The Jurassic piranha mimic, Dapedium: why even a little reconstruction matters

Finding a fossil is one thing. 
Cleaning the fossil to reveal the specimen in all of its wonder and detail is another thing. Recontructing a tracing of the fossil to its in vivo appearance is the final necessary step. For this specimen of the Early Jurassic deep-bodied fish, Dapedium, the preliminary steps were taken by Smithwick 2015. The final step was added here.

Figure 1. The skull of Dapedium (Early Jurassic) from Smithwick 2015, plus original tracing plus DGS colors and colors restored to in vivo positions.

Figure 1. The skull of Dapedium (Early Jurassic) from Smithwick 2015, plus original tracing plus modified tracing to match photo, plus DGS colors plus colors restored to in vivo positions. Note the several bones that are homologs to the squamosal (magenta) and jugal (cyan) in tetrapods.

Earlier Dapedium was added
to the large reptile tree (LRT, 1654+ taxa), but this skull provides more data in the form of this higher resolution image.

Figure 3. Dapedium with skull bones colorized and reconstructed using DGS methods.

Figure 2. Dapedium with skull bones colorized and reconstructed using DGS methods.

Dapedium granulatus, D. caelatum (Leach 1822; Thies and Hauff 2011; Lower Jurassic; UHH 2) nests between Lepisodetes and the extant gar, Lepisosteus, in the LRT. This piranha-mimic has a deep skull and small strong jaws. The tail is just barely heterocercal (Fig. 2).


References
Leach WE 1822. Dapedium politum. P. 45 in HT de la Beche Remarks on the geology of the south coast of England, from Bridport Harbour, Dorset, to Babbacombe Bay,
Devon. Volume 1, Transactions of the Geological Society of London, Series 2. Geological Society of London, London.
Smithwick F 2015. Feeding ecology of the deep-bodied fish Dapedium (Actinopterygii, Neopterygii) from the Lower Lias (Sinemurian) of Dorset, England. Palaeontology 58(2):293–311.

wiki/Dapedium

Was Vellbergia really a juvenile basal lepidosaur? Let’s check…

Earlier we looked at tiny Vellbergia
(Sobral, Simoes and Schoch 2020; Middle Triassic) represented by a disarticulated tiny skull (Fig. 1). The large reptile tree (LRT) nested this hatchling with the much larger adult Prolacerta (Fig. 1). The MPT was 20263 steps for 1654 taxa.

The LRT nesting ran counter to the SuppData cladogram
of Sobral, Simoes and Schoch 2020, who nested Vellbergia among basal lepidosaurs, the closest of which are shown here (Fig. 1). Earlier I did not show the competing lepidosaur candidates. That was an oversight rectified today.

Figure 1. Vellbegia compared to the lepidosaurs it would nest with if Prolacerta and all Archosauromorpha were deleted.

Figure 1. Vellbegia compared to the lepidosaurs it would nest with if Prolacerta and all Archosauromorpha were deleted. Gray areas on Vellbergia indicate restored bone that is lost in the fossil.

To test the lepidosaur hypothesis of relationships,
I deleted all Archosauromorph taxa, including Prolacerta, from the LRT to see where among the Lepidosauromorpha Vellbergia would nest. With no loss of resolution, Vellbergia nested between Palaegama and Tjubina + Huehuecuetzpalli at the base of the Tritosauria plus Fraxinisaura + Lacertulus (Fig. 1) at the base of the Protosquamata. The resulting MPT was 20276 steps, only 13 more than the Prolacerta hypothesis of interrelationships.

That is a remarkably small number considering the great phylogenetic distance between these taxa in the LRT.

Rampant convergence
is readily visible among the competing taxa (Fig. 1). No wonder Prolacerta was named “before Lacerta“, the extant squamate. According to Wikipedia, “Due to its small size and lizard-like appearance, Parrington (1935) subsequently placed Prolacerta between basal younginids and modern lizards. In the 1970s (Gow 1975) the close link between Prolacerta and crown archosaurs was first hypothesized.” That was prior to cladistic software and suffered from massive taxon exclusion.

