Resurrecting extinct taxa: Creodonta, Mesonychidae, Desmostylia and Gephyrostegidae

Taxonomy
“the branch of science concerned with classification, especially of organisms; systematics.”  Taxon: a taxonomic group of any rank, such as a species, family, or class.

The large reptile tree
(LRT, 1366 taxa) has resurrected several taxa (in this case, clades) long thought to be extinct.

Figure 1. Adding Sinopa to the LRT nests it here, between the extant quoll (Dasyurus) and the extant Tasmanian devil (Sarcophilus).

Figure 1. Members of the traditionally extinct Creodonta include the extant quoll (Dasyurus) and the extant Tasmanian devil (Sarcophilus).

Creodonta
According to Wikipedia: “Creodonta” was coined by Edward Drinker Cope in 1875. Cope included the oxyaenids and the viverravid Didymictis but omitted the hyaenodontids. In 1880. he expanded the term to include MiacidaeArctocyonidaeLeptictidae (now Pseudorhyncocyonidae), OxyaenidaeAmbloctonidae and Mesonychidae. Cope originally placed creodonts within the Insectivora. In 1884, however, he regarded them as a basal group from which both carnivorans and insectivorans arose. Hyaenodontidae was not included among the creodonts until 1909. Over time, various groups were removed, and by 1969 it contained, as it does today, only the oxyaenids and the hyaenodontids.

In the LRT, Oxyaena and Hyaenodon are members of an extinct clade. However, Sinopa is considered a hyaenodontid, and it nests between the extant quoll (genus: Dasyurus) and the extant Tasmanian devil (genus: Sarcophilus). Sarkastodon is considered an oxyaenid and it nests as a sister to Sarcophilus. So… either the quoll and Tasmanian devil are living members of the Creodonta, or we’ll have to redefine the Creodonta.

Figure 1. Rorqual evolution from desmostylians, Neoparadoxia, the RBCM specimen of Behemotops, Miocaperea, Eschrichtius and Cetotherium, not to scale.

Figure 1. Rorqual evolution from desmostylians, Neoparadoxia, the RBCM specimen of Behemotops, Miocaperea, Eschrichtius and Cetotherium, not to scale.

Desmostylia
According to Wikipedia: “Desmostylians are the only known extinct order of marine mammals. The Desmostylia, together with Sirenia and Proboscidea (and possibly Embrithopoda), have traditionally been assigned to the afrotherian clade Tethytheria, a group named after the paleoocean Tethys around which they originally evolved. The assignment of Desmostylia to Afrotheria has always been problematic from a biogeographic standpoint, given that Africa was the locus of the early evolution of the Afrotheria while the Desmostylia have only been found along the Pacific Rim. That assignment has been seriously undermined by a 2014 cladistic analysis that places anthracobunids and desmostylians, two major groups of putative non-African afrotheres, close to each other within the laurasiatherian order Perissodactyla.”

In the LRT, desmostylians are indeed derived from anthracobunids, which, in turn, are derived from hippos and mesonychids. Mysticeti, the clade of baleen whales are derived from desmostylians. So… baleen whales are extant desmostylians.

Figure 3. Four mesonychids to scale. Here Mesonyx, Anthracobune, Paleoparadoxia and Hippopotamus are compared.

Figure 3. Four mesonychids to scale. Here Mesonyx, Anthracobune, Paleoparadoxia and Hippopotamus are compared.

Mesonychidae
According to Wikipedia, “Mesonychidae is an extinct family of small to large-sized omnivorouscarnivorous mammals closely related to cetartiodactyls (even-toed ungulates & cetaceans) which were endemic to North America and Eurasia during the Early Paleocene to the Early Oligocene. The mesonychids were an unusual group of condylarths with a specialized dentition featuring tri-cuspid upper molars and high-crowned lower molars with shearing surfaces. They were once viewed as primitive carnivores, like the Paleocene family Arctocyonidae, and their diet probably included meat and fish. In contrast to this other family of early mammals, the mesonychids had only four digits furnished with hooves supported by narrow fissured end phalanges.”

