;
Fig. 1g, h
), Apateon lies on the batrachian stem. Under both versions of the PH (polyphyly hypothesis;
Fig. 1e, f
), Apateon lies on the caudate stem. Within the DH and the PH, both versions of each differ
in the position of Gymnophiona. Thus, despite the absence of any lepospondyl in our cranial ossifica-
tion sequence datasets, our taxonomic sample allows us to test all these competing hypotheses. The
appendicular datasets allow more direct tests of some of these hypotheses because they include two
lepospondyl taxa, which were likewise placed in trees representing the tested hypotheses (Fig. 1).
Sclerocephalus is the sister-group of Apateon under the LH (
Fig. 1d
), immediately rootward of it (on
the lissamphibian stem) under the TH (
Fig. 1c
) and likewise (but on the batrachian stem) under the
DH1 (
Fig. 1g
), on the caecilian stem under the DH2 (
Fig. 1h
) and the sister-group of Batrachia (including
Apateon) under both versions of the PH (Fig. 1e, f).
“Melanerpeton”humbergense (appendicular data only) is the sister-group of Apateon in all trees, except
under the hypothesis of branchiosaur paraphyly; Eusthenopteron (appendicular data only) forms the
outgroup in all trees.
The lepospondyls Microbrachis and Hyloplesion, from both of which only appendicular data are available,
form an exclusive clade (
Clack et al., 2019
;
Marjanović and Laurin, 2019
). This clade is the sister-group
of Lissamphibia (represented only by Batrachia because caecilians are lacking from the appendicular
datasets) under the LH, of Amniota under the TH and both versions of the DH (these three cannot be
distinguished due to the absence of caecilians) as well as under the PH1, and of Temnospondyli (including
Batrachia) under the PH2 (see the legend of Figure 1 for an explanation of these abbreviations).
The temnospondyl Micromelerpeton, from which likewise only appendicular data are available, forms
the sister-group of Apateon under the LH. The uncertainty over its phylogenetic position within Dis-
sorophoidea (as the sister-group to the rest, including anurans and urodeles: e.g.
Schoch, 2019
; as
the sister-group of Apateon +“Melanerpeton”humbergense: e.g.
Ruta and Coates, 2007
;
Marjanović and
Laurin, 2019
) generates two versions of the TH/DH1/DH2 tree for the appendicular dataset. We tested
both of these versions against that dataset, for a total of five trees.
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Lissamphibian origin and ossification sequences
Figure 3. Strict consensus of the most parsimonious trees obtained by analyzing cranial dataset 2.
Dataset 2 comprises 105 taxa and seven
characters (see
Table 1
). Note that several higher taxa whose monophyly is well-established are para- or polyphyletic here. Abbreviations: C., Coturnix; L.,
Larus. Asterisks meaningless.
To ensure that our analyses were not biased in favor of a given hypothesis, and in case that a con-
tinuous evolutionary model were favored, we initially adjusted the branch lengths such that the sum of
branch lengths was equal between the compared topologies and that the root was approximately at the
same age (in this case in the Tournaisian, the first stage of the Carboniferous). This was done for the trees
used to compare the hypotheses using the cranial dataset because if a model incorporating (variable)
branch length information had been selected, and if the trees representing the various hypotheses had
not all had the same total length (the sum of all branch lengths), the resulting distortions in branch lengths
created around the extinct taxa (whose height compared to extant taxa is specified by their geological
age) would have introduced another variable influencing the AICc. But given that the selected model
ignores branch lengths, this precaution turned out to be superfluous. We have therefore not made these
time-consuming adjustments to the additional trees we generated later to analyze the appendicular data.
Results
In the phylogenetic analysis of cranial data, a single tree island of 22,077 trees of 438 steps was found,
only once, so there might be more trees of that length and perhaps even shorter trees. Initially, an island
of 22,075 trees was found; we swapped on each of these in a subsequent run, which only recovered two
additional trees. Given that slightly longer trees did not differ much from those that we obtained, the low
quality of the results (poor congruence with the established consensus about the monophyly of major
clades such as squamates, birds, mammals and turtles) and the fact that about four full days of computer
time had been spent on analysis of the cranial data, we did not pursue that search further. As expected,
the strict consensus tree is poorly resolved (
Fig. 3
). The majority-rule consensus (not shown, but available
in the
Supplementary information
) is more resolved but not necessarily better because much of the
additional resolution contradicts the established consensus. For the appendicular matrix, 22,757 trees of
164 steps were found. Their strict consensus (
Fig. 4
) deviates even more from the established consensus
than the tree obtained from cranial data.
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Lissamphibian origin and ossification sequences
Figure 4. Strict consensus of the most parsimonious trees obtained by analyzing appendicular dataset 3.
Dataset 3 comprises 62 taxa and seven
characters (see Table 1). The phylogenetic signal in these data seems to be lower than in the cranial data. See Fig. 3 for abbreviations.
This visual assessment of phylogenetic signal through an examination of the consensus trees (
Figs. 3,
4
) is congruent with the test based on squared-change parsimony and random taxon reshuffling (
Laurin,
2004
). Indeed, the latter indicates that the phylogenetic signal in the cranial data is fairly strong, with a
probability of less than 0.0001 that the observed covariation between the data and the tree reflects a
random distribution (none of the 10,000 random trees generated were as short as the reference tree).
However, it is weaker, with a probability of 0.0017, for the appendicular data.
