Hindfins of Ichthyosaurus: effects of large sample size on ‘distinct’ morphological characters

Abstract

CORRECTION: Specimen NHMUK R11801, on TABLE 1, TABLE 3 and FIGURE 8, and discussed on p. 14, has been renumbered as NHMUK 38005. ____________________________________________________________________________________

The abundance of specimens of Ichthyosaurus provides an opportunity to assess morphological variation without the limits of a small sample size. This research evaluates the variation and taxonomic utility of hindfin morphology. Two seemingly distinct morphotypes of the mesopodium occur in the genus. Morphotype 1 has three elements in the third row: metatarsal two, distal tarsal three and distal tarsal four. This is the common morphology in Ichthyosaurus breviceps , I. conybeari and I. somersetensis . Morphotype 2 has four elements in the third row, owing to a bifurcation. This morphotype occurs in at least some specimens of each species, but it has several variations distinguished by the extent of contact of elements in the third row with the astragalus. Two specimens display a different morphotype in each fin, suggesting that the difference reflects individual variation. In Ichthyosaurus , the hindfin is taxonomically useful at the genus level, but species cannot be identified unequivocally from a well-preserved hindfin, although certain morphologies are more common in certain species than others. The large sample size filled in morphological gaps between what initially appeared to be taxonomically distinct characters. The full picture of variation would have been obscured with a small sample size. Furthermore, we have found several unusual morphologies which, in isolation, could have been mistaken for new taxa. Thus, one must be cautious when describing new species or genera on the basis of limited material, such as isolated fins and fragmentary specimens.

Full text

doi:10.1017/S0016756818000146
Hindfins of Ichthyosaurus: effects of large sample size on
‘distinct’ morphological characters
JUDY A. MASSARE∗†& DEAN R. LOMAX‡
∗Department of the Earth Sciences, SUNY College at Brockport, Brockport, New York 14420, USA
‡School of Earth and Environmental Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
(Received 4 August 2017; accepted 1 February 2018; first published online 14 March 2018)
Abstract – The abundance of specimens of Ichthyosaurus provides an opportunity to assess morpho-
logical variation without the limits of a small sample size. This research evaluates the variation and
taxonomic utility of hindfin morphology. Two seemingly distinct morphotypes of the mesopodium oc-
cur in the genus. Morphotype 1 has three elements in the third row: metatarsal two, distal tarsal three
and distal tarsal four. This is the common morphology in Ichthyosaurus breviceps,I. conybeari and I.
somersetensis. Morphotype 2 has four elements in the third row, owing to a bifurcation. This morpho-
type occurs in at least some specimens of each species, but it has several variations distinguished by
the extent of contact of elements in the third row with the astragalus. Two specimens display a different
morphotype in each fin, suggesting that the difference reflects individual variation. In Ichthyosaurus,
the hindfin is taxonomically useful at the genus level, but species cannot be identified unequivocally
from a well-preserved hindfin, although certain morphologies are more common in certain species
than others. The large sample size filled in morphological gaps between what initially appeared to be
taxonomically distinct characters. The full picture of variation would have been obscured with a small
sample size. Furthermore, we have found several unusual morphologies which, in isolation, could
have been mistaken for new taxa. Thus, one must be cautious when describing new species or genera
on the basis of limited material, such as isolated fins and fragmentary specimens.
Keywords: Lower Jurassic, Ichthyosauria, Ichthyosauridae, tarsus, intraspecific variation, pathology
1. Introduction
Lower Jurassic ichthyosaurs are known from thou-
sands of specimens, although not all of them are suffi-
ciently complete to assign to a species. Most Lower
Jurassic genera can, however, be distinguished from
one another by a few key postcranial elements, such
as the forefin, humerus, coracoid or pelvic girdle,
in the absence of a skull. In fact, the genus Ichthy-
osaurus has an unusual forefin that easily distinguishes
it from all other genera. The anterior digital bifurc-
ation (shared only with Protoichthyosaurus, Lomax,
Massare & Mistry, 2017) and a prominent digit V gives
the forefin a unique morphology (Motani, 1999). Fore-
fins of Ichthyosaurus are extremely variable in the di-
git that bifurcates (digit II or III), the location of the
bifurcation on the digit, the number of bifurcations
in the fin and the relative size of the branches of the
bifurcation, even within a species. Although an isol-
ated forefin can be identified as Ichthyosaurus, species
assignment within the genus usually requires a well-
preserved skull. Thus, identification of postcranial fea-
tures that are unique to a particular species of Ich-
thyosaurus can provide additional criteria for species
identification that can be applied to partial skeletons or
ones with poorly preserved skulls.
†Author for correspondence: jmassare@brockport.edu
Hindfins have been largely ignored in ichthyosaur
taxonomy, probably in part because they are less fre-
quently preserved than skulls and anterior portions of
the postcranium. Hindfins are much smaller than fore-
fins in some Jurassic ichthyosaurs, and so are more
vulnerable to disarticulation and transport by currents
before fossilization. When compared with the fore-
fin, even comprehensive phylogenetic analyses incor-
porate relatively few hindfin characters (e.g. Maxwell,
Zammit & Druckenmiller, 2012; Fischer et al.2013;
Ji et al.2016). This research explores the potential for
using hindfin morphology to distinguish species of the
Lower Jurassic genus Ichthyosaurus.
The genus presently has six species: Ichthyosaurus
communis,I. breviceps,I. conybeari,I. anningae,I.
larkini and I. somersetensis, distinguished largely by
features of the skull (McGowan, 1974; Lomax &
Massare, 2015,2017; Massare & Lomax, 2017). Ich-
thyosaurus is thus one of the most speciose genera of
parvipelvian ichthyosaurs. In fact, many of the gen-
era recognized recently are monotypic, making Ichthy-
osaurus seem even more unusual for its large num-
ber of species. Moreover, several new genera of par-
vipelvian ichthyosaurs are presently known from just
one specimen (e.g. Malawania, Fischer et al.2013;
Muiscasaurus, Maxwell et al.2016;Keilhauia, Delsett
et al.2017). In contrast, hundreds of well-preserved
specimens of Ichthyosaurus are in museum collections
worldwide, especially in the UK.
725
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Cambridge University Press 2018