Allometry vs. Isometry
One of the lepidosaurs shown above, Huehuecuetzpalli (Fig. 1), is known from both an adult and juvenile. The older and younger specimens were originally (Reynoso 1998) considered identical in proportion. Such isometry is an ontogenetic trait shared with other tritosaur lepidosaur clade members, including pterosaurs. On the other hand, if Vellbergia was a hatchling of Prolacerta, some measure of typical archosauromorph allometry should be readily apparent… and it is… including incomplete ossification of the nasals, frontals and parietals along with a relatively larger orbit and shorter rostrum, giving Vellbergia a traditional ‘cute’ appearance appropriate for its clade.

Size
Sobral, Simoes and Schoch considered Vellbergia a juvenile, but it is similar in size to the adult lepidosaurs shown here (Fig. 1). On the other hand, Vellbergia is appropriately smaller than Prolacerta, in line with its hatchling status.

Time
Remember also that Vellbergia is from the Middle Triassic. Prolacerta is from the Early Triassic. They were not found together and some differences are to be expected just from the millions of years separating them.

For comparison: another juvenile Prolacerta,
this time from Early Triassic Antarctica (Spiekman 2018; AMNH 9520), is much larger than Vellbergia from Middle Triassic Germany (Fig. 2), but just as cute. Note the relatively larger orbit and shorter rostrum compared to the adult Prolacerta (Fig. 1), traits likewise found in Vellbergia.

Figure 2. Small Prolacerta specimen AMNH 9520 from Spiekman 2018 compared to scale with Vellbergia.

Figure 2. Small Prolacerta specimen AMNH 9520 from Spiekman 2018 compared to scale with Vellbergia. Sclerotic rings (SCL) identified by Spiekman 2018 are re-identified as pterygoids here.

Generally
crushed, disarticulated and incomplete juvenile specimens of allometric taxa are difficult to compare with adults. Even so, what is left of hatchling Vellbergia tends to resemble the larger juvenile and adult specimens of Prolacerta more than hatchling Vellbergia resembles the similarly-sized adult lepidosaurs it nests with in the absence of Prolacerta from the taxon list.

Phylogenetic analysis is an inexact science.
Nevertheless no other known method breaks down and rebuilds thousands of taxa more precisely. Only taxon exclusion appears to trip up workers at present.


References
Gow CE 1975. The morphology and relationships of Youngina capensis Broom and Prolacerta broomi Parrington. Palaeontologia Africana, 18:89-131.
Parrington FR 1935. On Prolacerta broomi gen. et sp. nov. and the origin of lizards. Annals and Magazine of Natural History 16, 197–205.
Reynoso V-H 1998. Huehuecuetzpalli mixtecus gen. et sp. nov: a basal squamate (Reptilia) from the Early Cretaceous of Tepexi de Rodríguez, Central México. Philosophical Transactions of the Royal Society, London B 353:477-500.
Sobral G, Simoes TR and Schoch RR 2020. A tiny new Middle Triassic stem-lepidosauromorph from Germany: implications fro the early evolution of lepidosauromorphs and the Vellberg fauna. Nature.com Scientific Reports 10, Article number: 2273.
Spiekman SNF 2018. A new specimen of Prolacerta broomi from the lower Fremouw Formation (Early Triassic) of Antarctica, its biogeographical implications and a taxonomic revision. Nature.com/scientificreports (2018)8:17996

wiki/Prolacerta

Another clade no longer extinct: Acanthodii (spiny sharks)

The LRT has invalidated several traditional clades
like Parareptilia, Ornithodira and Cetacea. The LRT has also resurrected and supported a few long forgotten clades like Enaliosauria and Volitantia. The LRT also recovered extant members for traditionally extinct clades like Placodermi (catfish), Desmostylia (mysticete whales) and Pareiasauria (turtles).

Key to phylogenetic analysis
is the idea that it does not matter if certain clade members lack one trait or another, or have traits shared by taxa that are not clade members. It only matters that a clade is determined by a unique suite of hundreds of traits compared to all other tested clades. That makes it important to test as many taxa as possible to minimize the possibility that any pertinent taxa are excluded and no inappropriate taxa are included. Monophyletic clade members include all descendants of a last common ancestor.