The speciational model of evolution, in which all branch lengths are equal, has overwhelming support
among cranial data, whether or not the Permian temnospondyl Sclerocephalus (
Table 2
) or the squamosal
(
Table 3
) are included (including Sclerocephalus adds a second temnospondyl genus, but given that the
timing of ossification of the squamosal is unknown in Sclerocephalus, including it requires excluding the
squamosal from the analysis as described in the Methods section); the five other examined models have
AICc weights < 10
−11
. For the appendicular data, the speciational model also has the most support, but
that support is not as strong and varies depending on which dataset is analyzed (seven characters or
four) and under which phylogenetic hypothesis. In three of the four tests performed, support for the
second-best model, the non-phylogenetic/equal model, varied between 5% and 19% (Table 4).
Two main conclusions can be drawn from these tests (
Tables 2–4
). First, given that both of the best-
supported models imply equal branch lengths, actual time represented by branches can be ignored, so
we compare support of the six competing topologies using only the best-supported model (speciational).
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Lissamphibian origin and ossification sequences
Table 2. Support (AICc and AICc weights) for six evolutionary models given our reference tree (LH) and
dataset 1 (see Table 1).
Dataset 1 comprises six cranial characters (nasal, parietal, squamosal, maxilla, ptery-
goid, and exoccipital) scored in 107 taxa, including the temnospondyl Sclerocephalus. This was performed on the
tree representing the LH (lepospondyl hypothesis), but doing this on other trees leads to similar results. Numbers
presented with four significant digits; best values in boldface. "Distance" refers to keeping the original branch length
(which represent evolutionary time), "equal" sets all branch lengths (internal and terminal) to 1, "free" infers them
from the data. Abbreviations: k, number of estimable parameters; L, likelihood; wi, weight; ∆
i
, difference of AICc
from that of the Pure-Phylogenetic / Equal model.
Evolutionary model AIC L k AICc ∆iAICc wi(AICc)
Pure-Phylogenetic / Distance -584.4 293.2 1-583.4 641.2 5.85 E−140
Pure-Phylogenetic / Equal (speciational) -1225.6 613.8 1 -1224.6 0 1.000
Pure-Phylogenetic / Free 2.000 E10 -1.000 E10 486 2.000 E10 2.000 E10 <E−165
Non-Phylogenetic / Distance -473.6 237.8 1-472.6 752.0 4.97 E−164
Non-Phylogenetic / Equal -959.9 481.0 1-958.9 265.7 2.02 E−58
Non-Phylogenetic / Free 2.000 E10 -1.000 E10 244 2.000 E10 2.000 E10 <E−165
Table 3. Support (AICc and AICc weights) for six evolutionary models given our reference tree (LH) and
dataset 2 (see Table 1).
Dataset 2 comprises seven cranial characters (nasal, parietal, squamosal, premaxilla,
maxilla, pterygoid, and exoccipital) and 105 taxa, excluding Sclerocephalus. Abbreviations and boldface as in
Table 2
.
Evolutionary model AIC L k AICc ∆iAICc wi(AICc)
Pure-Phylogenetic / Distance -715.9 359.0 1-714.9 683.5 < E−26
Pure-Phylogenetic / Equal (speciational) -1399.5 700.7 1 -1398.5 0 1.000
Pure-Phylogenetic / Free 2.000 E10 -1.000 E10 306 2.000 E10 2.000 E10 0
Non-Phylogenetic / Distance -580.6 291.3 1-579.6 818.8 < E−26
Non-Phylogenetic / Equal -1106.0 554.0 1-1105.0 293.5 2.278 E−98
Non-Phylogenetic / Free 2.000 E10 -1.000 E10 244 2.000 E10 2.000 E10 <E−26
This simplifies the discussion, because it means that the original branch lengths are irrelevant (under
that model, all branch lengths are equal); unfortunately, the branch length (evolutionary time) data were
needed to reach this conclusion. Thus, the only remaining variable is the topology. Second, model fitting,
along with the test based on squared-change parsimony and random taxon reshuffling, indicates that
the phylogenetic signal in the cranial data is strong, but that it is noticeably weaker in the appendicular
data (this is shown mostly by the non-negligible support for the non-phylogenetic/equal model). Thus,
comparisons of the fit of the various phylogenetic hypotheses for the cranial data should be more reliable
than for the appendicular data. However, given that for several Paleozoic taxa (most importantly both of
the sampled lepospondyls), comparisons can be performed only for the appendicular data, these were
performed as well.
Using the speciational model, the AICc weights of the six compared topologies indicate that there is
strong support in the cranial data for the LH (lepospondyl hypothesis), with an AICc weight of 0.9885
when Sclerocephalus is included (
Table 5
) and 0.8848 when the squamosal is included instead (
Table 6
).
Of the other topologies, the TH (temnospondyl hypothesis) was by far the best supported, with an AICc
weight of 0.01144 (with Sclerocephalus) or 0.1056 (with the squamosal), which is 86.44 or 8.38 times less
than for the LH. Both versions of the DH (diphyly hypothesis) and of the PH (polyphyly hypothesis) have
negligible support (AICc weights < 0.01 when the squamosal is included, < 0.0001 when Sclerocephalus
is included). The least support is found for the PH2 when Sclerocephalus is included, and for the DH1
when the squamosal is included. In both cases, the recently proposed DH2 (
Pardo et al., 2017a
) fares
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Lissamphibian origin and ossification sequences
Table 4. AICc weights showing relative support for six evolutionary models given our appendicular datasets
(3 and 4; see Table 1) and various hypotheses.
Because of the number of analyses presented below, only the
AICc weights are presented (best values in boldface). Abbreviations: DH, diphyly hypothesis (both versions); LH,
lepospondyl hypothesis; TH, temnospondyl hypothesis.