In the LRT, mesonychids include hippos and baleen whales. So, they are extant mesonychids. On the other hand, Arctocyonidae includes Arctocyon, which nests in the unrelated marsupial clade, Creodonta (see above). Certain other traditional mesonychids, like Sinonyx and Andrewsarchus, are not mesonyhids, but nest with the elephant shrew, Rhychocyon, close to tenrecs.

Figure 1. Silvanerpeton and Gephyrostegus to the same scale. Each of the two frames takes five seconds. Novel traits are listed. This transition occurred in the early Viséan, over 340 mya. Gephyrostgeus is more robust and athletic with a larger capacity to carry and lay eggs.

Figure 1. Silvanerpeton and Gephyrostegus to the same scale. Each of the two frames takes five seconds. Novel traits are listed. This transition occurred in the early Viséan, over 340 mya. Gephyrostgeus is more robust and athletic with a larger capacity to carry and lay eggs.

Gephyrostegidae
According to Wikipedia, “Gephyrostegidae is an extinct family of reptiliomorph tetrapods from the Late Carboniferous including the genera GephyrostegusBruktererpeton, and Eusauropleura.”

In the LRT, Gephyrostegus is the last common ancestor of the Amniota (= Reptilia). So… gephyrostegids include all living mammals, archosaurs (crocs + birds) and lepidosaurs.

References

wiki/Gephyrostegidae
wiki/Mesonychidae
wiki/Desmostylia

Eusauropleura: now identified as a late-surviving basalmost reptile

The newest addition
to the large reptile tree (LRT, 1341 taxa) is Eusauropleura digitata (originally Sauropleura, Cope 1868; Romer 1930; Carroll 1970; Late Carboniferous, 310 mya; AMNH 6865; Figs. 1, 2) nests as a late-surviving basalmost reptile in the LRT.

The genesis for this genus
in the earliest Carboniferous is based on the more derived Silvanerpeton from the Viséan (335 mya). A dense layer of belly scales (not shown en masse), a larger manual digit 5, and a taller ilium, among other traits, distinguish this specimen from Gephyrostegus. A larger manus, ischium and giant caudal transverse processes (ribs) relative to the torso are unique traits among close relatives. Note the lack of ribs in the lumbar area, where large amniote eggs develop before they are laid. The eggs were relatively large based on the greater depth of the ischium.

Figure 1. Eusauropleura in situ and slightly reconstructed. Manus reconstruction with PILs enlarged.

Figure 1. Eusauropleura in situ and slightly reconstructed. Manus reconstruction with PILs enlarged.

Basal to Eusauropleura
are taxa close to the Reptilomorpha – Lepospondyli split. These include Eucritta and Utegenia (Fig. 2) all derived from the Late Devonian reptilomorph, Tulerpeton. This affirms the primitive state of basalmost reptiles, derived from Devonian tulerpetids. Further affirmation comes from the observation that the central vertebral elements of Eusauropleura “are very thin-walled, forming little more than a husk around the large notochord,” according to Carroll 1970.

Figure 2. Eusauropleura to scale with ancestral and descendant taxa including Eucritta, Utegenia, Silvanerpeton and Gephyrostegus, the last common ancestor of all reptiles.

Figure 2. Eusauropleura to scale with ancestral and descendant taxa including Eucritta, Utegenia, Silvanerpeton and Gephyrostegus, the last common ancestor of all reptiles. Note, none of these specimens preserves ossified carpals.

First considered a microsaur
(Cope 1868), then a gephyrostegid (Romer 1930, 1950; Carroll 1970), Eusauropleura was identified as more primitive than Gephyrostegus (Carroll 1970), but still terrestrial, not aquatic and close to the ancestry of reptiles, but not itself a reptile.