Some of the earliest known fish fossils
belong to spiny sharks (e.g. Mesacanthus, Fig. 1; clade: Acanthodii) in the Early Devonian. Other disarticulated specimens attributed to spiny sharks (scales and spines) are found in Early Silurian strata.

Instead of fin rays or lobe fins,
spiny sharks have sharp dentine spines trailing unreinforced membranes. Like sharks, acanthodian skeletons were made of cartilage because they do not preserve well. Unlike sharks, the scales were made of bone-like material. Due to taxon exclusion and the reliance on just a few traits (= Pulling a Larry Martin) acanthodians have been traditionally difficult to nest in prior phylogenetic analyses.

Figure 1. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. Distinct from other spiny sharks, this one lacks large cheek plates, as in the extant Notopterus (Fig. 3).

Figure 1. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. Distinct from other spiny sharks, this one lacks large cheek plates, as in the extant Notopterus (Fig. 3).

With the addition of the spiny shark Mesacanthus
(Early Devonian, Fig. 1) the large reptile tree (LRT, 1654+ taxa; subset Fig. 2) nests spiny sharks between anchovies, like the extant Engraulis, and palaeoniscids like Pteronisculus (Early Triassic).

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

Spiny sharks are also basal
to Perleidus (Triassic) and Notopterus (extant; Fig. 3) among tested taxa in the LRT.

Figure 3. The extant knife fish or featherback, Notopterus, is an extant descendant of spiny sharks in the LRT.

Figure 3. The extant knife fish or featherback, Notopterus, is an extant descendant of spiny sharks in the LRT. Note the spiny pectoral fins and similar skull morphology.

According to Wikipedia:
“Burrow et al. 2016 provides vindication by finding chondrichthyans to be nested among Acanthodii, most closely related to Doliodus and Tamiobatis. A 2017 study of Doliodus morphology points out that it appears to display a mosaic of shark and acanthodian features, making it a transitional fossil and further reinforcing this idea.”

That’s only true due to taxon exclusion,
according to the LRT (Fig. 2) where Doliodus (Fig. 4) is closer to Akmonistion and Xenacanthus.

Figure 1. Doliodus skull and pectoral region with lateral reconstruction at right. Note the narrow pectoral region relative to the wide spread occiput. Apparently this fish had a narrower body than head.

Figure 4. Doliodus skull and pectoral region with lateral reconstruction at right. Note the narrow pectoral region relative to the wide spread occiput. Apparently this fish had a narrower body than head.

Does a spiny shark have to have spiny fins?
No. Several taxa unrelated to spiny sharks, like catfish, have more or less spiny fins and sometimes these give rise to ray fins, as they do in Notopterus (Fig. 3) for the pectoral fins, but not the pelvic fins.

Taxa in the LRT are nested at nodes based on hundreds of traits
more closely shared with sister and cousin taxa than with more distantly related taxa in toto.

Key to scoring included taxa
is the creation of a reconstruction (Fig. 1) that moves disarticulated bones back to their in vivo positions. That simply must be done to understand the data.

Mesacanthus mitchelli (renamed with Traquair 1888; Early Devonian, 410 mya; 3cm) is a basal acanthodian transitional to Notopterus and Perleidus. So spiny sharks are not extinct. Distinct from other tested spiny sharks, Mesacanthus has open cheeks exposing the quadrate and hyomandibular. Soft tissue preserves membranes posterior to all the spines.


References
Baron MG 2015. An investigation of the genus Mesacanthus (Chordata: Acanthodii) from the Orcadian Basin and Midland Valley areas of Northern and Central Scotland using traditional morphometrics. PeerJ. 3: e1331. doi:10.7717/peerj.1331
Burrow C; den Blaauwen J; Newman M and Davidson R 2016. “The diplacanthid fishes (Acanthodii, Diplacanthiformes, Diplacanthidae) from the Middle Devonian of Scotland”. Palaeontologia Electronica 19 (1): Article number 19.1.10A.
Traquair RH 1888. Notes on the nomenclature of the Fishes of the Old Red Sandstone of Great Britain. Geol. Magazine (3)5:507–517.