J. A. MASSARE & D. R. LOMAX
Most of the fairly complete skeletons of Ichthy-
osaurus were collected in the nineteenth century,
mainly from the west Dorset coast (Lyme Regis–
Charmouth area) and quarries in Somerset (near the
village of Street), although the genus is known from
elsewhere (Massare & Lomax, 2017). This study ex-
amined nearly 100 specimens of Ichthyosaurus with
hindfins that are sufficiently well preserved to provide
morphological information. The hindfins display two
distinct morphologies of the mesopodium (tarsus), as
a result of a bifurcation of digit II. The large num-
ber of specimens has resulted in a more complete pic-
ture of hindfin variation, which would not have been
possible with a small sample size. What could have
been interpreted as unique morphologies are actually
end-members of a continuum of individual variation.
This has implications for identifying unique diagnostic
characters (autapomorphies) based on limited fossil
material.
Institutional abbreviations: ANSP – Academy of
Natural Science of Drexel University, Philadelphia,
Pennsylvania, USA; BGS – British Geological Survey,
Keyworth, Nottingham, UK; BELUM – Ulster Mu-
seum, National Museum of Northern Ireland, Belfast,
UK; BRLSI – Bath Royal Literary and Scientific In-
stitution, UK; BRSMG – Bristol City Museum and
Art Gallery, UK; BRSUG – University of Bristol,
UK; BU – Lapworth Museum of Geology, University
of Birmingham, UK; CAMSM – Sedgwick Museum,
Cambridge University, UK; CC – Cliffe Castle Mu-
seum, Bradford, UK; CCHC – Charmouth Heritage
Coast Centre, Dorset, UK; CLC – Cheltenham Ladies
College, UK; CHMUS – Charterhouse School Mu-
seum, Surrey, UK; CMNH – Cleveland Museum of
Natural History, Ohio, USA; DBYMU – Derby Mu-
seum and Art Gallery, UK; DORCM – Dorset County
Museum, UK; FMNH – Field Museum, Chicago,
Illinois, USA; GLAHM – Hunterian Museum, Univer-
sity of Glasgow, UK; GLRCM – Gloucester City Mu-
seum, UK; HEM – Haslemere Educational Museum,
Surrey, UK; LEICT – Leicester Arts and Museums
Service (New Walk Museum and Art Gallery), UK;
MANCH – Manchester Museum, The University of
Manchester, UK; LMG – Leeds Museums and Gal-
leries, UK; NHMUK (formerly BMNH) – The Nat-
ural History Museum, London, UK; NLMH – Nieder-
sächsisches Landesmuseum (Lower Saxony State
Museum), Hannover, Germany; NMING – National
Museum of Ireland, Dublin, Ireland; NMW – National
Museum of Wales, Cardiff, UK; NOTNH – Notting-
ham Natural History Museum (Wollaton Hall), UK;
OUMNH – Oxford University Museum of Natural
History, UK; PETMG – Peterborough Museum and
Art Gallery, UK; RAMM – Royal Albert Memorial
Museum, Exeter, UK; ROM – Royal Ontario Mu-
seum, Toronto, Canada; SHEFFM – Weston Park Mu-
seum, Sheffield, UK; SMNS – Staatliches Museum
für Naturkunde (Stuttgart State Museum of Natural
History), Germany; SOMAG (formerly AGC) – Al-
fred Gillett Collection, cared for by the Alfred Gillett
Trust (C & J Clark Ltd), Street, UK; TM – Teylers
Museum, Haarlem, Netherlands; TTNCM – Somer-
set County Museum, Taunton, UK; USNM – National
Museum of Natural History, Smithsonian Institution,
Washington DC, USA; WARMS – Warwickshire Mu-
seum, Warwick, UK; WFIS – Wagner Free Institute of
Science, Philadelphia, Pennsylvania, USA; WOSMG –
Worcester City Art Gallery and Museum, UK; YPM –
Peabody Museum, Yale University, New Haven, Con-
necticut, USA.
2. Materials
We examined 99 specimens of Ichthyosaurus with
at least one hindfin that provided morphological in-
formation (Table 1). Twenty-four specimens had two
informative hindfins. Most hindfins were associated
with a skeleton or significant portion of a skel-
eton, but some specimens were isolated or associ-
ated with pelvic bones and/or a few centra (ANSP
10123, BRLSI GP1870, GLAHM V1165, GLRCM
1987.45 (formerly GLRCM 938), MANCH L9664,
NHMUK OR35567, NHMUK R5790, NHMUK
R5918, NOTNH FS3450, OUMNH J.29351, OUMNH
J.95338 and RAMM 124–2009). Other specimens with
partial hindfins exist, but they are too incomplete
and/or too disarticulated to provide morphological in-
formation. Many of the specimens examined, how-
ever, were not sufficiently complete to assign to species
(Table 1).
Six specimens were not included in the analysis be-
cause the authenticity of the arrangement of bones
in the hindfins is questionable (Tab le 2). BRSMG
Cb4997 is a fairly complete skeleton of I. somerseten-
sis with both the left and right hindfin preserved. All
of the fin elements, however, are set in a filler mater-
ial. We cannot assess whether the current position of
the individual bones accurately reflects their original
arrangement, so this specimen was not included in this
analysis. Likewise, the hindfins of LMG.B.1843.4 have
been rebuilt, and so the specimen was not included. A
similar situation occurs in BRLSI M3551, a compos-
ite specimen in which both hindfins were probably ad-
ded to the specimen (Massare & Lomax, 2016a). The
hindfins themselves are on opposite sides of the ver-
tebral column and surrounded by plaster. It is unclear
whether the individual bones are in plaster or matrix. A
proximal bone of one fin (?left) is too large to belong
with the other elements and it is lying on the ‘mat-
rix’ rather than within it. Furthermore, the femur is
cracked across the shaft and the proximal portion is
rotated relative to the distal portion. Because of these
issues, both fins of BRSLI M3551 were excluded from
this analysis. A fourth questionable hindfin is NMING
F8772, in which the posterior branch of digit II has
some elements that are notched and oriented with the
notches facing posteriorly rather than anteriorly. Con-
sidering that only elements of the leading edge can
be notched, it is clear that these elements have been
altered from their original orientation. This calls into
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Hindfins of Ichthyosaurus
Table 1. Specimens that were used in this study with at least one informative hindfin
Specimen No. Species Morph Location
ANSP 10123 2 possibly Dorset†
ANSP 15766 I. somersetensis 1 Somerset
ANSP 17429 ?1 possibly Dorset†
BGS 955 I. breviceps 2 Dorset
BGS 956 I. conybeari 1 Dorset
BGS RGSCU1091 1 Somerset
BRLSI GP1870 possible composite 1
BRLSI M3559 I. somersetensis 1 & 2 possibly Somerset†
BRLSI M3572 composite 1
BRSMG Ce16611 I. somersetensis 2 Somerset
BRSUG 25300 I. larkini 2 Somerset
BU 5289 I. communis 2
CAMSM J35186 2 Leicestershire
CAMSM J35187 I. communis np Dorset
CAMSM J35188 I. communis 1 Dorset
CAMSM J35289 I. ?communis 2 Dorset
CAMSM J59574 I. somersetensis 2 Somerset
CAMSM J59575 composite 2 Somerset
CAMSM X50224 [formerly TN910] I. conybeari np Dorset
CAMSM X50187 I. breviceps 2 Dorset
CC BMT 03526 2 Dorset
CCHC2 I. breviceps 1 Dorset
CHMUS 2015.0102 I. ?communis 2
CLC1 I. somersetensis 2 Cotswolds area, possibly Gloucestershire
CMNH 11088 2 Dorset
DBYMU 1877-16 2 Leicestershire
DORCM G1 2 Dorset
FMNH P25027 I. communis 2 Dorset
GLAHM V1030 2 possibly Somerset†
GLAHM V1032 2 Dorset
GLAHM V1165 1 Dorset
GLAHM V1179 2 Dorset
GLRCM 1987.45 [formerly 938] 1 & 2
HEM GF.6.3190 ?1 Dorset
LEICT G125.1992 2 Leicestershire
MANCH L.9664 2 possibly Leicestershire†
NHMUK OR120 I. anningae 2 Dorset
NHMUK OR2001∗I. ?breviceps 2 Dorset
NHMUK OR2013 I. somersetensis 1 Somerset
NHMUK OR2013∗I. somersetensis 1 Somerset
NHMUK OR2017 1 Somerset
NHMUK OR2024∗1 Somerset
NHMUK OR2025∗1 Somerset
NHMUK OR8165 2 Leicestershire
NHMUK OR14563 I. ?somersetensis 1 possibly Somerset†
NHMUK OR14565 I. somersetensis 2 Somerset
NHMUK OR14567 I. somersetensis 1 Somerset
NHMUK OR35567 2 Dorset
NHMUK OR36256 I. communis 2 Dorset
NHMUK OR41849 2 Dorset
NHMUK OR43006 I. breviceps 1 Dorset
NHMUK OR85791 2 Dorset
NHMUK R44 I. somersetensis 1 Somerset
NHMUK R1073 I. communis 2 Dorset
NHMUK R1162 I. communis 2 Dorset
NHMUK R1212 [display specimen] 2 Dorset
NHMUK R1696 2 Dorset
NHMUK R3372 I. somersetensis 1 Somerset
NHMUK R5790 I. conybeari 2 Dorset
NHMUK R5918 2 Dorset
NHMUK R5595 I. larkini 2 Somerset
NHMUK R10028 I. anningae np ?Dorset
NHMUK R11199 2 Dorset
NHMUK R11801 I. ?conybeari 2 possibly Dorset†
NHMUK R15907 I. conybeari np Dorset
NLMH 106234 I. somersetensis 2 Somerset
NMW 91.29G.1 2 Dorset
NMW 92.5G.1 I. conybeari np Somerset
NMW 33.401.G2 2
NOTNH FS3450 2 probably Somerset†
NOTNH FS4940 2 Nottinghamshire
NOTNH FS13759 2
OUMNH J.10325 2 Somerset
OUMNH J.10342/p 2 Somerset
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J. A. MASSARE & D. R. LOMAX
Table 1. Continued.
Specimen No. Species Morph Location
OUMNH J.13592 2 Dorset
OUMNH J.13799 I. somersetensis 2 Somerset
OUMNH J.29351 2 Dorset
OUMNH J.29352 2 Dorset
OUMNH J.95338 2 Dorset
PETMG R174 I. communis 2 Dorset
RAMM 57/2009 2
RAMM 124/2009 I. conybeari 2 Dorset
ROM 26029 I. somersetensis 2 probably Somerset†
SMNS 58275 I. conybeari 2 Dorset
SOMAG/GEO/11 I. larkini 2 Somerset
SOMAG/GEO/16 I. somersetensis ?1 Somerset
SOMAG/GEO/17 I. somersetensis 1 Somerset
TM F002727 I. ?breviceps 1 Dorset
TTNCM 120-1996 I. ?somersetensis 2 Somerset
TTNCM 166-1992 2 Somerset
TTNCM 8349 ?1 Somerset
TTNCM 8359 2 probably Somerset†
TTNCM 8373 I. somersetensis 2 Somerset
USNM V4967 I. ?conybeari 2 Dorset
WFIS case 64 2 Dorset
WARMS G6188 I. ?somersetensis 1 Warwickshire
WOSMG No 3 2 Worcestershire
YPM 4198 I. somersetensis 1
YPM 9204 I. ?communis 2
Note: Two specimens display a different morphotype on each fin. Location is based on museum records or specimen labels, except
as noted. Species identifications based on evaluations by authors.
Abbreviations: Morph – morphotype; np – mesopodium not preserved. ? species or morphotype identification is uncertain.
†assessment by the authors, based on preservation.
Table 2. Composite, unreliable or reconstructed specimens
Specimen No. Comments
BELUM display
specimen (no number)
Both hindfins surrounded by plaster. Fins appear to be forefins.
BRLSI GP1870 Hindfin block might not belong with associated material.
BRLSI M3551 Hindfins probably added to specimens; elements set in plaster so arrangement unreliable.
BRLSI M3572 Hindfins might not belong with anterior skeleton.
BRSMG Cb4997 Elements reset on both fins so arrangement unreliable.
BRSUG 23500 Distal portion of both fins are reconstructed using a modelling material for the elements, but proximal portions
are in matrix.
CAMSM J59575 Hindfin probably does not belong with anterior skeleton.
LMG.B.1843.4 Elements reset on both fins so arrangement unreliable.
NHMUK R5595 Left fin may have been reset into plaster incorrectly or has post-mortem displacement of digit II.
NHMUK R5790 Femur reversed relative to the rest of the fin.
NHMUK OR14567 Hindfin surrounded by painted plaster, but arrangement seems authentic; inaccessible for close examination.
NMING F8772 Some elements anteroposteriorly reversed and reset, so arrangement unreliable.
SHEFFM: H93.188 Section distal to astragalus/calcaneum set in plaster and orientation reversed, so distal arrangement unreliable.
SMNS 58275 Some elements distal to the mesopodium on left fin reconstructed and reset, so arrangement unreliable.
USNM 15152 Femur too large for tibia and fibula; distal fin does not belong with proximal fin.
WOSMG No. 3 Elements distal to mesopodium (right fin) or first phalangeal row (left fin) are painted on plaster.
Some composite specimens were included in this study because hindfins were unaltered and preserved in matrix. See text for additional
information.
question whether other elements have been reset into
a different arrangement as well. Thus, this specimen
was also excluded from the analysis. Another speci-
men excluded is USNM 15152, a composite of prob-
ably three individuals. The entire fin is surrounded by
plaster and it is unclear whether individual bones are
in matrix. The femur is too large for the size of the
tibia and fibula, and the elements of the distal half of
the fin are probably from a forefin, or at least from a
larger individual than the proximal fin. The last speci-
men excluded is an unnumbered skeleton on display at
BELUM. Both hindfins are surrounded by plaster, and
the portions distal to the femora appear to be forefins
that were added to the specimen.
Three other specimens included in this research
might be parts of composite specimens (Tabl e 2).
BRLSI GP1870 comprises three blocks of matrix: one
with a hindfin, another with a portion of forefin and
the third with some articulated caudal vertebrae, all of
which are set in plaster within a wooden frame. The
lack of direct association suggests that they might not
belong to the same individual, but this is impossible
to determine solely by visual examination. BRLSI
M3572 is another suspicious specimen in which the
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Hindfins of Ichthyosaurus
skeleton appears to be pieced together from more than
one individual (Massare & Lomax, 2016a). In partic-
ular, the caudal portion of the vertebral column and
the hindfins might not belong to the same individual
as the skull and pectoral region or the dorsal vertebral
column. One hindfin is adjacent to articulated caudal
centra whereas the other is on an isolated block of mat-
rix, surrounded by plaster. The two hindfins have the
same morphology and the matrix appears similar, so
they probably belong to the same individual, although
not necessarily the same individual as the anterior skel-
eton. In both BRLSI GP1870 and BRLSI M3572, the
hindfins themselves are in blocks of matrix and appear
to maintain the original arrangement of elements, so
the specimens are included in the analysis. CAMSM
J59575 is almost certainly a composite specimen in
which the anterior and posterior portions of the skel-
eton do not belong to the same individual (Massare &
Lomax, 2016a). The bone colour is different on the
presacral skeleton compared to the hindfin and caudal
centra, and the blocks containing these bones are set in
plaster and separated from the presacral portion of the
skeleton. Although the hindfin of CAMSM J59575 is
surrounded entirely by plaster, the hindfin might be at
least partially in matrix. The arrangement of bones is
consistent with what occurs in other specimens, and so
this specimen was included in the analysis. Because of
the issue of composites, these three specimens could
not be assigned to a species.
Six other specimens were included in the study, but
have been partially reconstructed, and require some ex-
planation (Table 2). SHEFFM: H93.188 has both hind-
fins preserved, but the left (upper) is disarticulated and
does not provide any useful information, whereas the
right (lower) has been incorrectly rebuilt, in part. The
mesopodium is almost complete, with space for meta-
tarsal two, which is missing. Distal to this, the fin is
articulated, but set in plaster, with only portions of
each digit preserved. The reset piece is, however, re-
versed relative to the rest of the fin, with the anterior
edge facing posteriorly, identified by notching. Fur-
thermore, the bone colour is different from the main
specimen, so its authenticity can be questioned. We
considered only the mesopodium morphology in this
study. BRSUG 25300, the holotype of I. larkini, also
has some reconstruction (Lomax & Massare, 2017).
The distal portions of both fins have been reconstruc-
ted from plaster and have a much darker colour than
the bone, but the proximal portion of each fin is au-
thentic and provides useful morphological data. Simil-
arly, some of the phalanges distal to the mesopodium
of the left hindfin of SMNS 58275 have been restored
and partially reset. NHMUK OR14567 is a practically
complete skeleton that has the right hindfin preserved.
The hindfin is surrounded by paint, which makes it
suspicious. It is also on display, high on the wall and
behind glass, so a close inspection is not possible. Nev-
ertheless, the arrangement of bones appears to be au-
thentic and so we include it here. The distal portion
of each fin of WOSMG No. 3 has been painted to
illustrate a complete hindfin. The mesopodium, how-
ever, is original bone imbedded in matrix, and so we
have included the specimen in this study. Finally, in
NHMUK R5790, an isolated hindfin, the femur is re-
versed relative to the rest of the hindfin, with the an-
terior end pointing posteriorly. The femur has probably
been reset incorrectly.
3. Hindfin development and homology
The primitive reptilian hindfin has been greatly modi-
fied in parvipelvian ichthyosaurs. As in all other Mer-
riamosauria, parvipelvians have lost digit I in the hind-
fin (Ji et al.2016). The tibia and fibula are polygonal
bones that lack a shaft or even an anteroposterior con-
striction. Polygonal tarsal and metatarsals have broad
contacts with adjacent bones, sometimes interlocking,
and thus make the determination of homologies chal-
lenging. The problem is complicated by the presence
of accessory digits (e.g. Ophthalmosaurus,Platyptery-
gius) or digital bifurcations (e.g. Ichthyosaurus,Pro-
toichthyosaurus). Additionally, Ichthyosaurus has lost
digit V in the hindfin, as have other Lower Jurassic ich-
thyosaurs (e.g. Leptonectes,Eurhinosaurus), although
Lomax & Massare (2017) incorrectly identified a digit
VinI. larkini and I. somersetensis. In reptiles, digit I
is the first to be lost when digit reduction occurs, and,
if reduction continues, digit V is the next one to be lost
(Bakker et al.2013). Because of digital bifurcations,
however, Ichthyosaurus has more than just these three
digits in the hindfin (see Section 6b).
Because the limbs of ichthyosaurs are so derived
compared to those of terrestrial reptiles, three termin-
ologies have been used to identify elements of the
mesopodium in parvipelvian hindfins. The three ele-
ments in the second row (immediately distal to the tibia
and fibula) have been referred to as the centrale, as-
tragalus and calcaneum (Caldwell, 1997); tibiale, in-
termedium or astragalus, and fibulare (Zammit, Norris
& Kear, 2010; Massare & Lomax, 2016b; Lomax &
Massare, 2017); or distal tarsal two, astragalus and cal-
caneum (Fernández, 2007; Maxwell, 2012). McGowan
& Motani (2003, p. 57) noted the lack of consensus and
used two different terminologies in diagrams of hind-
fins (McGowan & Motani, 2003, figs 67, 70).
The important difference, however, is not merely
in assigning names of bones. The identification of
the anterior bone in the second row as the tibiale or
distal tarsal two has implications for development of
the fin, because the process that forms each bone is
quite different. In embryonic development, the tibiale
forms from segmentation of the tibia, whereas tarsal
two would form in the digital arch from distal tarsal
three (Shubin & Alberch, 1986). In extant reptiles (cro-
codiles, turtles, lizards), a condensation that will even-
tually become the femur segments distally and bifurc-
ates to form condensations that will become the tibia
and fibula. The fibula condensation similarly bifurc-
ates into condensations for an intermedium and fib-
ulare (Shubin & Alberch, 1986). Although Fabrezi,
729