Evolutionary model 7 characters, LH 7 characters, LH 4 characters, LH 4 characters, TH/DH
Pure-Phylogenetic / Distance 5.1857 E−149 2.340 E−70 1.227 E−52 2.646 E−52
Pure-Phylogenetic / Equal 1 0.9335 0.94459 0.8139
Pure-Phylogenetic / Free < E−179 1.598 E−277 4.012 E−158 3.002 E−155
Non-Phylogenetic / Distance 7.515 E−179 4.843 E−52 2.162 E−42 7.262 E−42
Non-Phylogenetic / Equal 2.14914 E−64 6.648 E−25.541 E−20.1861
Non-Phylogenetic / Free < E−179 <E−179 <E−179 <E−179
Table 5. Support (AIC and AICc weights) for the six topologies, reflecting the six hypotheses about the origin
of extant amphibians, under the speciational model (called Pure-Phylogenetic / Equal in Tables 2–4), with
dataset 1 (see Table 1).
Dataset 1 includes six cranial characters (nasal, parietal, squamosal, maxilla, pterygoid, and
exoccipital) and 107 taxa (including, among Paleozoic taxa, Apateon and Sclerocephalus). Abbreviations and boldface
as in
Table 2
, except ∆
i
: difference of AICc from that of the LH. Hypotheses from top to bottom: TH: monophyletic
origin among temnospondyls; LH: monophyletic origin from lepospondyls; DH1: diphyletic origin, caecilians from
lepospondyls and batrachians from temnospondyls, as in
Anderson et al. (2008)
; DH2: diphyletic origin, (batrachians
and caecilians from different temnospondyls:
Pardo et al., 2017a
); PH1: triphyletic (polyphyletic) origin with anurans
and urodeles from different temnospondyls, caecilians from lepospondyls, and lepospondyls closer to Amniota
than to Batrachia (
Fröbisch et al., 2007
); PH2: triphyletic (polyphyletic) origin as above, but with lepospondyls and
caecilians closer to temnospondyls than to amniotes (
Milner, 1993
), reflecting the well-established lissamphibian
monophyly among extant taxa (e.g., Feng et al., 2017;Irisarri et al., 2017).
Hypothesis AIC L AICc ∆iAICc wi(AICc)
TH -1217 609.4 -1215 8.919 0.01144
LH -1226 613.8 -1224 0 0.9885
DH1 -1204 602.9 -1202 21.90 1.738 E−5
DH2 -1195 598.3 -1193 31.01 1.827 E−7
PH1 -1194 597.9 -1192 31.86 1.196 E−7
PH2 -1193 597.4 -1191 32.89 7.143 E−8
second-worst by a small margin. Notably, the DH1 contradicts the modern consensus on lissamphibian
monophyly (
Fig. 1g
), while the PH2 and the DH2 fulfill this constraint from the molecular but not the
paleontological point of view, having lissamphibian monophyly with respect to amniotes but not with
respect to temnospondyls (Fig. 1f, h).
A slightly different dataset (only 84 taxa, but eight cranial characters –excluding the squamosal but
including the frontal and the vomer –and Apateon sequences for both species from Erdesbach rather
than Obermoschel) provides even stronger support for the LH, with an AICc weight of 0.9935 (
Table 7
).
The next best-supported topology, which simultaneously represents the TH, DH1 and DH2 (due to the
absence of caecilians from this dataset), has an AICc weight of only 0.0065.
The appendicular data are available in far more Paleozoic taxa than the cranial data; these include Scle-
rocephalus haeuseri,Archegosaurus decheni, and the non-branchiosaurid “branchiosaur”Micromelerpeton
credneri among temnospondyls, the lepospondyls Hyloplesion longicaudatum and Microbrachis pelikani,
and the tristichopterid finned stem-tetrapodomorph Eusthenopteron foordi, in addition to the same two
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Lissamphibian origin and ossification sequences
Table 6. Support (AIC and AICc weights) for the six topologies, reflecting the six hypotheses about the origin
of extant amphibians, for dataset 2 (see Table 1).
Dataset 2 includes seven cranial characters (nasal, parietal,
squamosal, premaxilla, maxilla, pterygoid, and exoccipital) and 105 taxa, excluding Sclerocephalus (among Paleozoic
taxa, only Apateon is present). Abbreviations, boldface and hypotheses as in Tables 2 and 5.
Hypothesis AIC L AICc ∆iAICc wi(AICc)
TH -1395 698.6 -1394 4.251 0.1056
LH -1399 700.7 -1398 0 0.8848
DH1 -1384 693.1 -1383 15.203 4.42 E−4
DH2 -1385 693.6 -1384 14.315 6.89 E−4
PH1 -1387 694.5 -1386 12.404 1.792 E−3
PH2 -1390 695.8 -1388 9.792 6.615 E−3
Table 7. Support for the various hypotheses about amphibian origins for dataset 5 (see Table 1).
Dataset 5
includes eight cranial characters (frontal added) and 84 taxa, with Apateon sequences from Erdesbach (in addition to
Sclerocephalus among Paleozoic taxa). Abbreviations, boldface and hypotheses as in
Tables 2
and
5
. Because of the
taxon sample, only three topologies can be tested.
Hypothesis AIC L AICc ∆iAICc wi(AICc)
LH -1296 649.0 -1294 0 0.9935
TH, DH1, DH2 -1286 644.0 -1284 10.061 6.493 E−3
PH1, PH2 -1274 638.0 -1272 22.038 1.628 E−5
species of Apateon as for the cranial datasets, A. caducus and A. pedestris. Analysis of these data (seven
characters: humerus, radius, ulna, ilium, femur, tibia and fibula) yields surprising results, with the PH2
having the most support, with an AICc weight of 0.7978 when using the dataset of seven bones (
Table 8
).