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/osteoglossiformes

 

‘The Tree of Life is Messed Up’ ~ YouTube video

The SciShow
produced a YouTube video (below) that discusses the problem with the Linnean (1758, 1766) method of taxonomy (naming clades) that included nested ranks such as phyla, class, order, genus and species. SciShow also described the problems (quirks) inherent in describing groups based on shared characteristics vs the last common ancestor method of assessing monophyly vs DNA results.

DNA results produce false positives
over deep time (not always, but often enough) compared to phenomic (trait-based) studies.

The last common ancestor method
trumps the shared characters method because convergence is rampant in Vertebrata. And no one wants to ‘Pull a Larry Martin.’

A wide gamut online phylogenetic analysis
of vertebrates has been building (1654+ taxa at present) over the past nine years at 
the large reptile tree. It includes extinct and extant taxa from Cambrian lancelets to modern humans enabling one to trace the ancestry of every included taxa and to list all the tested members of monophyletic clades.

This will become important in the next post
as the LRT resurrects yet another clade long thought to be extinct. Extant clade members don’t have some of the ‘key’ characters that characterize this clade. That doesn’t matter because in phylogenetic analysis there are no ‘key’ characters.

Tristychius: basal to angel sharks, a Carboniferous suction-feeder

Figure 1. CT scans of Tristychius skull from Coates et al. 2019.

Figure 1. CT scans of Tristychius skull from Coates et al. 2019.

Tristychius arcuatus (Agassiz 1837; Early Carboniferous; 60cm est.; Figs. 1, 2) was a small ancestor to angel sharks (Fig. 2) and a sister to dogfish with a short torso, large pectoral and pelvic fins and large dorsal spines. The nares pointed anteriorly, as in Megachasma. Teeth are nearly absent with only a few in the anterior dentary. The postorbital is absent in other taxa, but strongly developed here. Tabulars are absent. Note the low position of the gill slits. Note the large anterior gill bars (= labial cartilages) that restrict jaw depression and create lateral walls for the open jaws.

Figure 1. Tristychius, a basal shark from the Early Carboniferous, compared to descendant taxa, two angel sharks.

Figure 2. Tristychius, a basal shark from the Early Carboniferous, compared to extant descendant taxa, the wobbegong shark and the angel shark.

Coates et al. 2019 re-described this taxon
with regard to the evolution of suction feeding 50 million years before the bony fish equivalent. They considered Tristychius a hybodont shark, like Hybodus, but the large reptile tree (LRT; 1653+ taxa) nests it basal to angel sharks, like Squatina. The robust dorsal fin spines that both taxa have are therefore convergent.

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

References
Agassiz L 1837. Recherches Sur Les Poissons Fossiles. Tome III (livr. 8-9). Imprimérie de Petitpierre, Neuchatel viii-72
Coates MI, Tletjen K, Olsen AM and Finarelli JA 2019. High performance suction feeding in an early elasmobranch. Science Advances 2019:5: eaax2742.

wiki/Squatina
wiki/Smoothback Angelshark
wiki/Tristychius

Rhombichthys: a Late Cretaceous tarpon anglefish-mimic

Here’s an odd little fish with long sternal scutes
phylogenetically misplaced by the original authors according to the large reptile tree (LRT, 1652+ taxa).

Figure 1. Rhombichthys, a tiny Late Cretaceous tarpon with deep scutes creating a sternum.

Figure 1. Rhombichthys, a tiny Late Cretaceous tarpon with deep scutes creating a sternum. Semi-hidden pelvic fins are in blue here. Orange dot marks the cloaca.

Rhombichthys intoccabilis (Khalloufi, Zaragüeta-Bagils and Le, Cenomian, Late Cretaceous; 25cm) was described as a clupeomorph (= anchovies and kin) with a deep, scute-covered belly, but here nests with the extant tarpon and, more distantly, the extant piranha (above). Juveniles are also known and they lack deep scutes and high dorsal fins.

Figure 2. Rhombichthys skull in situ, as originally traced, as originally reconstructed and as traced here using DGS methods. See figure 1 for a reconstruction with bones replace to their in vivo positions.

