Sawskates (Rajiformes, Sclerorhynchoidei) and the concept of pristification

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Sawskates (Rajiformes, †Sclerorhynchoidei)
and the concept of pristification
Tyler Greenfield
tgreenfield999@gmail.com @TylerGreenfieId incertaesedisblog.wordpress.com

What is a saw?
●In anatomical terms: a saw is an
elongated, dorsoventrally-compressed
rostrum with lateral rows of enlarged
denticles.
●In simplified terms: a saw is a long,
flattened snout with rows of large scales
on both sides.
●The rostral denticles are modified dermal
denticles (‘scales’) and not oral teeth.
●Three groups of cartilaginous fishes
evolved a saw: sawfishes (Pristoidei),
sawsharks (Pristiophoriformes), and
sawskates (†Sclerorhynchoidei) (Fig. 1).
Figure 1. A comparison between the saws of the
three groups that evolved them. Pristiophorus and
Pristis are redrawn from Lange et al. (2015).

What are the functions of a saw?
●In sawfishes and sawsharks, the saw is
used for both sensing and killing prey.
●The saw contains hundreds of
electroreceptors (ampullae of Lorenzini)
that detect the electrical fields emitted
by prey.
●The saw can be swiftly swiped from side
to side to cut or impale prey with the
denticles (Fig. 2).
●It can also be used to pin prey to the
substrate and move them towards the
mouth.
●It likely functioned the same in sawskates.
Figure 2. A largetooth sawfish bisecting a prey fish
with its saw. Screenshot from one of the
supplementary videos in Wueringer et al. (2012).

What are sawskates?
●Sclerorhynchoidei is an extinct suborder
of saw-bearing fishes that lived worldwide
during the Cretaceous (130–66 Ma).
●They were formerly considered sawfishes,
but are now known to be closest to skates
(suborder Rajoidei) (Fig. 4).
●Sclerorhynchoids were the first group to
evolve a saw, appearing before the
earliest sawsharks (~90 Ma) and
sawfishes (~60 Ma).
●They were also the most diverse, with five
families and over 20 genera (Fig. 5).
Figure 3. The head and saw of the sclerorhynchoid
Libanopristis hiram, from Wikimedia Commons
(Hectonichus, CC BY-SA 3.0).

What are sawskates? (cont.)
●Sclerorhynchoids are still often called
‘sawfishes’, but are distinguished from
them by several characteristics.
●I proposed the more accurate common
name ‘sawskates’ to refer to them.
●This name acknowledges their
phylogenetic relationship to skates.
●It is important for science communication
to not use common names that are
misleading about phylogeny.
Figure 3. The head and saw of the sclerorhynchoid
Libanopristis hiram, from Wikimedia Commons
(Hectonichus, CC BY-SA 3.0).

Figure 4.
A phylogenetic tree of sharks and
rays with highlighted clades that
convergently evolved saws. The
numbers indicate the order in
which these clades appeared in the
fossil record. The topology of
Batomorphii is based on
Villalobos-Segura et al. (2019;
2021) and the topology of Selachii
is based on Pavan-Kumar et al.
(2020). The sawshark and sawfish
silhouettes are modified from
Compagno (1984) and Ebert &
Stehmann (2013), respectively.

Figure 5.
A phylogenetic tree of
Sclerorhynchoidei with rostral
denticles from the type genus of
each family. Ptychotrygonids seem
to have lacked rostral denticles.
The topology is based on
Villalobos-Segura et al. (2021) and
the family taxonomy is based on
Greenfield (2021). The denticles of
Schizorhiza, Ischyrhiza, and
Onchopristis are redrawn from
Cappetta (2012) and
Sclerorhynchus is redrawn from
Arambourg (1940).

Rostral denticles
●In sawskates, the rostral denticles
attached directly to the edges of the
rostrum, superficially similar to
sawsharks.
●The bases of the denticles were probably
covered in skin like those of sawsharks.
●The denticles were replaced when lost,
resulting in extreme rostral asymmetry in
some genera (Onchopristis, Figs. 6 & 11).
●In sawfishes, the denticles are embedded
in alveoli in the rostrum and are usually
not replaced.
Figure 6. The head and saw of Onchopristis numida,
from Villalobos-Segura et al. (2021).

Lateral cephalic denticles
●Sawskates had additional rows of
enlarged denticles along the sides of the
head at the base of the rostrum (Fig. 7).
●Lateral cephalic denticles are also
convergently present in sawsharks.
●Their function is unknown, but they may
have been defensive.
●Sawfishes do not have lateral cephalic
denticles.
Figure 7. The lateral cephalic denticles of
Sclerorhynchus atavus, from Underwood et al.
(2016).

Dermal denticles
●Sawskates had at least two different types
of enlarged dermal denticles on their
bodies.
●The ‘thorn’ type is known from Ischyrhiza
and Onchopristis, while the ‘prickle’ type
is known from Sclerorhynchus (Fig. 8).
●Both types are also found in extant skates
and are used for defense.
●Their exact arrangement is unknown, but
they were likely situated in longitudinal
rows like in skates.
●Sawfishes have no enlarged dermal
denticles.
Figure 8. Dermal denticles from sclerorhynchoids.
Ischyrhiza from Sternes & Shimada (2019),
Onchopristis from Villalobos-Segura et al. (2021),
and Sclerorhynchus from Woodward (1889).