J. A. MASSARE & D. R. LOMAX
Abdala & Martínez Oliver (2007) recognized a con-
densation for the radiale in the forefin, a similar con-
densation does not occur in the hindfin (Shubin &
Alberch, 1986; Fabrezi, Abdala & Martínez Oliver,
2007). This contrasts with development in salamanders
and birds, where a condensation that will eventually
form the tibiale is present distal to the tibia (Shubin &
Alberch, 1986). The interpretation of homology, and
thus terminology, depends on the pattern of develop-
ment that ichthyosaurs followed.
Phylogenetic analyses have placed the Ichthyoptery-
gia with basal diapsids (Motani, Minoura & Ando,
1998) or among the parareptiles (Maisch, 2010). The
latter hypothesized an origin from a more primit-
ive ancestral form than the former. Both studies hy-
pothesized the origin of Ichthyopterygia before the
divergence of archosauromorphs (which includes cro-
codiles) and lepidosauromorphs (which includes liz-
ards). More recently, Scheyer et al. (2017, fig. 12)
proposed a phylogeny in which archosauromorphs,
lepidosauromorphs and Ichthyopterygia diverged at
the same time. All three phylogenetic hypotheses, how-
ever, place the group within the Reptilia. Key charac-
teristics of the reptilian tarsus are recognized in many
early reptiles (e.g. Rieppel, 1993; O’Keefe et al. 2006),
suggesting that the reptilian developmental pattern was
well established by the time the ichthyosaurs diverged.
The hindfin of the Middle Triassic ichthyosaur Mixo-
saurus retains all five digits, and both distal tarsals
one and two contact the tibia (Caldwell, 1997; Max-
well, 2012). Only two proximal tarsals (astragalus, cal-
caneum) occur in this primitive ichthyosaur, the typical
reptilian pattern. Interestingly, Caldwell (1997) sug-
gested that the most anterior, proximal tarsal in Sten-
opterygius could be a centrale that did not fuse with
the intermedium to form the astragalus, although he
acknowledged that it could also be distal tarsal two.
Whether hindfin development in parvipelvians differed
from the primitive reptilian pattern has yet to be estab-
lished (but see Zverkov, 2017).
This work uses a conservative interpretation, which
assumes that the ichthyosaurian tarsus developed in the
primitive reptilian pattern and that the pattern is re-
tained even in derived taxa. Thus, in parvipelvian ich-
thyosaurs, the second row of the hindfin includes distal
tarsal two, the astragalus and the calcaneum, and the
third row typically includes metatarsal two, distal tarsal
three and distal tarsal four. In Ichthyosaurus,however,
a bifurcation in digit II commonly occurs, resulting in
four elements in the third row (see Section 5). An is-
sue with this interpretation is that distal tarsals two
and three do not contact one another in many speci-
mens of Ichthyosaurus, as well as other parvipelvian
taxa (McGowan & Motani, 2003, fig. 70; Maxwell,
2012, fig. 1). However, in Mixosaurus, the distal tars-
als contact those of adjacent digits along a broad con-
tact. Such a contact is expected considering how the
distal tarsals develop from the digital arch (Shubin &
Alberch, 1986). In Ichthyosaurus, and many parvipel-
vians, however, metatarsal two is often between distal
tarsals two and three. Such changes in positions of
distal tarsals could be related to the difference in shape
of the tibia and fibula between Mixosaurus (elongated,
with a distinct shaft) and Ichthyosaurus (polygonal).
Alternately, a faster growth rate of metatarsal two rel-
ative to distal tarsal two could also result in separation
of the two distal tarsals.
A clarification of the term ‘contact’ is also required.
Ichthyosaurian fins often have gaps between elements
either because of post-mortem disarticulation or be-
cause of the presence of uncalcified tissue in life. Ju-
veniles, in particular, often lack an interlocking mosaic
of fin elements (Johnson, 1977; but see Lomax et al.
2017). In spite of gaps between bones, the presence of
facets on adjacent elements often indicates that bones
were in contact or have the potential to be in contact
if the gaps were eliminated by additional growth or
re-articulation. This work uses the term ‘contact’ in
an imprecise sense to mean both physically touching
and having the potential to physically touch an adja-
cent bone if gaps between elements are eliminated.
4. Femur morphology in Ichthyosaurus
In general, the femur of Ichthyosaurus is more elong-
ated than the humerus, with a narrow shaft and a
distal end that is flattened dorsoventrally and expan-
ded anteroposteriorly (Fig. 1). The amount of antero-
posterior expansion varies depending on the size and
orientation of the facets for the tibia and fibula. Thus,
in dorsal or ventral view, the distal end is wider than
the proximal end, but the difference in widths varies
within a species. Minor differences in the orientation
of the femur also influence the assessment of relative
widths. Ichthyosaurus anningae differs from the other
species in having a much less elongated femur in which
the distal and proximal ends are equally wide, making
the femur shape similar to that of a humerus (Fig. 1g).
This species can thus be distinguished from the others
by femur morphology, as well as the relative size of the
femur compared to the humerus (Lomax & Massare,
2015).
In Ichthyosaurus larkini and I. somersetensis, the
anterior edge of the shaft is nearly straight, although
some specimens have an expansion, or bulge, in at
least the distal half of the shaft that gives it a con-
vex curvature (e.g. BRSUG 25300, I. larkini; BRSMG
Cb4997, I. somersetensis;Fig. 1a–c). The posterior
edge of the shaft is concavely curved, giving the femur
a distinct, asymmetric shape in dorsal or ventral view.
This asymmetry can distinguish I. larkini and I. somer-
setensis from the other species, although not from each
other. In contrast, I. communis,I. breviceps and I.
conybeari have femora with more symmetric shapes
in dorsal or ventral views, owing, in part, to a similar
curvature of the anterior and posterior edges (Fig. 1d–
f). I. conybeari usually has a distinctly narrow head
and shaft, and a relatively broad expansion of the distal
end in dorsal or ventral view (Fig. 1f). The distal end
is much wider than the proximal end, more so than in
730