Figure 2. Rhombichthys skull in situ, as originally traced, as originally reconstructed and as traced here using DGS methods. See figure 1 for a reconstruction with bones replace to their in vivo positions.

Angelfish are superficially similar,
but angelfish have a large anal fin, no sternal scutes.

Figure 2. Tarpon (Megalops) skull with tetrapod skull colors added.

Figure 3. Tarpon (Megalops) skull with tetrapod skull colors added.

Tarpons also have a large anal fin (Fig. 4).
By contrast, the anal fin of Rhombichthys is a vestige. Sea horses and pipefish have a similar upturned mouth by convergence.

Figure 1. Tarpon (Megalops) skeleton.

Figure 4. Tarpon (Megalops) skeleton is robust, capable of anchoring large, strong swimming muscles.

Which came first?
The tarpon body plan has more primitive traits. Tarpon-like fish are known from the Niobrara Sea (early Late Cretaceous). The last common ancestor of Rhombichthys and tarpons likely goes back to the Devonian based on their location in the LRT.

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.


References
Khalloufi B, Zaragüeta-Bagils R and Lelièvre H 2010. Rhombichthys intoccabilis, gen. et sp. nov. (Ellimmichthyiformes, Clupeomorpha, Teleostei), from the Cenomanian (Upper Cretaceous) of Ein Yabrud, Middle East: anatomial descriptions and phylogenetic implications. Journal of Vertebrate Paleontology. 30 (1): 57–67.

wikii/Rhombichthys

The preoperculum and the ‘preoperculum’ are not homologous

Added Feb. 23, 2020:
A suggestion for the second ‘preoperculum’ that does not contact the hyomandibular and quadrate: Call it what it is: a post squamosal.

According to Wikipedia,
The preoperculum is a crescent-shaped structure that has a series of ridges directed posterodorsally to the organisms canal pores. The preoperculum can be located through an exposed condyle that is present immediately under its ventral margin; it also borders the operculum, suboperculum, and interoperculum posteriorly.”

Figure 4. Basal to the four-eyed fish, sea robin and gurnard in the LRT is this taxon, Seriola sonata, the banded rudder fish. Note the lack of cheek bones and the tentative appearance of the palatine outside of the palate.

Figure 1. Seriola sonata, the banded rudder fish has a typical preoperculum for a teleost fish. The squamosal and quadratojugal are not present here.

Figure 2. Eusthenopteron skull showing some changes from the Cheirolepis skull.

Figure 2. Eusthenopteron skull showing some changes from the Cheirolepis skull.

The wiki-authors are referring to the majority
of preoperculum appearances in vertebrates (Fig.1 ), ignoring the others (Figs. 2-3).

Figure 3. Subset of the LRT focusing on basal vertebrates and highlighting the appearance of the preoperculum in various ways. Note the reappearance of the preoperculum in the tetrapods within the  Ossinodus clade.

Figure 3. Subset of the LRT focusing on basal vertebrates and highlighting the appearance of the preoperculum in various ways. Note the reappearance of the preoperculum in the tetrapods within the  Ossinodus clade.

A preoperculum can also appear
as a bone posteriorly bordering the squamosal and quadratojugal (Figs. 2-4). Intervening taxa have no preoperculum, so the various appearances of this bones are not homologous, but keep returning as reversals. Note the appearance of a preoperculum in most extant fish and in unrelated catfish, too.

Figure 9. Ossinodus is a close relative of Trimerorhachis in the LRT.

Figure 4. Ossinodus is a close relative of Trimerorhachis in the LRT.

Among tetrapods
the preoperculum makes a brief reappearance in the Ossinodus (Fig. 4) clade.

An angelfish-mimic salmon sister from the late Cretaceous

Cawley and Kriwet 2019 describe Flagellipinna rhomboides,
“a pycnodontiform fish, Flagellipinna rhomboides, gen. et sp. nov., from this locality based on four specimens. It is considered a member of the derived family Pycnodontidae due to the presence of a post parietal process. This taxon is distinct from other pycnodontids due to its diamond-shaped body, whip-like dorsal fin, postcloacal scales with forward-pointing spines, and acute anterior profile with a concave slope, giving it a ‘hunchback’ appearance. The prognathous snout armed with molariform teeth suggests that this pycnodont preyed on a variety of shelled animals from crevices. The prognathous snout armed with molariform teeth suggests that this pycnodont preyed on a variety of shelled animals from crevices.”