The TH, DH1 and DH2 with “branchiosaur”monophyly are collectively (they cannot be distinguished with
that taxonomic sample) the second-best hypotheses with that dataset, with an AICc weight of only 0.1874.
The least-supported hypothesis with these data is the TH/DH with “branchiosaur”polyphyly.
Using the other postcranial dataset with only four bones (radius, ulna, ilium, and femur) but with more
taxa (notably the branchiosaurid temnospondyl “Melanerpeton”humbergense) shows that intraspecific
variation in the postcranial ossification sequences of Apateon do not significantly impact our assessment
Table 8. Support (AICc weights) for the various hypotheses about amphibian origins according to dataset 3
(see Table 1).
Dataset 3 features seven appendicular characters (humerus, radius, ulna, ilium, femur, tibia and fibula)
and 62 taxa , including several Paleozoic taxa (the temnospondyls Archegosaurus decheni and Micromelerpeton credneri,
the lepospondyls Hyloplesion longicaudatum and Microbrachis pelikani, and the tristichopterid Eusthenopteron foordi) in
addition to Apateon (two species, A. caducus and A. pedestris) and Sclerocephalus haeuseri. The Apateon sequences
come from Obermoschel. Abbreviations, boldface and hypotheses as in
Table 5
, except that the TH and both variant
of the DH become indistinguishable, but the phylogenetic position of the "branchiosaur" Micromelerpeton can be
tested.
Hypothesis AIC L AICc ∆iAICc wi(AICc)
LH -885.0 443.5 -884.2 11.808 2.177 E−3
TH, DH (branchiosaur monophyly) -881.1 441.6 -880.3 2.897 0.1874
TH, DH (branchiosaur polyphyly) -886.4 444.2 -885.6 15.754 3.027 E−4
PH1 -888.5 445.3 -887.7 8.341 0.01232
PH2 -896.9 449.4 -896.1 0.000 0.7978
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Lissamphibian origin and ossification sequences
Table 9. Effect of the intraspecific variability in ossification sequences of Apateon on the support (AICc
weight; best values in boldface) for the various hypotheses about amphibian origins.
The dataset (number 4;
Table 1
) includes only four appendicular bones (radius, ulna, ilium, and femur) and 63 to 65 taxa but it allows testing
the impact of intraspecific variability in ossification sequences in Apateon, which are documented in two localities
(Erdesbach and Obermoschel). Because of the number of tests presented (15: five topologies x three sets of se-
quences), only the AICc weights are given. In all tests, the following Paleozoic taxa are present: Sclerocephalus haeuseri,
Archegosaurus decheni, "Melanerpeton"humbergense,Micromelerpeton credneri,Apateon (two species, A. caducus and A.
pedestris) among temnospondyls, Hyloplesion longicaudatum and Microbrachis pelikani among lepospondyls, and the
tristichopterid Eusthenopteron foordi. For abbreviations of the hypotheses, see Table 5.
Hypothesis Erdesbach and Obermoschel Erdesbach Obermoschel
LH 0.21407 0.20169 0.22657
TH, DH (branchiosaur monophyly) 0.05492 0.05265 0.05532
TH, DH (branchiosaur polyphyly) 0.03713 0.04285 0.03342
PH1 0.05653 0.05491 0.05638
PH2 0.63735 0.64790 0.62832
of the support for various hypotheses. Whether both sequences of Apateon (from the Erdesbach and
Obermoschel localities, which represent separate paleo-lakes) are included (treated as if they were distinct
taxa, such as subspecies), or whether either one of these is used in isolation, the PH2 retains the highest
support, with AICc weights of 0.62 to 0.65. The LH is a distant second, at 0.20–0.23, but still well ahead of
the TH/DH and the PH1, which all receive AICc weights between 0.03 and 0.06 (Table 9).
Discussion
Phylogenetic signal
In his discussion of previous phylogenetic conclusions from ossification sequences (e.g.,
Schoch and Car-
roll, 2003
),
Anderson (2007)
noted that ossification sequences seemed to abound in symplesiomorphies
and in autapomorphies of terminal taxa, while potential synapomorphies were scarce. This pessimism
was seemingly confirmed by
Schoch (2006)
in a paper that was published after Anderson’s
(2007)
book
chapter had gone to press: not only were many similarities in the cranial ossification sequences across
Osteichthyes found to be symplesiomorphies, but a phylogenetic analysis of cranial ossification sequences
did not recover Mammalia, Sauropsida, Amniota or Lissamphibia as monophyletic. Along with these
results,
Schoch (2006)
dismissed another: the position of the temnospondyl Apateon caducus (the only in-
cluded extinct taxon) outside the tetrapod crown-group, i.e. the lepospondyl hypothesis on lissamphibian
origins (LH).
While ossification sequences alone may not provide enough data for a phylogenetic analysis, as shown
by our results (
Figs. 3, 4
), there is clearly a phylogenetic signal because the taxa are not randomly scattered
over the tree. Specifically, our datasets (with much larger taxon samples than in
Schoch, 2006
) fit some
tree topologies much better than others. Both the tests using CoMET and squared-change parsimony
with random taxon reshuffling overwhelmingly support the presence of a strong phylogenetic signal in
the cranial data; the null hypothesis of the absence of a phylogenetic signal can be rejected in both cases,
given that it has a probability of < 10
−97
for the cranial and < 10
−4
for the appendicular dataset. We
conclude that the cranial dataset contains a strong phylogenetic signal, and are therefore cautiously
optimistic about future contributions of ossification sequences to phylogenetics. We are less optimistic
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Lissamphibian origin and ossification sequences
about the appendicular sequence data, which both tests suggest contains less phylogenetic signal.