Hindfins of Ichthyosaurus
Figure 1. (Colour online) Femora of the six species of Ichthyosaurus, anterior to the left. (a) I. larkini (BRSUG 25300) right femur
in ventral view. (b) I. somersetensis (NLMH 106234, reversed) right femur in dorsal view. (c) I. somersetensis (TTNCM 8373) left
femur in dorsal view. (d) I. communis neotype (NHMUK R1162) left femur in dorsal view. (e) I. breviceps holotype (NHMUK
OR43006) left femur in dorsal view. (f) I. conybeari (NMW 93.5G.2, reversed), ?right femur in dorsal view. (g) I. anningae holotype
(DONMG:1983.98) ?left femur in dorsal view. Scale bars in (a–e) equal 3 cm. Scale bar in (f) equals 1 cm. Scale bar in (g) equals
2 cm. Scale is estimated for (d) and (e) because specimens are on display behind glass.
other species, and this feature can sometimes distin-
guish I. conybeari from the other species.
In all species of Ichthyosaurus, the ventral process
of the femur is centrally located and is more prominent
than the dorsal process (Maxwell, Zammit & Drucken-
miller, 2012, fig. 1). The dorsal process, however, ap-
pears to be offset anteriorly in all species, although it
can sometimes be slightly more centrally located, and
its prominence also varies. For example, one specimen
of Ichthyosaurus somersetensis (NHMUK OR2013∗)
has a dorsal process that is offset anteriorly; whereas
another specimen (SOMAG/GEO/17) has a dorsal pro-
cess that is slightly more centrally located, although
still anteriorly offset. Such a minor difference could be
the result of the femur orientation. Thus, the processes
are not particularly useful in referring a specimen to
species.
The relative size of the tibia and fibula are not use-
ful in distinguishing species except for I. conybeari,
which has a much larger fibula than tibia, both prox-
imodistally and anteroposteriorly. I. somersetensis of-
ten has a fibula that is somewhat larger, at least antero-
posteriorly, than the tibia (e.g. NHMUK OR2013∗,
NLMH 106234), but the difference is not of the mag-
nitude seen in I. conybeari. In general, the relative
sizes of the tibia and fibula vary within a species, al-
though in I. anningae, no sufficiently well-preserved
specimens are known. The facets on the femur for the
tibia and fibula are approximately the same length,
even in I. conybeari. Variations in the size of facets
arise mainly because of differences in the orientation
of the femur. The angle between the facets also var-
ies within species. The most striking example is in I.
somersetensis, where in some specimens, a proximal
deflection of the fibula facet occurs, with the angle
between the facets being nearly 90°in some speci-
mens (e.g. BRSMG Cb4997, TTNCM 8373; Fig. 1c).
This feature is not ontogenetic, because neither the
731

J. A. MASSARE & D. R. LOMAX
Figure 2. Ichthyosaurus hindfin morphotypes. (a) Morphotype 1 (NHMUK OR43006), with three elements in the third row. (b)
Morphotype 2 (NHMUK R1162), with four elements in third row. Metatarsal two-b and distal tarsal three have equal contact with the
astragalus. Dashed line indicates inferred outline of metatarsal two-a that is partially overlain. (c) Morphotype 2 variation (NLMH
106234, right hindfin, reversed), with distal tarsal three having broad contact with astragalus and metatarsal two-b having minor to
no contact. (d) Morphotype 2 variation (NHMUK R5595 right hindfin, reversed), with three elements in contact with the astragalus.
Anterior is to the left in all specimens. Scale bars in (a), (b) and (d) equal 2 cm. Scale bar in (c) equals 3 cm. Scales in (a), (b) and (d)
are estimates because the specimens are on display behind glass. Abbreviations: 2 – distal tarsal two; 3 – distal tarsal three; 4 – distal
tarsal four; a – astragalus; c – calcaneum; F – fibula; iia – anterior metatarsal of digit II; iib – posterior metatarsal of digit II; T – tibia.
smallest (NHMUK R44) nor the largest (NLMH
106234 (Fig. 1b), NHMUK 2013∗) specimens of the
species display this feature. Notably, in the holotype of
I. larkini, one femur has facets at nearly a right angle
(Fig. 1a), but the other femur does not, so the angle is
likely related to differences in orientation.
5. Mesopodium morphology in Ichthyosaurus
The mesopodium of Ichthyosaurus has one of two
morphotypes, distinguished by the number of elements
in the third row. Morphotype 1 has three elements:
metatarsal two, distal tarsal three and distal tarsal four,
which are immediately distal to distal tarsal two, the
astragalus and the calcaneum, respectively (Fig. 2a).
Nearly all specimens showing Morphotype 1 have a
bifurcation in the first phalangeal row of digit II. The
exception is WARMS G6188, in which the right hind-
fin has the usual bifurcation in the first phalangeal row,
but the left hindfin has the proximal bifurcation in the
third phalangeal row. A bifurcation of phalanges that
results in additional lines of elements is polyphalangy
(Cooper & Dawson, 2009). Morphotype 2 has a bifurc-
ation in the metatarsal row of digit II, resulting in four
elements in the third row (Fig. 2b–d). Thus the third
row has two metatarsal elements associated with digit
II, distal tarsal three and distal tarsal four. A bifurcation
in the metatarsal row resulting in an additional meta-
tarsal is polydactyly (Cooper & Dawson, 2009). Our
contention is that in Ichthyosaurus, the bifurcation pro-
cess is essentially the same, but the position at which
it occurs on digit II is the result of individual vari-
ation. Thus, the distinction between polydactyly and
polyphalangy is not useful for the unusual hindfin (and
forefin) morphology found in Ichthyosaurus.
Should Ichthyosaurus be separated into two gen-
era, one displaying polydactyly and the other not? This
might have been the conclusion were fewer specimens
available for examination, but in reality, the situation is
more complex. Morphotype 1 is the common morpho-
logy in I. breviceps,I. conybeari and I. somerseten-
sis. However, all six species of Ichthyosaurus show
Morphotype 2 in at least some specimens. Ichthy-
osaurus communis and I. larkini seem to show this
morphotype exclusively, but the latter species is known
from relatively few specimens. The only specimen of
I. anningae with the proximal hindfin exposed also
shows this morphology. Because three species show
both morphotypes, we argue that the difference is the
result of individual variation. Without the exception-
ally large sample size, this might not have been recog-
nized. Moreover, two of the 24 specimens that preserve
both hindfins (BRLSI M3559, GLRCM 1987.45) show
a different morphotype on each fin. Neither of these is
a composite specimen, although composites occur in
historic collections (Massare & Lomax, 2016a). This
further suggests that the two morphotypes, although
seemingly distinct, reflect individual variation. Thus,
the difference between Morphotype 1 and 2 is largely
whether the bifurcation of digit II occurs in the first
phalangeal row of digit II (Morphotype 1) or in the
metatarsal row (Morphotype 2). Similarly, in the fore-
fin of Ichthyosaurus, the proximal bifurcation can be in
the metacarpal row or one of the phalangeal rows (not
always the first) of either digit II or digit III (Motani,
1999, fig. 6). Moreover, the position of the bifurcation
can vary between the left and right fin of an individual,
although rarely (e.g. SOMAG/GEO11, ANSP 15766).
So having a similar kind of variation in the hindfin is
not unexpected.
In limb development, a bifurcation results in two
equivalent branches, so in the hindfin, both branches
of the bifurcation represent digit II (Oster et al.1988;
Motani, 1999). In both the hindfin and forefin, how-
ever, one branch of the bifurcated digit can be oriented
such that it appears to be a continuation of the ori-
ginal digit (Fig. 3a, d; Motani, 1999, fig. 6c, d; Lomax,
Massare & Mistry, 2017, fig. 11a, b), but this is not al-
ways the case (Fig. 3b, c; Lomax, Massare & Mistry,
2017, fig. 11c, d). Furthermore, one branch can have
smaller elements than the other (Fig. 3a; Motani, 1999,
fig. 6e, f), although this is more common on distal
732

Hindfins of Ichthyosaurus
Figure 3. (Colour online) Variation in the orientation and size of subdigits resulting from a bifurcation of digit II. Anterior to the
left. (a) WARMS G6188 (Morphotype 1), right hindfin (reversed) in which subdigit-b is more in line with proximal elements of digit
II than is subdigit-a. Note that subdigit-a has somewhat smaller elements than subdigit-b. (b) NLMH 106234 (Morphotype 2), right
hindfin (reversed) showing the two subdigits of digit II symmetrically arranged with respect to distal tarsal two. Note the presence of
a third subdigit in the fourth phalangeal row. (c) SMNS 58275 (Morphotype 2), right hindfin, in which neither subdigit is in line with
proximal elements of digit II. (d) CLC1 (Morphotype 1), left hindfin in which subdigit-a is more in line with the proximal elements of
digit II than is subdigit-b. Scale bars in (a), (c) and (d) equal 2 cm. Scale bar in (b) equals 3 cm. Scale for (d) is estimated because the
specimen is on display behind glass. Abbreviations: 2 – distal tarsal two; 3 – distal tarsal three; 4 – distal tarsal four; a – astragalus;
c – calcaneum; F – fibula; iia – anterior metatarsal of digit II; iib – posterior metatarsal of digit II; sa – anterior subdigit of digit II; sb
– posterior subdigit of digit II; T – tibia.
bifurcations (Fig. 3b). Irrespective of size or orienta-
tion, this work considers the two branches to be equi-
valent subdigits of digit II. In Morphotype 2, owing to
the bifurcation, two metatarsals are associated with di-
git II, referred to herein as metatarsal two-a and two-b.
This results in four metatarsals in the hindfin. Neither
metatarsal one nor five is present, as might have been
inferred from four metatarsal elements if only a por-
tion of the variation in the hindfins was examined.
An alternate explanation, however, is that the posterior
branch of the bifurcation is a continuation of digit II,
whereas the anterior branch is a supernumerary digit.
This interpretation is consistent with the posterior to
anterior sequence of branching of the digital arch dur-
ing limb development (Shubin & Alberch, 1986), so
it also has some basis in development. However, there
are a few cases where the anterior branch seems to be
the continuation of digit II (e.g. Fig. 3d), which is why
we favour interpreting the two subdigits as equal.
Three variations occur in Morphotype 2, mainly dif-
fering in the number and extent of contact of ele-
ments in the third row with the astragalus (Fig. 2b–
d). Some specimens have two small metatarsals in
digit II that are directly distal to distal tarsal two, with
metatarsal two-b having little or no contact with the as-
tragalus (Figs 2c, 4a, b). Distal tarsal three has a broad
contact with the astragalus similar to that which oc-
curs in Morphotype 1. In some specimens of I. com-
munis, on the other hand, both metatarsal two-b and
distal tarsal three have fairly broad contacts with the
astragalus. The contacts are approximately the same
length (Fig. 2b). These two variations seem to be dis-
tinct morphologies, and if only a specimen or two
were known, one could conclude that each species had
a diagnostic hindfin morphology. However, although
many specimens of I. somersetensis have little or no
contact between metatarsal two-b and the astragalus,
others have contacts with the astragalus of varying
amounts (Fig. 4a–c). Similarly, although some speci-
mens of I. communis have two equally large elements
and broad, symmetric contacts of metatarsal two-b and
distal tarsal three with the astragalus, other specimens
of I. communis have a distal tarsal three contact that
is somewhat longer than the metatarsal contact (e.g.
Fig. 4d). The size of distal tarsal three relative to the
adjacent metatarsal seems to be the controlling factor
in the extent of contact with the astragalus, and could
reflect individual variation in relative growth rates.
Thus, the seemingly distinct morphologies of I. somer-
setensis and I. communis are end-members of a con-
tinuum of variation in the relative size of the contacts
between metatarsal two-b, distal tarsal three and the
astragalus. In fact, two specimens of Ichthyosaurus
(OUMNH J.29351, NHMUK OR41849) preserve two
hindfins, each with a different variation of Morphotype
2. Neither is a composite specimen.
When Morphotype 2 occurs in I. somersetensis,
typically distal tarsal three has a broad contact with
the astragalus, with metatarsal two-b having only a
minor contact, if any, with the astragalus (Figs 2c, 4a,
b). A similar morphology also occurs in I. conybeari
(SMNS 58275, NHMUK R5790) and I. breviceps
733