So what is a reptile?
As determined here in 2011, there is no list of traditional reptile skeletal traits that upholds the reptile status of Gephyrostegus. There is a new list. Irregardless of skeletal traits, only the nesting of Gephyrostegus as the last common ancestor of all reptiles in the LRT tells us it was laying eggs with an amnion, the ONLY trait needed to determine its reptile status. Silvanerpeton, from the earlier Viséan, was likely also a reptile because phylogenetic descendants of late-surviving Gephyrostegus are also found in coeval Viséan strata. Reptiles are that old. Given that the last common ancestor of Silvanerpeton and Gephyrostegus must also be a late-surviving member of that basalmost reptile radiation, whether the amnion was fully developed or not, something we may never know given the fragility of an amniotic membrane over 300 million years in stone. Earlier workers did not enter Eusauropleura, Silvanerpeton and Gephyrostegus into a wide gamut phylogenetic analysis and so did not recover a last common ancestor status for these amphibian-like reptiles.

Another specimen attributed to Eusauropleura
AMNH 6860 (Moodie 1909, Carroll 1970), is a bit more jumbled, more incomplete and more difficult to reconstruct. A complete ilium with an elongate posterior process is easy to see in this specimen. Such a process provides attachment points for more than one sacral rib, a traditional reptile trait, but this is difficult to determine in the scattered remains of the fossil. And is this really Eusauropleura?

Yet another specimen attributed to Eusauropleura
PU 16815 is an isolated pectoral girdle, bones lacking in the other specimens and therefore not readily comparable.

Scales
According to Carroll 1970, “Scales, both dorsal and ventral, are conspicuous in these specimens [Gephyrostegus and Eusauropleura]. The body was protected by heavy, oblong scales, overlapping to form a chevron pattern, between the pelvic and pectoral girdles. Were they not associated with the skeleton, they would be difficult to distinguish from those of [more primtive] embolomeres. Laterally the scales assume a more oval outline, become thinner, smaller and less extensively overlapping. The dorsal scales are small, thin and round. Where worn, all the scales exhibit a pattern of fine ridges, running parallel with the margins. These form a pattern of concentric ridges in the dorsal scales, similar to that of [more primitive] discosauriscids. Except for the heavier ossification of the dorsal scales, those of Eusauropleura are generally similar to those of Gephyrostegus.”

References
Carroll RL 1970. The ancestry of reptiles. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 257 (814):267–308. DOI: 10.1098/rstb.1970.0026
Cope ED 1868. Synopsis of the Extinct Batrachia of North America. Proceedings of the Academy of Natural Sciences of Philadelphia 1868:208-221.
Romer AS 1930. The Pennsylvanian tetrapods of Linton, Ohio. Bulletin of the American Museum of Natural History. 59 (2):144–147.
Romer AS 1950. The nature and relationships of the Paleozoic microsaurs: American Journal of Science 248:628-654.

wiki/Eusauropleura

Diplovertebron vs. Gephyrostegus

Updated June 13, 2017 with the realization that Watson’s 1926 Diplovertebron is the same specimen as Gephyrostegus watsoni (bohemicus). 

This blog had its genesis in a reader comment
that considered the taxon, Diplovertebron congeneric with the coeval Gephyrostegus bohemicus and G. watsoni (Fig. 1), echoing earlier authors. Although there may be some confusion here (see below), and several specimens have been attributed to Gephyrostegus by various authors, the specimen illustrated and labeled by Watson 1926 (Fig. 1) is not one of them, unless it was drawn very poorly. If anyone has in situ skeletal material, please send it along for an update.

Gleaning data from several papers, provided that update. 

Part of my confusion
lies in the Wikipedia article on Diplovertebron, which states it was 60 cm in length, at least 5x larger than the one illustrated by Watson and far larger than any of its sister taxa. There may be a paper I am unfamiliar with at present that clarifies the matter.

So far, I have not found it. 60 cm may be an error.

The Westphalian (310 mya) tetrapods
include some reptile-like amphibians and some amphibian-like reptiles. This strata is 30 million years younger than the Viséan, where members from the first great radiation of reptiles can be found. Several late-survivors of earlier radiations can still be found in Westphalian strata.

Earlier G. watsoni nested among basal archosauromorpha, apart from G. bohemicus at the base of the Reptilia and separated by Eldeceeon. So the three taxa in figure 1 are separated from each other by intervening genera and therefore cannot be congeneric.