Fins
●In sawskates, the pectoral and pelvic fins
were adjoined (Fig. 9).
●Both dorsal fins were located behind the
pelvic fins.
●The caudal fin was reduced and possibly
absent in some species.
●In sawfishes, there is a sizable gap
between the pectoral and pelvic fins.
●The first dorsal is located either in front of
or over the pelvic fins, while the second
dorsal is located behind the pelvic fins.
●The caudal fin is well-developed.
Figure 9. Complete specimens of Sclerorhynchus
atavus, from the MNHN collections database
(Jocelyn Falconnet, CC BY-SA 4.0).

Ecology
●Based on the arrangement and sizes of
their fins, sawskates were more benthic
than sawfishes or sawsharks.
●They may have been ambush predators
similar to wobbegongs and angelsharks
(Fig. 10).
●Prey could be dispatched with a burst of
speed and slashing with the saw.
●The diversity of oral teeth suggests a
variety of diets, ranging from durophagy
to piscivory.
●Their colorations were probably spotted
or mottled for camouflage.
Figure 10. A Japanese angelshark striking out from
the seafloor, from Wikimedia Commons (Ryo Sato,
CC BY-SA 2.0).

Figure 11.
My reconstruction of Onchopristis numida, in particular IPUW 353500 described by
Villalobos-Segura et al. (2021). Missing, distorted, or displaced elements were
restored after specimens of Onchopristis numida, Libanopristis hiram, and
Sclerorhynchus atavus described by Stromer (1925), Cappetta (1980), and
Underwood et al. (2016).

The concept of pristification
●Etymology: Ancient Greek pristis (πρίστις, ‘saw’/‘sawyer’) + Latin ficatio (‘becoming’/‘making’)
●Definition: Pristification is the repeated, convergent evolution of the saw in cartilaginous fishes.
●Other forms: pristify, pristified
●The structure of this term is based on others used for examples of convergent evolution (e.g.,
‘carcinization’ for crabs).
●Unlike other examples, it refers to the convergent evolution of a structure and not a bauplan.
●Previously the ‘pristobenthic ecomorphotype’ was proposed, but this is erroneous since it implies
that all saw-bearing fishes have the same bauplan.
●The bauplans of sawfishes and sawsharks are similarly shark-like, but sawskates have a bauplan
more like other rays.

The concept of pristification (cont.)
●Pristification is intended to be useful shorthand for both scientific literature and science
communication.
●Using a single term, rather than a complicated phrase, makes the concept more accessible to
laypeople.
●Saw-bearing fishes are an important example for teaching about convergent evolution and the
ecological pressures that cause it.
●Pristification recognizes both the similarities and distinctions between sawfishes, sawsharks, and
sawskates.

Additional candidates
●The Carboniferous ctenacanth shark
Bandringa had an elongated rostrum and
lateral cephalic denticles (Fig. 12).
●It lacked rostral denticles but an unknown
relative could have evolved them.
●This hypothetical pristified bandringid
would be the first example outside of
Neoselachii (crown group sharks + rays).
●Schizorhiza is usually considered a
sclerorhynchoid, but it might represent a
distinct clade of saw-bearing rays.
●This was suggested because of its unique
rostral denticle replacement pattern.
Figure 12. A reconstruction of Bandringa rayi and its
lateral cephalic denticles, modified from Sallan &
Coates (2014).

References
●Arambourg, C. (1940). Le groupe des Ganopristinés. Bulletin de la Société Géologique de France, 5e Série, 10, 127–147.
https://doi.org/10.2113/gssgfbull.S5-X.3-6.127
●Cappetta, H. (1980). Les Sélaciens du Crétacé Supérieur du Liban. II: Batoïdes. Palaeontographica, Abteilung A, 168(5–6),
149–229.
●Cappetta, H. (2012). Handbook of Paleoichthyology. Volume 3E. Chondrichthyes. Mesozoic and Cenozoic Elasmobranchii: Teeth.
Verlag Dr. Friedrich Pfeil.
●Compagno, L.J.V. (1984). FAO Species Catalogue. Vol. 4. Sharks of the World. An Annotated and Illustrated Catalogue of Shark
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the North Atlantic. Food and Agriculture Organization of the United Nations.
●Greenfield, T. (2021). Corrections to the nomenclature of sawskates. Bionomina, 22(1), 39–41.
https://doi.org/10.11646/bionomina.22.1.3
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145–160.
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References (cont.)
●Sternes, P.C., & Shimada, K. (2019). Paleobiology of the Late Cretaceous sclerorhynchid sawfish, Ischyrhiza mira
(Elasmobranchii: Rajiformes), from North America based on new anatomical data. Historical Biology, 31(10), 1323–1340.
https://doi.org/10.1080/08912963.2018.1452205
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Skelettrest des Pristiden Onchopristis numidus Haug sp. Abhandlungen der Bayerischen Akademie der Wissenschaften,
Mathematisch-Naturwissenschaftliche Abteilung, 30(6), 1–22.
●Underwood, C.J., Smith, M.M., & Johanson, S. (2016). Sclerorhynchus atavus and the convergent evolution of rostrum-bearing
chondrichthyans. Geological Society, London, Special Publications, 430, 129–136. https://doi.org/10.1144/SP430.7
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sclerorhynchid sawfish: Asflapristis cristadentis gen. et sp. nov., and implications for the phylogenetic relations of the
Sclerorhynchoidei (Chondrichthyes). Journal of Systematic Palaeontology, 17(21), 1847–1870.
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