J. A. MASSARE & D. R. LOMAX
Figure 4. (Colour online) The continuum of variation in Morphotype 2, all with four elements in the third row. (a) CAMSM J59574,
distal tarsal three with broad contact with astragalus and metatarsal two-b with no contact. (b) BRSMG Ce16611, distal tarsal three
with broad contact with astragalus and metatarsal two-b with minor contact. (c) OUMNH J.13799 (reversed), distal tarsal three with
broad contact with astragalus and metatarsal two-b with much shorter contact with astragalus. (d) NHMUK R1073, distal tarsal three
with slightly longer contact with astragalus than that of metatarsal two-b. Scale bars in (a) and (c) equal 3 cm. Scale bar in (b) equals
4 cm, based on a cast of the specimen at LEICT. Scale bar in (d) equals 2 cm but is estimated because the specimen is on display
behind glass. Abbreviations: 3 – distal tarsal three; a – astragalus; iib – metatarsal of posterior subdigit of digit II.
(CAMSM X50187, NHMUK OR2001∗), although less
commonly than Morphotype 1. Without several speci-
mens of each species, the less common morphology
would not have been evident, and mesopodium mor-
phology might have been considered a species-specific
character rather than a matter of individual variation.
In a third variation of Morphotype 2, metatarsal two-
b, distal tarsal three and distal tarsal four have fairly
broad and nearly equal contacts with the astragalus,
at least on well-articulated specimens (e.g. NHMUK
R5595, NHMUK OR8165 left fin). Metatarsal two-b
is more proximally located such that it nearly separ-
ates distal tarsal two from the astragalus (Fig. 2d). It
appears to be the common morphology of I. larkini,
but only four specimens of the species are known,
three of which preserve a hindfin. Lomax & Massare
(2017, p. 12) misidentified the digit count in I. larkini
and identified the element that almost separates tarsal
two (their tibiale) from the astragalus as the proximal
element of digit III, whereas it is actually metatarsal
two-b. Two additional hindfins have this morphology
(NOTNH: FS3450 and NOTNH: FS4940). They could
be I. larkini, but additional diagnostic material is lack-
ing, so they cannot be assigned unequivocally to the
species. As in the other variations of Morphotype 2,
the relative size of the contacts with the astragalus var-
ies considerably. The contact between the astragalus
and distal tarsal three is longer than the contact with
metatarsal two-b and with distal tarsal four in NOTNH
FS4940 and BRSUG 25300, compared to Figure 2d. In
fact, two variations of this morphology are preserved
in BRSUG 25300 (the holotype of I. larkini): in one
hindfin distal tarsal four has a significant contact with
the astragalus, whereas in the other fin, tarsal four has
only a slight, if any, contact with the astragalus, even
accounting for taphonomic displacement. A slight con-
tact between distal tarsal four and the astragalus oc-
curs in Morphotype 1 and other variations of Morpho-
type 2, so it is not unique to the I. larkini variation
of Morphotype 2. One specimen of Ichthyosaurus that
preserves two hindfins (NHMUK OR8165; Fig. 5a, b)
has the I. larkini variation in one hindfin and another
variation of Morphotype 2 in the other fin; specific-
Figure 5. (Colour online) Specimens with two different mor-
phologies in the hindfins. (a) NHMUK OR8165, left hindfin
(reversed) showing one variation of Morphotype 2, with four
elements in the third row and three elements in contact with
the astragalus. (b) NHMUK OR8165, right hindfin showing an-
other variation of Morphotype 2, with four elements in the third
row and one element (distal tarsal three) in broad contact with
the astragalus. (c) GLRCM 1987.45, ?left hindfin showing one
variation of Morphotype 2, with four elements in the third row
and three elements in contact with the astragalus. Specimen had
previously been identified as a forefin, but the propodial shape
and size indicate that it as a hindfin. (d) GLRCM 1987.45, ?right
hindfin (reversed) showing Morphotype 1, with three elements
in the third row and distal tarsal three in broad contact with
the astragalus. Scale bars equal 3 cm, but the scales for (a) and
(b) are estimates because the specimens are on display behind
glass. Abbreviations: 2 – distal tarsal two; 3 – distal tarsal three;
a – astragalus; ii – metatarsal two; iia – metatarsal of the an-
terior subdigit of digit II; iib – metatarsal of posterior subdigit of
digit II.
ally, metatarsal two-b does not have a broad contact
with the astragalus nor does it nearly separate it from
distal tarsal two. This suggests that the morphology
of I. larkini is another variation of Morphotype 2, but
we have not yet identified specimens of other species
with this variation. Although this variation seems to be
734