The sizable effect on nodal estimates and inferred heterochronies of intraspecific variation found
by
Sheil et al. (2014)
in lissamphibians could raise doubts about the robustness of our findings. We
have been able to incorporate infraspecific variability in only two terminal taxa (Apateon caducus and
A. pedestris), but Apateon has played a prominent role in discussions about the significance of cranial
ossification sequences on the origins of extant amphibians (
Schoch and Carroll, 2003
;
Schoch, 2006
;
Germain and Laurin, 2009
). Thus, incorporation of intraspecific variability in Apateon is presumably much
more important than in extant taxa, even though variability in the latter would obviously add to the
analysis and should be tackled in the future. The variability in Apateon should be exempt from two sources
of artefactual variability in ossification sequences discussed by
Sheil et al. (2014)
, namely the way in
which the specimens were collected (there can be no lab-raised specimens in long-extinct taxa) and the
fixing method used (in this case, fossilization under quite consistent taphonomic conditions). The finding
that the results are very similar no matter whether we used the Apateon sequences from Erdesbach,
Obermoschel, or both, we find very similar results (
Table 9
), is reassuring. In this case, intraspecific
variation has negligible impact. However, future studies should attempt to assess the effect of more
generalized incorporation of intraspecific variability (in a greater proportion of the OTUs).
Of course, these results do not preclude functional or developmental constraints from applying to the
same data. This phenomenon has been documented, among other taxa, in urodeles, whose development
has often been compared with that of temnospondyls (e.g.,
Schoch and Carroll, 2003
;
Schoch, 2006
;
Fröbisch et al., 2007
;
Germain and Laurin, 2009
;
Fröbisch et al., 2015
). For instance,
Vorobyeva and
Hinchliffe (1996)
documented the larval functional constraints linked to early forelimb use that may cause
an early development of manual digits 1 and 2, compared with other tetrapods, as briefly discussed below.
However, in the case of our seven cranial characters, there is no evidence of functional constraints. This
is a little-investigated topic, but all these bones apparently form a single developmental module of the
urodele skull (
Laurin, 2004
). For the appendicular data, functional constraints might explain the more
subdued phylogenetic signal, but this will have to be determined by additional research.
The finding that the postcranial characters that we analyzed contain relatively little phylogenetic signal
may raise doubts about the claims that have been made about the phylogenetic implications of other such
data. Specifically,
Carroll et al. (1999)
stated that the neural arches ossify before the centra in frogs and
temnospondyls, but not in salamanders, caecilians or lepospondyls. When it was found that the centra do
ossify first in a few cryptobranchoid salamanders,
Carroll (2007, p. 30)
took this as “strong evidence that
the most primitive crown-group salamanders had a sequence of vertebral development that is common
to frogs and labyrinthodonts [including temnospondyls] (but distinct from that of lepospondyls)”.In fact,
apart from tail regeneration in Hyloplesion and Microbrachis (where the centra ossify before the neural
arches:
Olori, 2015
;
Fröbisch et al., 2015
;
Vos et al., 2018
), only one incompletely ossified vertebral
column (referred to Utaherpeton) is known of any putative lepospondyl. “In this specimen, [...] five neural
arches [...] have ossified behind the most posterior centrum”(
Carroll and Chorn, 1995, 40–41
). Carroll’s
(2007, p. 85)
claim that “the centra always ossified prior to the arches”in lepospondyls is therefore rather
puzzling.
Fröbisch et al. (2007)
and
(2015)
pointed out that the first two digital rays (digits, metapodials and
distal carpals/tarsals) ossify before the others (“preaxial polarity”) in salamanders and the temnospondyls
Apateon,Micromelerpeton and Sclerocephalus, while the fourth ossifies first (“postaxial polarity”) in am-
niotes, frogs and “probably”(
Fröbisch et al., 2015, pp. 233, 234
) the lepospondyls Microbrachis and
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Lissamphibian origin and ossification sequences
Hyloplesion. This latter inference, however, is based only on a delay in the ossification of the fifth ray that is
shared specifically with sauropsid amniotes (
Olori, 2015
). Ossification sequences (however partial) of the
other four rays in any lepospondyl are currently limited to the tarsus of Batropetes, which clearly shows
preaxial polarity (
Glienke, 2015, fig. 6O–S
;
Marjanović and Laurin, 2019
), and that of the putative (but
see
Clack et al., 2019
) lepospondyl Sauropleura, in which likewise the second distal tarsal ossified before
all others (
Marjanović and Laurin, 2019
). Outside of temno- and lepospondyls,
Marjanović and Laurin
(2013, 2019)
presented evidence that preaxial polarity is plesiomorphic, widespread and dependent on
the use of the still developing limbs for locomotion, which would explain why it was independently lost
in amniotes and frogs and reduced (the second ray still forms first, but the delays between the rays are
much reduced so that all form nearly at the same time) in direct-developing salamanders as well as in the
limb regeneration of terrestrial postmetamorphic salamanders (
Kumar et al., 2015
). It may be relevant
here that the PH2 (
Fig. 1f
), favored by our appendicular data, groups exactly those sampled taxa in a
clade that are known to have preaxial polarity in limb development. To sum up, neither our own analyses
nor the previous works that we cited above demonstrated conclusively that ossification sequences of
postcranial elements provide reliable clues about the origin of extant amphibians.
In contrast, we are reasonably confident about our results on the cranial ossification sequences. Given
the phylogenetic signal we have found in our cranial datasets, we think that ossification sequence data
should eventually be added to phenotypic datasets for analyses of tetrapod phylogeny. Indeed, an
analysis of amniote phylogeny using data from organogenesis sequences (coded using event-pairing
in Parsimov) already exists (
Werneburg and Sánchez-Villagra, 2009
). The usefulness of such data for
phylogenetic inference was further tested, with encouraging results, by
Laurin and Germain (2011)
, and
the present analysis adds additional support for it.