Figure 1. One of four specimens of Flagellipinna from Cawley and xx, 2019.

Figure 1. One of four specimens of Flagellipinna from Cawley and Kriwet 2019.

From the introduction:
“Pycnodontiformes, or pycnodonts, were a successful order of neopterygian fishes with a rich fossil record that ranged from the Late Triassic to the Eocene.”

Figure 2. Flagellipinna skull in situ, original tracing, DGS colors applied and reconstruction. Small skull is Salmo the salmon, not to scale.

Figure 2. Flagellipinna skull in situ, original tracing, DGS colors applied and reconstruction. Small skull is Salmo the salmon, not to scale.

In the large reptile tree (LRT, 1652 taxa) Flagellipinna nests closely with Salmo, the extant salmon (Fig. 2), a basal teleost with sharp teeth. Salmo and kin were not mentioned in the text, but the authors’ reference list includes four citations related to salmonidae. This hypothesis of interrelationships may be novel. If you run across an earlier reference, please let me know so I can promote that citation.


References
Cawley JJ and Kriwet J 2019. A new genus and species of pycnodontid fish Flagellipinna rhomboides, gen. et sp. nov. (Neopterygii, Pycnodontiformes), from the Upper Cretaceous (Cenomanian) of Lebanon, with notes on juvenile form and ecology. Journal of Vertebrate Paleontology 39(2): e1614012

wiki/Flagellipinna (not listed yet)

Tiny Vellbergia: a juvenile Prolacerta, not a stem-lepidosauromorph

Sobral, Simoes and Schoch 2020
report on a new, tiny, Middle Triassic reptile, Vellbergia bartholomei (Figs. 1, 3) known from disarticulated skull material. Without creating a reconstruction, they considered Vellbergia a stem-lepidosauromorph different from other stem-lepidosauromorphs.

Figure 1. Vellbergia in situ, original line drawing, DGS colors apple and reconstruction. Note the quadrate break occurs during taphonomic crushing of the curved bone. Scale bar = 5mm. So the skull is about 1.5cm in length, quite tiny.

Figure 1. Vellbergia in situ, original line drawing, DGS colors apple and reconstruction. Note the quadrate break occurs during taphonomic crushing of the curved bone. The nasal, frontal and parietals are incomplete due to their juvenile state. Scale bar = 5mm. So the skull is about 1.5cm in length, quite tiny.

The authors report:
“Important morphological attributes of Vellbergia, most notably the elongate and slender jaw bones, the deeper post-dentary region of the jaw relative to the anterior region, and the far posteriorly reaching maxillary tooth row can be found on some other early diverging diapsid species, such as Prolacerta and Youngina, thus showing these features were retained into the early part of the lepidosauromorph evolutionary history as well.”

Prolacerta.

Figure 2. Prolacerta. Note the relative lengths of the manual and pedal lateral digits.

After phylogenetic analysis
in the large reptile tree (LRT, 1653+ taxa) Vellbergia nests with Prolacerta (Figs. 2, 3). A reconstruction (Figs. 1, 3) demonstrates a close affinity. Based on size and the smooth, incomplete, open sutures of the specimen, this is a juvenile. So the genus ‘Vellbergia’ is a junior synonym. The authors did not include Prolacerta in their published cladogram, but did list it in their suppdata.

Figure 3. Prolacerta and 'Vellbergia' to scale.

Figure 3. Prolacerta and ‘Vellbergia’ to scale.

Taxon exclusion
The Sobral, Simoes and Schoch taxon list did not include enough taxa to produce the basal dichotomy splitting Archosauromorpha from Lepidosauromorpha in the Viséan following their last common ancestor, Silvanerpeton. Prolacertiformes (= Protorosauria) nest in the Archosauromorpha and converge with Lepidosauriformes in many ways, hence the traditional confusion.

The LRT is available online.
Problems like this can be avoided by using ReptileEvolution.com and the LRT to double-check work before submission.