Hindfins of Ichthyosaurus
unique to I. larkini, too few specimens are known to
adequately evaluate it. To complicate the picture, an-
other specimen with two hindfins (GLRCM 1987.45;
Fig. 5c, d) has one fin with the ‘I. larkini’ morphology
but the other fin displays Morphotype 1, with three
elements in the third row. Unfortunately, GLRCM
1987.45 is not sufficiently complete to assign to a spe-
cies. Thus, Morphotype 1 might occur in I. larkini,or
equally likely, the I. larkini variation of Morphotype 2
might occur in another species. This suggests that this
variation of Morphotype 2 cannot be used on its own
to unequivocally refer a specimen to I. larkini.
6. Other features of the hindfin
6.a. Notching of anterior elements
Notching on one or more elements in the anterior
digit of the hindfin occurs in many Lower Jurassic
ichthyosaurian genera including Ichthyosaurus. Notch-
ing has been interpreted as a retained primitive fea-
ture homologous to the shaft in long bones of the
tetrapod limb (Motani, 1999) or as a structure to pro-
tect nerves or blood vessels (Johnson, 1979; Cald-
well, 1997). Maxwell, Scheyer & Fowler (2014) found
that periosteal bone is present at the anterior end of
notched elements, whereas it is lacking at the pos-
terior end. They suggested that this related to increased
stresses on the leading edge of the forefin. Thus, notch-
ing might be related to the way the fin was used dur-
ing locomotion. The slight correlation between the
number of notched elements and the size of the indi-
vidual suggests an ontogenetic component of notching
(Johnson, 1977; Maxwell, Scheyer & Fowler, 2014)
and supports the hypothesis that notching is related to
fin use throughout life. In Ichthyosaurus, notching is
more common in the hindfin than in the forefin (Mas-
sare & Lomax, 2016b). In Stenopterygius, notching is
more variable in the hindfin than in the forefin (Max-
well, Scheyer & Fowler, 2014).
Massare & Lomax (2016b) argued that notching in
the fins of Ichthyosaurus probably had no taxonomic
significance in elements other than the tibia, but this
study has found that some species of Ichthyosaurus
have notched elements whereas others do not. Ichthy-
osaurus somersetensis almost always has notching in
the hindfin. Of 21 specimens of I. somersetensis recog-
nized in this research (Table 1), only one (ROM 26029)
might lack notching in the hindfin. Distal tarsal two
lacks a notch, but the anterior element in the metatarsal
row is probably missing and the first phalanx is dam-
aged. Most specimens of I. somersetensis have a notch
in the distal tarsal or metatarsal or both, and sometimes
in one or two phalanges as well. All known specimens
of I. larkini with a hindfin show notching in at least one
element, but with so few specimens, it is impossible
to say whether this always occurs. On the other hand,
no known specimens of I. communis,I. breviceps or
I. anningae have notched elements in the hindfin (or
the forefin). Only two specimens of I. anningae,how-
ever, preserve incomplete hindfins (NHMUK OR120
and NHMUK R10028), and although neither shows
notching, with so few specimens, the variation in this
feature cannot be assessed. Ichthyosaurus somerseten-
sis is the common species from Somerset, whereas I.
communis is the common species from Dorset (i.e. the
Lyme Regis–Charmouth area). Ichthyosaurus larkini is
known only from Somerset, whereas I. breviceps and
I. anningae are exclusively from Dorset. As a result
of the difference in species abundances, notching of
anterior elements of the hindfins is rare on specimens
from Dorset, but occurs commonly on specimens from
Somerset, as noted by McGowan (1974) and Massare
& Lomax (2016b).
Ichthyosaurus conybeari, which is an uncommon
species from both Somerset and Dorset (McGowan,
1974; Massare & Lomax, 2016b), almost always has
a notched tibia, the only species of Ichthyosaurus to
show notching in the zeugopodium. A notched tibia
is present in very small specimens (CAMSM X50224,
femur length =1.3 cm), even in the absence of notch-
ing in the forefin. A larger specimen (BGS 956, femur
length 1.9 cm) has a notched tibia and distal tarsal
two, as well as forefin notching. A much larger spe-
cimen (SMNS 58275, femur length =3.5 cm) also has
a notched tibia and two notched elements in the right
forefin. A notched tibia appears to be a unique char-
acter for the species, at least within Ichthyosaurus.If
this is the case, then OUMNH J.10301 could also be
referred to I. conybeari. This would be the largest ex-
ample of the species known, with an estimated femur
length greater than 6 cm. OUMNH J.10301 is a his-
toric specimen in which the matrix block with the hind-
fin is isolated from at least two other blocks, all within
a wooden frame. The entire specimen might be a com-
posite (Massare & Lomax, 2016a). The picture is more
complicated, however. Skull and humerus morphology
suggest that NHMUK R11801 and USNM V4697 are
probably I. conybeari, but both lack a notched tibia
in the articulated hindfin that is preserved. The latter,
however, has an odd morphology in the hindfin (see
Section 7). So although the presence of a notched tibia
is indicative of I. conybeari, it would appear that the
lack of a notched tibia does not completely exclude
that possibility. Although both I. somersetensis and I.
larkini have notched elements in the hindfin, the tibia
is never notched in those species.
6.b. Number of digits
An issue arises in how to characterize the number of di-
gits in the hindfin given that digit II can have a bifurc-
ation in the metatarsal row. Applying the definition of
digits as elements arising from a metatarsal/metacarpal
(McGowan, 1972) creates complications in species
diagnoses. For example, specimens of I. conybeari
with Morphotype 1 would have three digits corres-
ponding to metatarsals two, three and four; whereas
specimens with Morphotype 2 would have four
digits, corresponding to metatarsals two-a, two-b, three
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J. A. MASSARE & D. R. LOMAX
and four. Irrespective of the morphotype, all speci-
mens of I. conybeari have similar hindfins, with five
columns of elements (including a distal bifurcation
of digit II). An additional complication is that the
fourth digit arising from the metatarsal bifurcation is
neither digit I nor digit V, making Ichthyosaurus dis-
tinctly different from a taxon with digit I or V in the
hindfin (e.g. Mixosaurus). A strict application of the
definition creates potential errors in phylogenetic ana-
lysis as well because the four digit hindfin in some
species of Ichthyosaurus is not plesiomorphic. Cooper
et al. (2007, fig. 1) distinguished between digital
rays (metatarsal plus phalanges) and digits (phalanges
only), but those definitions create a similar issue in
characterizing hindfins of Ichthyosaurus.
Thus, the standard terminology does not work well.
All Ichthyosaurus species have three primary digits
in the hindfin, digits II, III and IV, with digit II hav-
ing two subdigits resulting from a proximal bifurc-
ation in either the first phalangeal row (Morphotype
I) or the metatarsal row (Morphotype 2). Most speci-
mens have a third subdigit of digit II, resulting from
a distal phalangeal bifurcation. Subdigits arise from
bifurcations. They are distinct from pre- and post-
axial accessory digits, which do not arise from the
tarsus/carpus (McGowan, 1972) nor from a digital bi-
furcation. Previous workers have largely not distin-
guished between primary digits and subdigits, count-
ing them together to describe the number of digits in
the hindfin (e.g. Lomax & Massare, 2017) and fore-
fin (e.g. McGowan & Motani, 2003;Jietal.2016;
Massare & Lomax, 2017). Similarly, following Cooper
et al. (2007) and Cooper & Dawson (2009), we use the
term ‘digits’ here to include both primary digits and
subdigits. We exclude accessory digits from the counts
that follow.
Thus, the common morphology of the hindfin of all
species of Ichthyosaurus, for both morphotypes, has
a total of five digits, the result of bifurcations in di-
git II. The two morphotypes are consistent, although
different, in the position of the proximal bifurcation,
either in the first phalangeal row (Morphotype 1) or in
the metatarsal row (Morphotype 2). Usually a second
bifurcation occurs distally, which results in five di-
gits. However, the fin must be complete to the fifth
phalangeal row in order to confirm the digit count be-
cause the distal bifurcation often occurs in the fourth or
fifth phalangeal row of digit II (e.g. NHMUK R3372,
NLMH 106234, OUMNH J.29351), especially in I.
somersetensis.
Exceptions to this pattern occur. Two specimens of
Ichthyosaurus have six digits. PETMG R174 (I. com-
munis) and YPM 9204 (possibly I. communis)havebi-
furcations in the metatarsal, first or second phalanx,
and fifth or sixth phalanx of digit II. Less unusual
occurrences are specimens with fairly complete hind-
fins that have only four digits, although this could be
a preservation issue. Collection or preparation could
have missed a small distal bifurcation. In particu-
lar, the lack of a distal bifurcation on historic spe-
cimens needs to be evaluated carefully. The anterior
edge of some hindfins is at the edge of the mat-
rix, presenting the possibility that some fin elements,
especially small ones, could have been accidentally
overlooked or intentionally removed to show a bet-
ter defined fin. Thus, a distal bifurcation, especially
one with small elements, cannot be entirely ruled out
on historic specimens with only four digits, even in
what appears to be a fairly complete hindfin. That be-
ing said, some specimens that are well preserved and
sufficiently complete have hindfins that lack the distal
bifurcation (e.g. BRLSI GP1870, CAMSM J59574,
NHMUK R11199, NHMUK OR43006, TTNCM 8359
and WARMS G6188; possibly also CAMSM J35188
and NHMUK OR2013 (not OR2013∗)). Those that can
be assigned to a species are either I. breviceps or I.
somersetensis, but several specimens cannot be identi-
fied to species.
Although some forefins of Ichthyosaurus have a
pre-axial (anterior) accessory digit (e.g. NHMUK
OR43006), hindfins seem to lack them. Sometimes one
subdigit of digit II has larger elements than the other,
possibly as a result of unequal division of embryonic
tissue or different growth rates. This does not change
the developmental equivalence of the two subdigits.
When this occurs, however, it is most often the anterior
subdigit that is smaller (Fig. 3a). Poor preservation or
disarticulation could make it difficult to distinguish a
subdigit from a pre-axial accessory. This is especially
problematic for distal bifurcations if one subdigit has
small, rounded elements (Fig. 3b), if the proximally
adjacent element lacks a distinct facet or if only a few
elements are present. It is often a judgement call as to
whether small anterior elements belong to a subdigit
or a pre-axial accessory digit. We have not been able
to unequivocally identify a pre-axial accessory digit on
any hindfin (contrary to Massare & Lomax, 2014), so
we conclude that pre-axial accessory digits are absent
or rare on hindfins of Ichthyosaurus.
In contrast, post-axial (posterior) accessory digits
are easier to recognize. The elements are round or
oval, whereas digit IV always has larger, rectangular
phalanges, at least in the proximal half of the fin. The
presence of one post-axial accessory digit seems to be
the typical condition in hindfins of Ichthyosaurus, al-
though they are present on less than half of the spe-
cimens examined. A post-axial accessory digit occurs
in all species, possibly even I. anningae (NHMUK
R10028). Post-axial accessory digits are often poorly
preserved, incomplete or absent because of decisions
made during preparation, especially on historic speci-
mens. So, the lack of a post-axial accessory needs to
be viewed initially with some scepticism.
7. Unusual hindfin morphologies in Ichthyosaurus
Several hindfins show unusual morphologies, some of
which might be pathology (Table 3 ). Had there been
fewer or less complete specimens to examine, some
of these morphologies could have been interpreted as
736

Hindfins of Ichthyosaurus
Table 3. Hindfins with unusual morphologies
Specimen Morphotype Unusual features
ANSP 17429 1 co-ossification occurs
BGS 955 2 supernumerary elements present
BRSMG Ce16611 left 2 supernumerary elements present
BRSUG 25300 right 2 bifurcation element displaced proximally; four elements in
third row
BU 5289 2 middle two digits displaced proximally
CAMSM J35187 2 co-ossification occurs
NHMUK OR41849 ?right 2 astragalus separates tibia and fibula and almost contacts femur
NHMUK R5595 left 2 both branches of digit II displaced proximally
NHMUK R5790 2 middle two digits displaced proximally
NHMUK R5918 2 both branches of digit II displaced proximally
NHMUK R11801 2 astragalus separates tibia and fibula and almost contacts femur
NMW 91.29G.1 2 astragalus separates tibia and fibula and almost contacts femur
OUMNH J.29352 2 calcaneum enlarged posteriorly
PETMG R174 2 enlarged, proximodistally elongated calcaneum; astragalus
does not contact tibia
RAMM 57/2009 2 three elements contact tibia; four elements in second row
SMNS 58275 right 2 supernumerary element present
TTNCM 8359 2 co-ossification occurs
USNM V4967 2 supernumerary element anterior to tibia; three elements
contact femur
WARMS G6188 left 1 supernumerary element present
Note: ‘left’ and ‘right’ indicate the left or right hindfin on specimens that preserve both.
Figure 6. (Colour online) An example of gradation in morphologies in proximal fin elements. Anterior to the left. (a) The hindfin of
PETMG R174 has an oval, proximodistally elongated calcaneum and the tibia and astragalus lack contact. (b) The hindfin of OUMNH
J.29352 has a similarly shaped astragalus, a short contact between the tibia and astragalus and a calcaneum that is anteroposteriorly
elongated only in the posterior portion. (c) The hindfin of CHMUS 2015.0102 (reversed) has a calcaneum, astragalus and fibula shaped
similarly to (b) but the relative sizes and arrangement of elements are more like that of a typical Ichthyosaurus hindfin. Image courtesy
of Charterhouse School Archive. Scale bar in (a) equals 3 cm. Scale bars in (b) and (c) equal 1 cm. Abbreviations: a – astragalus;
c – calcaneum; F – fibula; T – tibia.
sufficiently unique to be a new taxon. PETMG R174,
a specimen of Ichthyosaurus communis, lacks a con-
tact between the astragalus and the tibia (Fig. 6a). In
addition, the calcaneum is proximodistally elongated
and anteroposteriorly short. It is posterior to the fib-
ula for much of its length. The hindfin otherwise has
the Morphotype 2 arrangement of elements. A less ab-
errant, but similar, arrangement occurs in OUMNH
J.29352. The astragalus has a minor contact with the
tibia and the calcaneum is proximodistally elongated
only in the posterior portion and it is not anteroposteri-
orly short (Fig. 6b). The calcaneum is, however, sim-
ilarly posterior to most of the fibula. Another speci-
men, CHMUS 2015.0102, has a posteriorly expanded,
but small, calcaneum and a posteriorly reduced fibula
(Fig. 6c), but not to the extent displayed in OUMNH
J.29352. The astragalus has a broader contact with the
tibia and fibula. Thus, the three specimens display a se-
quence of morphologies that grade from a highly un-
usual arrangement to one that is more similar to the
typical morphology (compare Fig. 6ctoFig. 4b, c). It
suggests that even the morphology of PETMG R174 is
just an odd variation.
Another unusual hindfin morphology is that of
RAMM 57/2009, which is clearly Ichthyosaurus based
on the rest of the skeleton. The hindfin has four ele-
ments in the second row, with three elements in con-
tact with the tibia (Fig. 7a). In addition to the facet for
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J. A. MASSARE & D. R. LOMAX
Figure 7. (Colour online) Another unusual morphology of the
hindfin. Anterior to the left. (a) RAMM 57/2009, left hindfin
showing two elements immediately distal to the tibia, possibly
the result of a bifurcation in digit II. The anterior subdigit curves
‘outwards’ (anteriorly) and metatarsal two-a is more proximally
located than metatarsal two-b. (b) NHMUK R11199, left hind-
fin, showing a bifurcation of digit II in the metatarsal row. The
anterior subdigit curves ‘outwards’ (anteriorly), and metatarsal
two-a is more proximally located than metatarsal two-b. Scale
bar in (a) equals 2 cm. Scale bar in (b) equals 1 cm but is estim-
ated because the specimen is on display behind glass. Abbrevi-
ations: 2 – distal tarsal two; iia – anterior metatarsal of digit II;
iib – posterior metatarsal of digit II; p – phalanx; T – tibia; ? –
homology unknown.
the astragalus, the tibia has two other facets, one more
anteriorly oriented than the other. This arrangement
could be interpreted as a bifurcation in the distal tarsal
row. The matrix is darker around the fin than elsewhere
on the block, so it is possible that it was reset, but the
facets on adjacent elements suggest that the arrange-
ment is authentic. Taphonomic displacement is pos-
sible, but the facets and the lack of disarticulation sug-
gest that this is not displacement. No other hindfin of
Ichthyosaurus shows this morphology in which the bi-
furcation of digit II seems to have occurred more prox-
imally than usual. The anterior branch of digit II curves
‘outwards’ as if displaced anteriorly by the posterior
branch. Also, for a given position on the proximal
half of the fin, there are more elements in the anterior
branch than in the posterior branch of digit II (Fig. 7a).
Both of these features also occur in DORCM G1, BGS
955 and NHMUK R11199 (Fig. 7b), although these
specimens have the usual Morphotype 2 bifurcation in
the metatarsal row. In all three specimens, the relative
position of the proximal elements is the same: meta-
tarsal two-a is slightly more proximally located than
metatarsal two-b (Fig. 7a, b). The major difference is
that the bifurcation apparently occurs in a more prox-
imal row in RAMM 57/2009 than in the others. As
with the previous example, the unusual morphology of
RAMM 57/2009 shares similarities with fins having a
more typical morphology.
In NHMUK R11801, a small articulated skeleton,
the astragalus is positioned so far proximally that it
separates the tibia and fibula and almost contacts the
femur. Distal tarsal two and the calcaneum seem to
be pushed anteriorly and posteriorly, respectively, by
metatarsal two-b and distal tarsal three (Fig. 8a). Disar-
ticulation and displacement can produce this arrange-
ment (e.g. BU 5289; Fig. 8b), but NHMUK R11801
is not disarticulated. In isolation, the arrangement of
elements could be interpreted as having three ele-
ments in the first row and four elements in the second
and third rows, thus an entirely new morphology. In
fact, a contact between the astragalus and femur oc-
curs in Maiaspondylus (Druckenmiller & Maxwell,
2010, table b1) and Platypterygius hercynicus (Kolb
& Sander, 2009; Fischer, 2012). Other derived ich-
thyosaurs (Aegirosaurus,Brachypterygius,Maiaspon-
dylus) have a comparable contact between the hu-
merus and the intermedium in the forefin (Bardet &
Fernández, 2000; McGowan & Motani, 2003; Max-
well & Caldwell, 2006). However, the morphology of
NHMUK R11801 is yet another ‘extreme’ variation
of a common arrangement of Morphotype 2. NMW
91.29G.1 also has an astragalus that nearly contacts the
femur, but metatarsal two-b is somewhat smaller than
in NHMUK R11801, and so the calcaneum and tarsal
two are closer to the astragalus (Fig. 8c). The second
row of three elements is better defined, and distal tarsal
two and the calcaneum are not displaced outwards as
much. This is also the case in one fin of NHMUK
OR41849, where the astragalus practically contacts the
femur, separating the tibia from the fibula. The other
fin also shows the separation but the astragalus is not as
proximally located. In NHMUK R1162 the astragalus
is positioned slightly more distally (Fig. 2b), and the
tibia and fibula are closer together, a more typical ar-
rangement of elements.
USNM V4967 is another unusual morphology,
where a fairly large, supernumerary element is anterior
to the tibia and in contact with the femur (Fig. 9a).
The tibia is smaller than usual relative to the fibula,
so it is possible that the supernumerary bone restricted
its growth. Maxwell (2012, p. 550) reported a some-
what similar, but more common, anomaly in Stenop-
terygius quadriscissus, in which a single element con-
tacts the femur anterior to the tibia. USNM V4967 is
otherwise a typical example of Morphotype 2, with
a proximal bifurcation in the metatarsal row and a
more distal bifurcation in the second phalangeal row.
Other features of the skeleton identify this specimen
as Ichthyosaurus, but as with PETMG R174, RAMM
57/2009, NMW 91.29G.1 and NHMUK R11801, the
hindfin might have been considered a new taxon had
it occurred in isolation. This makes it clear that frag-
mentary or isolated material does not usually provide
sufficient information to justify a new taxon (e.g. as in
Brusatte et al.2015).
Several specimens have smaller, more distally loc-
ated supernumerary elements in the fin, in the form of
a small round bone between two of the digits (Fig. 9b–
d). These elements are usually associated with digit II.
BRSMG Ce16611 has a supernumerary element in di-
git II, between the first and second phalanges, and a
second, smaller isolated element anterior to it (Fig. 9b).
BGS 955 has a small, round supernumerary element in
digit II between the first and second phalangeal rows
and another anterior to the second phalanx of digit
738