Indirect support for the lepospondyl hypothesis from temnospondyls
The strong support for the lepospondyl hypothesis that we have found in cranial data is surprising
because cranial ossification sequence data, especially those of the Permo-Carboniferous temnospondyl
Apateon, have often been claimed to contradict the LH (lepospondyl hypothesis,
Fig. 1d
). Similarities
between Apateon and extant urodeles, in particular the supposedly “primitive”hynobiid Ranodon, have
often been emphasized (
Schoch and Carroll, 2003
;
Schoch and Milner, 2004
;
Carroll, 2007
;
Schoch,
2014a
). However, other studies have already raised doubts about some of these claims (e.g.,
Schoch,
2006
;
Anderson, 2007
;
Germain and Laurin, 2009
).
Schoch (2006)
and
Anderson (2007)
concluded that
most characters shared between Apateon and urodeles were plesiomorphies.
Germain and Laurin (2009)
also demonstrated that, far from being very similar to the ancestral urodele morphotype (contra
Schoch
and Carroll, 2003
or
Carroll, 2007
), the cranial ossification sequence of Apateon was statistically signif-
icantly different from that of the hypothetical last common ancestor of all urodeles (as suspected by
Anderson, 2007
). However, these earlier studies did not clearly show which of the various hypotheses on
lissamphibian origins the ossification sequences of Apateon spp. –or the newly available partial sequence
(
Werneburg, 2018
) of the phylogenetically distant temnospondyl Sclerocephalus –supported most. This is
what we have attempted to do here.
Unfortunately, the development of lepospondyls is too poorly documented to be incorporated into
the cranial analyses, but we included two lepospondyls in analyses of appendicular data. These analyses
weakly favor a polyphyletic origin of extant amphibians, with both temno- and lepospondyls in the am-
phibian clade, a hypothesis that has not been advocated seriously for decades (
Milner, 1993, fig. 5B
) as
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Lissamphibian origin and ossification sequences
far as we know. However, given the moderate phylogenetic signal in these data, we view these results
with skepticism.
Olori (2011)
, using event-pairing with Parsimov (
Jeffery et al., 2005
) and PGi (
Harrison
and Larsson, 2008
), analyzed lepospondyl postcranial ossification sequences and concluded that support
for the three hypotheses that she tested (TH/DH with two different positions for Micromelerpeton, and LH)
did not differ significantly. By contrast, our analyses of the postcranial data indicate a stronger support
for polyphyly (PH2) than for the TH/DH, which is only a distant second (
Table 8
) or third (behind PH2
and LH;
Table 9
) depending on the analyses.
Olori (2011)
performed no statistical test of phylogenetic
signal of her data, though a related test (performing phylogenetic analyses on the data) yielded trees
(
Olori, 2011, fig. 5.5–5.7
) that are largely incongruent with the established consensus, in which most large
taxa (Mammalia, Testudines, Lissamphibia, etc.) are para- or polyphyletic. Olori’s
(2011)
results, like ours,
support the conclusion that the phylogenetic signal in postcranial ossification sequence data is low.
Given the current limitations in the availability of developmental data in Paleozoic stegocephalians,
we hope to have demonstrated that cranial ossification sequences of amniotes, lissamphibians and
temnospondyls provide support for the LH that is independent of the phylogenetic analyses of
Laurin
(1998)
,
Pawley (2006, appendix 6)
or
Marjanović and Laurin (2009, 2019)
. This independence is im-
portant because the cranial ossification sequence data cannot rival the morphological data in terms
of data availability, simply because growth sequences of extinct taxa are rare (
Sánchez-Villagra, 2012
),
but having a fairly independent line of evidence to investigate a major evolutionary problem is clearly
advantageous. We hope that the modest methodological progress made in this study will stimulate the
search for fossilized ontogenies (Cloutier, 2010;Sánchez-Villagra, 2010).
Acknowledgements
Jennifer Olori, two anonymous reviewers and the Recommender Robert Asher made helpful comments
that improved this paper. D. M. would further like to thank Ralf Werneburg for an electronic reprint of
his 2018 paper, Nadia Fröbisch for discussion of limb development in salamanders, and Daniel Field for
discussion of molecular divergence times and the fossil record.
Additional information
Funding
This work was supported by the CNRS (Centre National de la Recherche Scientifique) and the French
Ministry of Research (unnumbered recurring grants to the CR2P, for ML).
Competing interests
The authors declare that they have no personal or financial conflict of interest relating to the content
of this study. ML is a Recommender for PCIPaleo.
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Lissamphibian origin and ossification sequences
Author contributions
ML designed the study, supervised the data collection, analyzed the data and wrote much of the
draft; OL collected most of the ossification sequence data; DM added data to the database (mostly of
Paleozoic taxa), updated the timetrees, participated in the writing, and drafted Figure 1.
Data availability
All data used in this study are available as Supplementary material (see below)
Supplementary information
Data matrices and trees used in this analysis can be downloaded in NEXUS format for Mesquite from:
https://www.biorxiv.org/content/10.1101/352609v3.supplementary-material.
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Lissamphibian origin and ossification sequences
Appendix
Sources of data for ossification sequences
Empty cells indicate that these data are unavailable. Three methods were examined, and we used the
one for which most data were available (position in the ossification sequence, last column).