References
Sobral G, Simoes TR and Schoch RR 2020.
A tiny new Middle Triassic stem-lepidosauromorph from Germany: implications fro the early evolution of lepidosauromorphs and the Vellberg fauna. Nature.com Scientific Reports 10, Article number: 2273.

https://doi.org/10.1038/s41598-020-58883-x
https://www.nature.com/articles/s41598-020-58883-x

 

 

Reassessment of Scleromochlus: Bennett 2020

SC Bennett 2020
followed Benton 1999 and others (citations below) in giving us a closer look at Scleromochlus taylori (Woodward 1907; Late Carnian, Late Triassic ~217 mya, 18 cm long; Figs. 1, 2), a tiny biped crocodylomorph derived from a sister to Gracilisuchus and Saltopus according to the large reptile tree (LRT, 1650+ taxa; Fig. 3).

Bipeds of the Triassic

Figure 1. Bipeds of the Triassic. Top to bottom: Cosesaurus, Scleromochlus, Marasuchus and Tropidosuchus. Each represents a distinct lineage of bipeds with bipedal sister taxa. This version of Scleromochlus was published in Peters 2002, based on Benton 1999.

Unfortunately,
despite the firsthand examination of this taxon, Bennett ignored sister taxa recovered by the LRT (Figs. 3, 4). His cladograms failed to recover a single node on which to nest Scleromochlus. In essence, he still doesn’t know what Scleromochlus is, despite his best efforts (see below for Bennett’s self assessment).

From Bennett’s 2020 introduction
“The first specimens were briefly described and named by Woodward (1907), who interpreted Scleromochlus as a small bipedal running or leaping dinosaur. Huene (1914) described the specimens more thoroughly and interpreted Scleromochlus as an arboreal climbing and leaping pseudosuchian close to the origin of pterosaurs. Swinton (1960), Brodkorb (1971) and Martin (1983) discussed Scleromochlus in relation to the origin of birds, whereas Padian (1984) suggested that Huene had it only half right and interpreted Scleromochlus as a digitigrade bipedal cursor close to the origin of pterosaurs and dinosaurs, a view that has gained general acceptance (Gauthier, 1986; Sereno, 1991; Benton, 1999; Fraser, 2006; Brusatte et al., 2010). Despite that, Bennett (1996, 1997) argued that Huene had only the other half right and Padian had it all wrong and that Scleromochlus was an arboreal leaper not close to pterosaurs.”

True to Bennett’s curse,
“You will never be published, and if you are published, you will not be cited,” Bennett 2020 did not cite Peters 2002, who wrote, “Among recent workers, Padian (1984), Sereno (1991) and Benton (1999) noted pterosaur similarities in the bipedal diapsid, Scleromochlus. The homoplasy is striking (Table I). However, figures by Benton (1999), which are reconstructed here (Fig. 8D), show that this archosauriform had a low, wide skull, a deep antorbital fossa, a terminal naris, a short neck of only six or seven cervicals, a long lumbar region, a small manus, a broadly separated pubis and ischium, a fibular flange, a calcaneal heel and a spike-like, digit-less metatarsal V. These characters are not found in pterosaurs. They are synapomorphies of basal bipedal crocodylomorphs, such as Gracilisuchus (Romer, 1972) and Saltoposuchus (Huene, 1921; Sereno and Wild, 1992).”

Bennett 2020 failed to mention
or include Junggarsuchus, Pseudhesperosuchus, Gracilisuchus and Saltposuchus in his taxon list. He only mentioned Saltopus as a coeval predator. These are all bipedal basal crocodylomorpha, a clade  ignored by Bennett 2020.

Figure 2. From Bennett 2020 showing in dorsal view the skull of Scleromochlus with DGS overlays colorizing the bones. At right, Bennett's drawing of same.

Figure 2. From Bennett 2020 showing in dorsal view the skull of Scleromochlus with DGS overlays colorizing the bones. At right, Bennett’s drawing of same. A compression crack across the fragile frontal was identified as the only suture in the skull, between the nasal and frontal, by Bennett 2020.