Hindfins of Ichthyosaurus
Figure 8. (Colour online) Hindfins in which the astragalus separates the tibia and fibula and almost contacts the femur. (a) NHMUK
R11801, left hindfin, showing digit III and the posterior subdigit of digit II more proximally positioned than usual. The fin appears to
have three elements in the first row and four elements in the second row because tarsal two and the calcaneum are shifted anteriorly
and posteriorly, respectively. (b) BU5289, right hindfin (reversed), with a similar morphology as (a). However, it is probably due
to post-mortem displacement in this specimen. Note, that some elements are dorsoventrally rotated and partially buried. (c) NMW
91.29G.1, ?left hindfin (reversed), showing the astragalus almost in contact with the femur, but digit III and the posterior subdigit of
digit II do not displace distal tarsal two and the calcaneum as in (a). Scale bars in (a) and (b) equal 1 cm. Scale bar in (c) equals 2 cm.
Abbreviations: 2 – distal tarsal two; 3 – distal tarsal three; 4 – distal tarsal four; a – astragalus; c – calcaneum; F – fibula; iia – anterior
metatarsal of digit II; iib – posterior metatarsal of digit II; T – tibia.
Figure 9. (Colour online) Examples of supernumerary elements and co-ossifications. Anterior to the left. (a) USNM V4967, showing
a fairly large, supernumerary bone (arrow) that is anterior to the tibia and in contact with the distal end of the femur. (b) BRSMG
Ce16611, arrows point to two supernumerary elements in digit II. (c) BGS 955, arrows point to two small supernumerary elements.
(d) WARMS G6188, arrow points to a supernumerary element anterior to digit II. (e) ANSP 17429 (reversed), arrow points to a
co-ossification of elements from digits II and III, possibly metatarsal three and metatarsal two-b. (f) TTNCM 8359 (reversed), arrow
points to a co-ossification of the proximal element of the two metatarsals of digit II. The other hindfin (not figured) shows Morphotype
2. Scale bars in (a) and (b) equal 3 cm. Scale bars in (c) and (f) equal 1 cm. Scale bars in (d) and (e) equal 2 cm.
II (Fig. 9c). The right hindfin of SMNS 58275 has a
supernumerary element between the fourth and fifth
phalangeal rows of digit II. The left fin of WARMS
G6188 has a supernumerary element anterior to and
between phalanges one and two of digit II, proximal
to where the distal bifurcation occurs (Fig. 9d). Sim-
ilar supernumerary elements occur occasionally in the
forefins of Ichthyosaurus (pers. obs.) and have been re-
ported in forefins and hindfins of Stenopterygius (Max-
well, 2012), and in whales (Cooper & Dawson, 2009).
Another irregularity in a few hindfins is co-
ossification of adjacent elements, which has also been
reported in forefins of Ichthyosaurus (Motani, 1999;
Lomax, Massare & Mistry, 2017), Mixosaurus (Max-
well, 2012), Stenopterygius (Johnson, 1979; Maxwell,
2012), Undorosaurus (Arkhangelsky & Zverkov, 2014,
fig. 1c) and in the hindfin of Suevoleviathan (Maisch,
1998; pers. obs. SMNS 15390), as well as whales
(Cooper & Dawson, 2009). In the hindfin of Ichthy-
osaurus, a co-ossification can be recognized as a large,
anteroposteriorly elongated, irregular element. Thus,
in TTNCM 8359 (Fig. 9f), the elongated bone in di-
git II is probably a co-ossification of metatarsals two-a
and two-b. So what was originally a bifurcation in the
metatarsal row of Morphotype 2 mimics Morphotype
1 because of the co-ossification. ANSP 17429 (Fig. 9e)
739

J. A. MASSARE & D. R. LOMAX
Figure 10. (Colour online) Examples of post-mortem displacement that produces unusual morphologies. (a) NHMUK R5918, an
isolated hindfin, in which distal tarsal two has been rotated and moved proximally so that it is anterior to the tibia. This also shifts both
branches of digit II proximally relative to digit III. (b) BRSUG 25300, left hindfin, in which metatarsal two-b has been shifted so that it
is almost in contact with the tibia. Darker elements of the distal portion of the fin are entirely reconstructed. (c) NHMUK R5595, left
hindfin, in which distal tarsal two and both branches of digit II have been moved proximally, similar to (a). All scale bars equal 3 cm,
but scale for (c) is an estimate because the specimen is on display behind glass. Abbreviations: 2 – distal tarsal two; 3 – distal tarsal
three; 4 – distal tarsal four; a – astragalus; c – calcaneum; F – fibula; iia – anterior metatarsal of digit II; iib – posterior metatarsal of
digit II; T – tibia.
also shows two co-ossified elements between digits II
and III, possibly metatarsal three and metatarsal two-b,
but the fin is somewhat disarticulated and identification
of elements is difficult. The proximal portion of the fin
is missing in CAMSM J35187, but a co-ossification of
phalanges of the two branches of digit II occurs in the
third or fourth phalangeal row.
Post-mortem displacement of digits can also res-
ult in odd morphologies that could, in isolation, be
mistaken for new taxa. Four specimens have a por-
tion of the hindfin shifted proximally. In NHMUK
R5918, an isolated hindfin, distal tarsal two has been
rotated and moved anterior to the tibia and the two
branches of digit II have also been shifted proxim-
ally (Fig. 10a), probably a taphonomic effect. Note that
the space between the posterior branch of digit II and
digit III is wider than between digit III and IV, sug-
gesting post-mortem disarticulation. The left hindfin of
NHMUK R5595 (Fig. 10c), a nearly complete skeleton
of I. larkini (Lomax & Massare, 2017), shows a similar
shift, but distal tarsal two has not been rotated and di-
git II is not shifted proximally as much as in NHMUK
R5918. This, however, might not be taphonomic, but
instead, the result of decisions made during prepara-
tion, because most of the fin appears to be set in a filler
material. Unfortunately, the specimen is not accessible
for close examination. BRSUG 25300, the holotype of
I. larkini, also shows what might be post-mortem dis-
placement on the right hindfin (posterior fin on speci-
men). Metatarsal two-b has been displaced proximally
so that it is between distal tarsal two and the astragalus,
resulting in what could be interpreted as four elements
in the second row (Fig. 10b). Although metatarsal two-
b in the left fin of the specimen partly separates the
740