Taxa Standardization method (data type used)
Ontogenetic stages Snout-vent length (mm) Ossification sequence position
Actinopterygii
Amia calva Grande and Bemis (1998) Grande and Bemis (1998)
Clarias gariepinus Adriaens and Verraes (1998) Adriaens and Verraes (1998)
Danio rerio Cubbage and Mabee (1996) Cubbage and Mabee (1996)
Oryzias latipes Langille and Hall (1987)
Tristichopteridae
Eusthenopteron foordi Cote et al. (2002)
;
Leblanc and Cloutier
(2005)
Cote et al. (2002)
;
Leblanc and Cloutier
(2005)
Temnospondyli
Archegosaurus decheni Witzmann (2006) Witzmann (2006)
Apateon caducus (Erdesbach) Schoch (2004) Schoch (2004) Schoch (2004)
Apateon caducus (Obermoschel) Werneburg (2018) Werneburg (2018)
Apateon pedestris (Erdesbach) Schoch (2004) Schoch (2004)
Apateon pedestris (Obermoschel) Werneburg (2018) Werneburg (2018)
"Melanerpeton"humbergense Schoch (2004) Schoch (2004)
Micromelerpeton credneri Boy (1995)
;
Witzmann and Pfret-
zschner (2003)
;
Lillich and Schoch
(2007);Schoch (2009)
Boy (1995)
;
Witzmann and Pfret-
zschner (2003)
;
Lillich and Schoch
(2007);Schoch (2009)
Sclerocephalus haeuseri Lohmann and Sachs (2001)
;
Schoch
(2003)
;
Schoch and Witzmann (2009)
;
Werneburg (2018)
Lohmann and Sachs (2001)
;
Schoch
(2003)
;
Schoch and Witzmann (2009)
;
Werneburg (2018)
Lohmann and Sachs (2001)
;
Schoch
(2003)
;
Schoch and Witzmann (2009)
;
Werneburg (2018)
Lepospondyli
Hyloplesion longicaudatum Olori (2013) Olori (2013)
Microbrachis pelikani Olori (2013) Olori (2013)
Gymnophiona
Gegeneophis ramaswamii Müller et al. (2005) Harrington et al. (2013)
Hypogeophis rostratus Müller (2006) Harrington et al. (2013)
Urodela
Aneides lugrubis Wake et al. (1983) Wake et al. (1983)
Ambystoma macrodactylum Harrington et al. (2013)
Ambystoma maculatum Harrington et al. (2013)
Ambystoma mexicanum Laurin and Germain (2011) Harrington et al. (2013)
Ambystoma talpoideum Reilly (1987) Reilly (1987) Reilly (1987)
Ambystoma texanum Laurin and Germain (2011) Harrington et al. (2013)
Ambystoma tigrinum Harrington et al. (2013)
Amphiuma means Harrington et al. (2013)
Andrias japonicus Harrington et al. (2013)
Bolitoglossa subpalmata Ehmcke and Clemen (2000)
Dicamptodon tenebrosus Harrington et al. (2013)
Continued on next page.
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Lissamphibian origin and ossification sequences
Continued from previous page.
Taxa Standardization method (data type used)
Ontogenetic stages Snout-vent length (mm) Ossification sequence position
Eurycea bislineata Harrington et al. (2013)
Gyrinophilus porphyriticus Harrington et al. (2013)
Hemidactylium scutatum Harrington et al. (2013)
Lissotriton vulgaris Laurin and Germain (2011) Harrington et al. (2013)
Necturus maculosus Harrington et al. (2013)
Notophthalmus viridescens Reilly (1986) Reilly (1986) Harrington et al. (2013)
Onychodactylus japonicus Harrington et al. (2013)
Pleurodeles waltl Harrington et al. (2013)
Ranodon sibiricus Harrington et al. (2013)
Salamandra salamandra Harrington et al. (2013)
Salamandrella keyserlingii Harrington et al. (2013)
Siren intermedia Reilly and Altig (1996) Reilly and Altig (1996) Reilly and Altig (1996)
Triturus karelinii Harrington et al. (2013)
Anura
Alytes obstetricans Yeh (2002)
Ascaphus truei Harrington et al. (2013)
Anaxyrus boreas Gaudin (1978)
Bombina orientalis Harrington et al. (2013)
Bufo bufo Harrington et al. (2013)
Cornufer guentheri Harrington et al. (2013)
Ceratophrys cornuta Harrington et al. (2013)
Chacophrys pierotti Harrington et al. (2013)
Crinia signifera Harrington et al. (2013)
Dendrobates auratus Sá and Hill (1998) Sá and Hill (1998) Harrington et al. (2013)
Discoglossus sardus Púgener and Maglia (1997)
Eleutherodactylus coqui Harrington et al. (2013)
Eleutherodactylus nubicola Harrington et al. (2013)
Epidalea calamita Harrington et al. (2013)
Epipedobates tricolor Sá and Hill (1998) Sá and Hill (1998) Harrington et al. (2013)
Fejervarya cancrivora Harrington et al. (2013)
Hamptophryne boliviana Harrington et al. (2013)
Hyla versicolor Harrington et al. (2013)
Hylorina sylvatica Harrington et al. (2013)
Hymenochirus boettgeri Sá and Swart (1999)
Hypsiboas lanciformis Sá (1988) Sá (1988) Sá (1988)
Kassina senegalensis Harrington et al. (2013)
Leptodactylus chaquensis Harrington et al. (2013)
Osteopilus septentrionalis Trueb (1966)
Palaeobatrachus sp. Harrington et al. (2013)
Pelobates cultripes Harrington et al. (2013)
Philautus silus Harrington et al. (2013)
Phyllomedusa vaillanti Harrington et al. (2013)
Pipa myersi Yeh (2002)
Pipa pipa Trueb et al. (2000) Harrington et al. (2013)
Pseudacris regilla Harrington et al. (2013)
Pseudacris triseriata Harrington et al. (2013)
Pseudis platensis Harrington et al. (2013)
Pseudophryne bibronii Harrington et al. (2013)
Pyxicephalus adspersus Harrington et al. (2013)
Rana (Amerana) aurora Harrington et al. (2013)
Rana (Amerana) cascadae Harrington et al. (2013)
Rana (Amerana) pretiosa Harrington et al. (2013)
Rana (Amerana) temporaria Harrington et al. (2013)
Continued on next page.