Back to Bennett’s 2020 introduction
“In 2013 I came to suspect that Bennett (1997), too, had it at least half wrong. By happy coincidence, I had shortly before perfected my technique for studying small slab specimens, so I took another look at the evidence and after several years of study gained some confidence in interpreting the specimens. This article is not a thorough redescription of the osteology of Scleromochlus but rather is a reassessment of the osteological evidence that has been used to interpret Scleromochlus’s mode of life, locomotion, and phylogenetic relationships.”

Again, the major shortcoming
in Bennett’s phylogenetic analysis is taxon exclusion. And Bennett’s “perfected technique” is not perfect (Fig. 2). His outmoded freehand technique overlooks many bones and sutures.

Figure 1. Subset of the LRT focusing on the Crocodylomorpha, dorsal scutes, elongate proximal carpals, bipedality and clades.

Figure 1. Subset of the LRT focusing on the Crocodylomorpha, dorsal scutes, elongate proximal carpals, bipedality and clades.

Concluding Bennett’s 2020 introduction
“A principal component analysis of skeletal measurements of Scleromochlus and other vertebrates of known locomotor type was done to examine the locomotion of Scleromochlus, and it was found to plot with frogs. Based on osteological evidence, including previously overlooked evidence from the specimens, and the principal component analysis, Scleromochlus is interpreted as a sprawling quadrupedal hopper analogous to frogs. Phylogenetic analyses found that Scleromochlus was not an ornithodiran, but rather either within the Doswelliidae or outside the clade consisting of the most recent common ancestor of the Erythrosuchidae and Archosauria and all its descendants.”

Pretty vague…
If the best Bennett can do is nest tiny bipedal Scleromochlus with giant, quadrupedal  Doswellia OR Erythrosuchus, then Bennett should have added taxa to his cladogram. If Bennett would have just added archosaur taxa that were small, bipedal and with flat skulls and osteoderms, he would have nailed it.

Other than the proportions, size, skeletal details and osteoderms
of Scleromochlus, the anterior lean of the long quadrate is also a crocodylomorph trait overlooked by all prior workers, except Peters 2002 (Figs. 1,2). Bennett traced the quadrate in stereo, but identified it with a question mark (Fig. 2). Whenever that happens, the technique has not been, as Bennet reported, “perfected.’


References
Bennett SC 1996. The phylogenetic position of the Pterosauria within the Archosauromorpha. Zoolological Journal of the Linnean Society 118: 261–308.
Bennett SC 1997. The arboreal leaping theory of the origin of pterosaur flight. Historical Biology 12(3–4):265–290
Bennett SC 2020. Reassessment of the Triassic archosauriform Scleromochlus taylori: neither runner nor biped, but hopper. PeerJ 8:e8418 DOI 10.7717/peerj.8418
Benton MJ 1999. Scleromochlus taylori and the origin of the pterosaurs. Philosophical Transactions of the Royal Society London, Series B 354 1423-1446. Online pdf
Clark JM 2011. A new shartegosuchid crocodyliform from the Upper Jurassic Morrison Formation of western Colorado. Zoological Journal of the Linnean Society. 163 (s1): S152–S172.
Hone DWE and Benton MJ 2007. An evaluation of the phylogenetic relationships of the pterosaurs to the archosauromorph reptiles. Journal of Systematic Palaeontology 5:465–469.
Hone DWE and Benton MJ 2008. Contrasting supertree and total evidence methods: the origin of the pterosaurs. Zitteliana B28:35–60.
Padian K. 1984. The Origin of Pterosaurs. Proceedings, Third Symposium on Mesozoic Terrestrial Ecosystems, Tubingen 1984. Online pdf
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. Hist Bio 15: 277–301.
Senter P 2003. Taxon Sampling Artifacts and the Phylogenetic Position of Aves. PhD dissertation. Northern Illinois University, 1-279.
Sereno PC 1991. Basal archosaurs: phylogenetic relationships and functional implications. Journal of Vertebrate Paleontology 11 (Supplement) Memoire 2: 1–53.
Woodward AS 1907. On a new dinosaurian reptile (Scleromochlus taylori, gen. et sp. nov.) from the Trias of Lossiemouth, Elgin. Quarterly Journal of the Geological Society 1907 63:140-144.

wiki/Scleromochlus