Hindfins of Ichthyosaurus
astragalus and distal tarsal two, it is more distally posi-
tioned and is clearly an element of the third row. Post-
mortem displacement resulted in an odd morphology
in one of the two fins. A similar displacement occurs
in NHMUK R5790, a small, isolated hindfin. Meta-
tarsal two-b is between the astragalus and distal tarsal
two, allowing a similar interpretation of four elements
in the second row. The wide separation between ele-
ments in NHMUK R5790 makes it clear that this odd
arrangement is the result of post-mortem disarticula-
tion and displacement.
8. Discussion and conclusion
Ichthyosaurus has three primary digits (II, III, IV) in
the hindfin, a feature it shares with Temnodontosaurus,
Suevoleviathan,Leptonectes,Eurhinosaurus,Stenop-
terygius and Hauffiopteryx (Maisch, 1998; McGowan
& Motani, 2003; Maxwell, 2012, fig. 2). The hindfins
of the other Lower Jurassic genera, Excalibosaurus,
Wahlisaurus and Protoichthyosaurus, are too incom-
plete to determine the total digit count. Only Ichthy-
osaurus is known to have an anterior digital bifurc-
ation in the hindfin (but see below), which increases
the digit count to as high as six, although five is more
typical. The digital bifurcation can distinguish hindfins
of Ichthyosaurus from most genera. However, incom-
plete hindfins present a problem in recognizing Ich-
thyosaurus. Morphotype 1 is seen in other genera, so
if only the first three rows of elements are preserved,
Ichthyosaurus is not easily distinguished from other
Lower Jurassic genera, although the femora might be
distinctive. Thus, the taxonomic usefulness of hindfin
morphology has some limitations.
Another limitation is that the hindfin of Protoichthy-
osaurus shares similarities with that of Ichthyosaurus,
a sister taxon (Lomax, Massare & Mistry, 2017). Fur-
thermore, both genera have an anterior digital bifurca-
tion in the forefin, which suggests the possibility that
Protoichthyosaurus also has a bifurcation in the hind-
fin. Only three specimens of Protoichthyosaurus with
a hindfin or femur are known (Lomax, Massare &
Mistry, 2017). BRLSI M3555 is the only specimen
to preserve a complete femur, which has a long shaft
with a narrow proximal end and a very wide distal
end. It is similar to that of I. conybeari but the shaft
is more elongated and the distal expansion is broader.
The femur on SOMAG/GEO/12 is not as elongated
and the shaft is wider, although the proximal end is
damaged and poorly preserved. With only two spe-
cimens the variation in the elongation cannot be as-
sessed. Only SOMAG/GEO/12 preserves the mesopo-
dium, which displays Morphotype 1. The more distal
portion of the fin is partially disarticulated, and anterior
elements are too scattered to ascertain the presence or
absence of a bifurcation. A third specimen of Protoi-
chthyosaurus (BRLSI M3563) preserves a hindfin, but
the specimen is a composite. Only the right forefin and
associated scapula and ribs definitely belong together
and can be assigned to Protoichthyosaurus. The rest of
the skeleton might not belong to the same individual or
the same species (Massare & Lomax, 2016a; Lomax,
Massare & Mistry, 2017). Thus, based on currently re-
cognized specimens, the hindfin of Protoichthyosaurus
cannot be distinguished from that of Ichthyosaurus.
Within Ichthyosaurus, species cannot be identified
unequivocally from a well-preserved hindfin, although
certain morphologies are more common in some spe-
cies than others (Table 4). Morphotype 1 definitely oc-
curs in I. breviceps,I. conybeari and I. somersetensis,
although I. anningae and I. larkini are known from
few specimens, and so this morphotype could pos-
sibly occur in those species. Combined with the pres-
ence/absence of notching, I. breviceps,I. conybeari
and I. somersetensis can be distinguished with some
degree of confidence if a specimen displays Morpho-
type 1. A hindfin without notching is more likely in I.
breviceps than the others. A notched tibia almost cer-
tainly indicates I. conybeari.AnyIchthyosaurus hind-
fin with a notched tibia can be assigned to this spe-
cies, at least based on currently known specimens. A
notched distal tarsal two, metatarsal and/or phalanges
is most likely I. somersetensis. Other features of the
skull and postcranium, however, are needed to verify
these preliminary identifications.
The occurrence of Morphotype 2, in which the bi-
furcation is in the metatarsal row, presents more un-
certainty in identifications based only on a hindfin.
If metatarsal two-b nearly separates distal tarsal two
from the astragalus, and three elements contact the
astragalus (Fig. 2d), then the specimen could be I.
larkini, the only species that has been unequivocally
identified with this morphology. However, this vari-
ation might also occur in I. somersetensis, because
some specimens with this variation cannot be assigned
to a species. I. larkini and I. somersetensis have simil-
arly shaped femora as well. If distal tarsal three has a
broad contact with the astragalus and metatarsal two-
b has only a slight, if any, contact with the astragalus
(Fig. 2c), then the specimen could be I. somerseten-
sis,I. conybeari or I. breviceps. The presence of a
notched distal tarsal two or more distal notching would
most likely be I. somersetensis; a notched tibia would
indicate I. conybeari; and a lack of notching would
most likely be I. breviceps. A hindfin in which meta-
tarsal two-b and distal tarsal three have broad, nearly
equal contacts with the astragalus (Fig. 2b) indicates
that the specimen is most likely I. communis or I. an-
ningae, in the absence of notched elements. As with
Morphotype 1, additional features of the skull or hu-
merus shape are needed to confirm these preliminary
identifications. Thus, although morphotypes are not
unique to a species, combinations of hindfin charac-
ters can suggest the most likely species assignment
for a particular hindfin morphology. However, inter-
mediate morphologies exist in the extent of contact
of the astragalus and metatarsal two-b. In these cases,
the hindfin morphology has no taxonomic utility; other
features are required even for a preliminary species
assignment.
741

J. A. MASSARE & D. R. LOMAX
Table 4. Morphologic characteristics of hindfins of species of Ichthyosaurus
Species Morphotype Notched tibia Other notching Bifur Femur shape
I. communis 2 no no 2 or 3 elongated, symmetric
I. breviceps 1 or 2 no no 1 or 2 elongated, symmetric
I. conybeari 1 or 2 usually sometimes 2 elongated, symmetric
I. anningae 2 no no 2 almost as wide as long
I. larkini 2 no yes 2 elongated, asymmetric
I. somersetensis 1 or 2 no yes 1 or 2 elongated, asymmetric
Bifur =total number of bifurcations in digit II. Fins must be complete to at least the fifth phalangeal row to assess the presence or absence of
a distal bifurcation.
This research has also shown the effect of large
sample sizes on the identification of taxonomically dis-
tinct morphological characters. Without several fairly
complete specimens of each species, the occurrence
of both morphotypes in I. somersetensis,I. breviceps
and I. conybeari could not have been recognized. It
would have appeared that the two morphotypes were
diagnostic of two different sets of species within Ich-
thyosaurus. Hindfin morphology would have appeared
much less variable and much more distinct with fewer
specimens. The presence or absence of a bifurcation in
the metatarsal row could be seen as a major difference
among species or as individual variation, depending on
how many specimens are known/examined. This raises
doubts about the validity of new taxa based on limited
material.
Gaps between morphologies are much larger when
fewer specimens are available. With only one or two
specimens, intermediate morphologies that grade from
one form to another are unknown. More specimens res-
ult in more variation, and apparent gaps between mor-
phologies are filled in, as is the case here with hind-
fin morphology. Distinct characters can be more eas-
ily defined if only one or two specimens are known
because intermediate morphologies are missing. This
raises the question of whether a particular morpho-
logy is distinct or only seems to be because of a small
sample size. Character matrices and cladistic analyses
cannot answer that question.
Cladistics can sort out many characters simultan-
eously, but variation is not always assessed with the
same level of detail as defining the character states
themselves. One individual from a new location or an
unusual horizon can be examined in such detail that
often numerous ‘unique’ features can be identified. In
fact, it is probably easier to identify such features on
a single individual than on a dozen individuals, espe-
cially considering that multiple specimens will vary in
orientation, preservation and completeness. To put it
in a different context, a new specimen could look quite
different from the type specimens of known species,
but if one examines several specimens to get a better
picture of variation, the new specimen might be within
the variation of a known species. This is especially true
of isolated elements, e.g. isolated hindfins that appear
to have unique morphologies. Character matrices that
are based solely on type specimens will inevitably jus-
tify the erection of new taxa from isolated material.
Variation must be adequately assessed by comparing
many specimens, rather than just examining type spe-
cimens or citing exclusively from the literature. Other-
wise, the decision to erect a new taxon is not substan-
tially different from what was done in the nineteenth
century, when new species were erected on the basis of
a single individual (often from a new location or ho-
rizon) if it showed a small difference in morphology
from previously known specimens.
Acknowledgements. We thank the following individuals
and their respective institutions for access to and help with
specimens: E. Daeschler and E. Gilmore (ANSP); M. Simms
(BELUM); P. Shepherd, L. Neep and S. Harris (BGS);
M. Williams (BRLSI); D. Hutchinson, I. Gladstone and
the late R. Vaughan (BRSMG); C. Hildebrandt, J. Han-
son and M. Benton (BRSUG); J. Clatworthy (BU); M. Ri-
ley and S. Finney (CAMSM); G. McGowan (CC); P. Dav-
idson (CCHC); R. Roberts (CLC); S. Bailey (DBYMU);
P. Robinson (DONMG); J. Cripps (DORCM); W. Simpson
(FMNH); N. Clark and R. Smith (GLAHM); A. Smith and
D. Rice (GLRCM); R. Neller (HEM); M. Evans (LEICT);
D. Gelsthorpe and R. Petts (MANCH); N. Owen (LMG);
S. Chapman, P. Barrett, L. Steel and R. Hansen (NHMUK);
A. Richter (NLMH); N. Monaghan (NMING); C. Howells
(NMW); A. Smith (NOTNH); E. Howlett and H. Ketchum
(OUMNH); G. Wass and J. Liston (PETMG); K. Sey-
mour (ROM); A. McLean (SHEFFM); E. Maxwell (SMNS);
D. Parsons (TTNCM); H.-D. Sues, D. Bohaska and A.
Millhouse (USNM); J. Radley and L. McCoy (WARMS);
S. Glassman, (WFIS); G. Phillips and R. Roden (WOSMG);
D. Brinkman (YPM). We also thank R. Clark, C. Berry and
the Alfred Gillett Trust, Street, for access to the Alfred Gil-
lett Collection (SOMAG). We are grateful to C. Smith and
S. Jackson (CHMUS), A. McGee (CMNH), H. Morgen-
roth (RAMM) and H. Voogd (TM) for providing images of
specimens in their respective collections that we have not
been able to examine. Finally, we greatly appreciate the re-
views provided by N. Zverkov, E. Maxwell and J. Lawrence
Wujek, which significantly improved the paper. DRL’s re-
search was covered in part by a PGR, Dean’s Doctoral Schol-
arship Award from the University of Manchester.
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