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Lissamphibian origin and ossification sequences
Continued from previous page.
Taxa Standardization method (data type used)
Ontogenetic stages Snout-vent length (mm) Ossification sequence position
Rana (Pantherana) pipiens Kemp and Hoyt (1969)
Rhinophrynus dorsalis Harrington et al. (2013)
Shomronella jordanica Harrington et al. (2013)
Smilisca baudini Harrington et al. (2013)
Spea bombifrons Wiens (1989) Wiens (1989) Wiens (1989)
Spea multiplicata Harrington et al. (2013)
Triprion petasatus Harrington et al. (2013)
Uperoleia laevigata Harrington et al. (2013)
Xenopus laevis Harrington et al. (2013)
Triprion petasatus Harrington et al. (2013)
Uperoleia laevigata Harrington et al. (2013)
Xenopus laevis Harrington et al. (2013)
Mammalia
Bradypus variegatus Hautier et al. (2011)
Cavia porcellus Hautier et al. (2013)
Choloepus didactylus Hautier et al. (2011)
Cryptotis parva Koyabu et al. (2011)
Cyclopes didactylus Hautier et al. (2011)
Dasypus novemcinctus Hautier et al. (2011)
Dasyurus viverrinus Hautier et al. (2013)
Didelphis albuventris Oliveira et al. (1998) Oliveira et al. (1998)
Echinops telfairi Werneburg et al. (2013)
Elephantulus rozeti Hautier et al. (2013)
Eremitalpa granti Hautier et al. (2013)
Erinaceus amurensis Koyabu et al. (2011)
Felis silvestris Sánchez-Villagra et al. (2008)
Homo sapiens Hautier et al. (2013)
Heterohyrax brucei Hautier et al. (2013)
Loxodonta africana Hautier et al. (2012)
Macropus eugenii Hautier et al. (2013)
Macroscelides proboscideus Hautier et al. (2013)
Manis javanica Hautier et al. (2013)
Meriones unguiculatus Yukawa et al. (1999) Yukawa et al. (1999)
Mesocricetus auratus Hautier et al. (2013)
Mogera wogura Koyabu et al. (2011)
Monodelphis domestica Hautier et al. (2013)
Mus musculus Hautier et al. (2013)
Ornithorhynchus anatinus Weisbecker (2011)
Orycteropus afer Hautier et al. (2013)
Perameles nasuta Hautier et al. (2013)
Peromyscus melanophrys Hautier et al. (2013)
Procavia capensis Hautier et al. (2013)
Rattus norvegicus Hautier et al. (2013)
Rhabdomys pumilio Hautier et al. (2013)
Rousettus amplexicaudatus Hautier et al. (2013)
Sus scrofa Hautier et al. (2013)
Trachyglossus aculeatus Weisbecker (2011)
Talpa spp. Sánchez-Villagra et al. (2008)
Tenrec ecaudatus Werneburg et al. (2013)
Tamandua tetradactyla Hautier et al. (2011)
Tarsius spectrum Hautier et al. (2013)
Trichosurus vulpecula Weisbecker et al. (2008) Hautier et al. (2013)
Tupaia javanica Hautier et al. (2013)
Continued on next page.
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Lissamphibian origin and ossification sequences
Continued from previous page.
Taxa Standardization method (data type used)
Ontogenetic stages Snout-vent length (mm) Ossification sequence position
Squamata
Zootoca vivipara Hautier et al. (2013)
Lerista bougainvillii Hugi et al. (2012) Hugi et al. (2012)
Liopholis whitii Hugi et al. (2012) Hugi et al. (2012)
Hemiergis peronii Hugi et al. (2012) Hugi et al. (2012)
Saiphos equalis Hugi et al. (2012) Hugi et al. (2012)
Crocodylia
Alligator mississipiensis Rieppel (1993a) Rieppel (1993a)
Aves
Anas platyrhynchos Maxwell et al. (2010)
Cairina moschata Maxwell et al. (2010)
Coturnix coturnix Maxwell et al. (2010)
Coturnix coturnix (N&T) Maxwell et al. (2010)
Dromaius novaehollandiae Maxwell et al. (2010)
Dromaius novaehollandiae (YPM) Maxwell et al. (2010)
Gallus gallus Maxwell et al. (2010)
Gallus gallus (S&W) Maxwell et al. (2010)
Larus argentatus Maxwell et al. (2010)
Larus canus Maxwell et al. (2010)
Larus ridibundus Maxwell et al. (2010)
Meleagris gallopavo Maxwell et al. (2010)
Phalacrocorax auritus Maxwell et al. (2010)
Somateria mollissima Maxwell et al. (2010)
Stercorarius skua Maxwell et al. (2010)
Sterna hirundo Maxwell et al. (2010)
Struthio camelus Maxwell et al. (2010)
Testudines
Apalone spinifera Sánchez-Villagra et al. (2008)
Chelydra serpentina Rieppel (1993b) Rieppel (1990) and Rieppel (1993b) Rieppel (1993b)
Macrochelys temminckii Sánchez-Villagra et al. (2008)
Pelodiscus sinensis Sánchez-Villagra et al. (2008)
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