With present data, flawed though it may be
Diplovertebron nests in the large reptile tree (LRT) with Utegenia, at the base of the Lepospondyli, the clade that ultimately gives us frogs, like Rana, salamanders, like Andrias, and caecilians, like Dermophis.

Figure 1. Diplovertebron, Gephyrostegus bohemicus and Gephyrostegus watsoni. None of these are congeneric.

Figure 1. Diplovertebron, Gephyrostegus bohemicus and Gephyrostegus watsoni. to scale  None of these are congeneric. That’s because Watson’s drawing (upper left) was poorly drafted. 

Revised backstory:
Diplovertebron punctatum (Fritsch 1879, Waton 1926; DMSW B.65, UMZC T.1222a; Moscovian, Westphalian, Late Carboniferous, 300 mya) aka:  Gephyrostegus watsoniBrough and Brough 1967) and  Gephyrostegus bohemicus (Carroll 1970; Klembara et al. 2014) after several name changes perhaps this specimen should revert back to its original name as it nests a few nodes away from Gephyrostegus.

Derived from a sister to EldeceeonDiplovertebron was basal to the larger Solenodonsaurusand the smaller BrouffiaCasineria and WestlothianaDiplovertebron was a contemporary ofGephyrostegus bohemicus, Upper Carboniferous (~310 mya), so it, too, was a late survivor.

Overall smaller and distinct from Eldeceeon, the skull of Diplovertebron had a shorter rostrum, larger orbit and greater quadrate lean. The dorsal vertebrae formed a hump and had elongate spines. The hind limbs were much longer than the forelimbs. The tail is incomplete, but appears to have been short and deep.

Seven sphere shapes were preserved alongside this specimen. They may be the most primitive amniote eggs known.

Watson 1926 attempted a freehand reconstruction (see below) that was so different from this specimen that for a time it nested as a separate taxon, now deleted.

References
Brough MC and Brough J 1967. The Genus Gephyrostegus. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 252 (776): 147–165. doi:10.1098/rstb.1967.0006
Carroll RL 1970. 
The Ancestry of Reptiles. Philosophical Transactions of the Royal Society London B 257:267–308. online pdf
Fritsch A 1879. Fauna der Gaskohle und der Kalksteine der Permformation “B¨ ohmens. Band 1, Heft 1. Selbstverlag, Prague: 1–92.
Jaeckel O 1902. Über Gephyrostegus bohemicus n.g. n.sp. Zeitschrift der Deutschen Geologischen Gesellschaft 54:127–132.
Klembara J, Clack J, Milner AR and Ruta M 2014. Cranial anatomy, ontogeny, and relationships of the Late Carboniferous tetrapod Gephyrostegus bohemicus Jaekel, 1902. Journal of Vertebrate Paleontology 34:774–792.
Ruta M, Jeffery JE and Coates MI 2003. A supertree of early tetrapods. Proceedings of teh Royal Society, London B (2003) 270, 2507–2516 DOI 10.1098/rspb.2003.2524 online pdf
Watson DMS 1926. VI. Croonian lecture. The evolution and origin of the Amphibia. Proceedings of the Zoological Society, London 214:189–257.

wiki/Gephyrostegus
wiki/Diplovertebron

 

The Tiny Gephyrostegus and The Tiny Origin of the Reptilia

This page is no longer valid. Please see the update at:
https://pterosaurheresies.wordpress.com/2014/11/02/news-at-the-base-of-the-amniota-part-3-the-amniotic-egg/

And at: www.reptileevolution.com/gephyrostegus_watsoni.htm

Gephyrostegus-size

Figure 1. Two species of Gephyrostegus, the closest outgroup taxon to the Reptilia, in size comparisons to Cephalerpeton, the most primitive reptile. As you can see, Gephyrostegus watsoni was smaller than both. Whether G. watsoni (Fig. 2) was a juvenile or not (it is not a tadpole but certain tiny bones remain unossified), the fact remains that the earliest of reptiles were all smaller than Gephyrostegus bohemicus and they stayed small for several succeeding phylogenetic nodes. 

Carroll’s Hypothesis
As blogged earlier, Dr. Robert L. Carroll (1970) figured out that the earliest of all reptiles, the ones that biologically invented the amniotic egg, would be small specimens similar to Gephyrostegus. Carroll (2008) wrote: “On the basis of the present fossil record, all adequately known Palaeozoic reptiles appear to have had a common ancestry among the predecessors of the known gephyrostegids.” Carroll (2008) considered the possibility of a long ghost-lineage for gephyrostegids, unfortunately he did not undertake a cladistic analysis like the one here. Later Carroll (2009) dismissed his earlier hypothesis because, […gephyrostegids] lack many definitive features [of reptiles] and the best-known genus only occurs long after the appearance of unquestioned amniotes.” 

Gephyrostegus watsoni

Figure 2. Gephyrostegus watsoni. This tiny specimen retained an intermedium in the skull roof, a hallmark of non-reptiles. Small manual phalanages were poorly ossified. The pelvis likely connected to only one sacral vertebra, another non-reptile hallmark.

The Importance of Small Body Size
Carroll (2008) wrote, “Study of the earliest known reptiles and their closest relatives among contemporary amphibians indicates that the initial adaptation leading to the emergence of the class was assumption of a terrestrial habit, with accompanying small body size. The small body size of the immediate ancestors of reptiles would have made it possible for them to produce sufficiently small eggs that they could develop in damp places on land without initially being supported and protected by extraembryonic membranes.”

Carroll (1991) presumed the hatchling of tiny early reptiles would have been very large relative to the adult, skipping the tadpole stage and adapting to life immediately as an adult. He noted living salamanders that lay eggs on land have the largest hatchling to adult ratio. Carroll (1991) estimated the size of such an egg at 1 cm with an adult of 10 cm in snout-vent length. Cephalerpeton was exactly that size. Gephyrostegus watsoni (Fig. 2) was less than half that size.

The Origin of the Reptilia
Here Gephyrostegus is the closest outgroup taxon to the Reptilia and Cephalerpeton is the most primitive reptile (despite being 30 million years younger than the oldest known fossil reptiles, Westlothiana and Casineria). Another species, Gephyrostegus watsoni, was smaller than both. Whether G. watsoni was a juvenile or not (it is not a tadpole but certain tiny bones remain unossified), the fact remains that the earliest of reptiles were all smaller than Gephyrostegus bohemicus and basal reptile stayed small for several succeeding phylogenetic nodes.

Reptiles
Distinguished from basal tetrapods, basal reptiles lose their palatal fangs, the intertemporal (a tiny skull roof bone) and the occipital notch at the back of the skull. The occiput includes a large supraoccipital bone linking the skull roof bones to the back of the braincase. The pterygoid developed a transverse flange. The intercentra were reduced to crescents. The proximal tarsals integrated to form an astragalus and calcaneum, which was present in Gephyrostegus and reversed in Weslothiana.

Rapid Size Increase?
Carroll (2008) wrote: The rapid increase in body size in all lineages of Pennsylvanian reptiles indicates the prior development of an amniotic egg.” Perhaps not with reptiles first appearing some 340 million years ago (e.g. Casineria to Hylonomus) and not getting much bigger for the next 40 million years (e. g. Cephalerpeton to Milleretta).

A chronology of the basal Reptilia.

Figure 3. Click to enlarge. A chronology of the basal Reptilia.

Diadectids
Carroll (2008) was not able to establish the specific relationships of the Diadectidae. Here that clade is buried deep within the new Lepidosauromorpha.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Carroll RL 1970. The Ancestry of Reptiles. Philosophical Transactions of the Royal Society London B 257:267–308. online pdf.
Carroll RL 1991. The Origin of Reptiles in Origins of the Higher Groups of Tetrapods: Controversy and Consensus.  Schultze H-P and Trueb L (eds). Cornell University Press.
Carroll RL 2008. Problems of the Origin of Reptiles. Biological Reviews 44(3):393-431.
Carroll RL 2009. The Rise of the Amphibians 365 Million Years of Evolution. The Johns Hopkins University Press. 360 pp.

wiki/Gephyrostegus
wiki/Cephalerpeton