New paper on Eldeceeon, one of our earliest reptile ancestors

Ruta, Clack and Smithson 2020
bring us new two new specimens of the amphibian-like reptile, Eldeceeon (pronounced ‘L-D-C-on’, Smithson 1994; Viséan, 335mya), adding to the two previously described specimens (Fig. 1). These two are the basalmost taxa in the Archosauromorpha in the large reptile tree (LRT, 1725+ taxa, subset Figs. 4, 5). The authors confirm a relationship to Silvanerpeton (Fig. 2), the last common ancestor of all reptiles in the LRT. They share a deep pelvis for large egg laying and a large lumbar region  for egg-carrying in females. This trait is shared with males unless all specimens around this node found so far are all females.

Figure 3. Two specimens attributed to Eldeceeon that nest together.

Figure 1. The two earlier specimens attributed to Eldeceeon that nest together at the base of the Archosauromorpha. Note the extended lumbar region and deep pelvis ideal for laying large eggs on both specimens.

Unfortunately, 
the authors consider these taxa “either as the most plesiomorphic stem amniote clade or as a clade immediately crownward of anthracosauroids.” 

They didn’t test enough taxa to nest Elcedeceeon and Silvanerpeton as basal amniotes (= reptiles), nor did they test enough taxa to recover a basal dichotomy in the Viséan at the base of the Reptilia. One branch, the Archosauromorpha, gives rise to synapsids and non-lepidosaur diapsids. At its base, Eldeceeon is an amphibian-like reptile that laid (by phylogenetic bracketing) amniotic eggs.

From the abstract:
“A detailed account of individual skull bones and a revision of key axial and appendicular features are provided, alongside the first complete reconstructions of the skull and lower jaw and a revised reconstruction of the postcranial skeleton.”

Actually those first complete reconstructions were done here in 2014. Worse yet, the authors created by freehand one chimaera reconstruction (their figure 7e), not appreciating the distinctions between the two previously known specimens (Fig. 1).

From the abstract
“The late Viséan anthracosauroid Eldeceeon rolfei from the East Kirkton Limestone of Scotland is re-described. Information from two originally described and two newly identified specimens broadens our knowledge of this tetrapod. 

Figure 3. Four cladograms from Ruta, Clack and Smithson 2020 seeking a nesting place for included taxa.

Figure 2. Four cladograms from Ruta, Clack and Smithson 2020 seeking a nesting place for included taxa. Compare to Figure 4, a subset of the LRT. They need more outgroup taxa to solve their self-confessed phylogenetic problems.

From the Discussion
“The most vexing aspect of the Eldeceeon postcranium is the configuration of its rib cage, with long and curved ribs confined to the anterior half of its trunk. We hypothesise that the space between the most posterior trunk ribs and the pelvis was occupied by an unusually large puboischiofemoralis internus 2 (PIFI2).”

All basal amniotes have this sort of lumbar region. Gravid lizards use this space to carry eggs (Fig. 3). Ruta, Clack and Smithson overlook this possibility because their cladograms (Fig. 2) do not nest Eldeceeon and kin among the reptiles.

Figure 4. Extant lizards, A. gravid, B. in the process of laying eggs, C. with egg clutch.

Figure 3. Extant lizards, A. gravid, B. in the process of laying eggs, C. with egg clutch.

In the LRT Anthracosaurus
(Fig. 4) nests far from Eldeceeon (Fig. 5), Silvanerpeton, Gephyrostegus and other stem reptiles (= Reptilomorpha). Anthracosaurus nests in the same basal tetrapod clade as Ichthyostega and Proterogyrinus in the LRT. So taxon exclusion has mixed up the order of taxa in the cladogram of Ruta, Clack and Smithson 2020. More taxa solve such phylogenetic problems.

Other taxa are also adversely affected by taxon exclusion.
Ruta, Clack and Smithson report, “Eucritta melanolimnetes Clack, 1998 shares characters with groups as diverse as baphetids, temnospondyls, and anthracosaurs (Clack 2001); perhaps unsurprisingly, this combination of features has resulted in alternative phylogenetic placements for this taxon, either as a derived stem tetrapod or as a basal crown tetrapod shifting between alternate positions on either side of the lissamphibian–amniote dichotomy”. In the LRT (subset Fig. 2) Eucritta is a sister to Tulerpeton, the proximal outgroup clade to the Amniota + Gephyrostegus, which may be an amniote, too. It is the proximal outgroup to the Amniota in the LRT and includes all of the basal amniote traits.

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits. Eldeceeon arises after Silvanerpeton. Compare to cladograms in figure 2.

Eldeceeon rolfei (Smithson 1994) ~27 cm in total length, Early Carboniferous ~335 mya, is from the same formation that yielded Silvanerpeton and Westlothiana in the Viséan. Derived from a sister to TulerpetonEldeceeon was basal to Diplovertebron and Solenodonsaurus in the LRT (Fig. 5). Relative to G. bohemicus, the skull of Eldeceeon was shorter and taller. The dorsal ribs are missing from the posterior half of the torso. This is an adaption to carrying larger eggs in gravid females. The pectoral girdle was more gracile. yet still deep. These two specimens nest together, but are distinct enough to warrant distinct species names.

Figure 4. Subset of the LRT focusing on basal Archosauromorpha including Vaughnictis and Cabarzia nesting at the base of the Protodiapsid-Synapsid split. Note all the large varanopids nest together here in the Synapsida, separate from small varanopids in the Protodiapsida.

Figure 5. Subset of the LRTfrom 2019 focusing on basal Archosauromorpha including Vaughnictis and Cabarzia nesting at the base of the Protodiapsid-Synapsid split. Note all the large varanopids nest together here in the Synapsida, separate from small varanopids in the Protodiapsida.

Add taxa
to see the big picture. That always solves problems. Taxon exclusion continues to be the number one problem in paleontology.

Don’t create reconstruction chimaeras.
That never works out well. Too often the chimaera is created freehand.

The LRT is free, online and worldwide,
just so workers can check out the current list of sister taxa pertinent to any taxon under study. Someday it will be used. Not this time, but someday.

More on Anthracosaurus
and the traditional clade ‘Anthracosauria’ follows below the References. This clade turns out to be much smaller than current textbooks and lectures might indicate. Anthracosaurus is a terminal taxon leaving no descendants tested in the LRT.


References
Ruta M, Clack JA and Smithson TR 2020.
 A review of the stem amniote Eldeceeon rolfei from the Viséan of East Kirkton, Scotland. Earth and Environmental Science Transactions of The Royal Society of Edinburgh (advance online publication)
DOI: https://doi.org/10.1017/S1755691020000079
Smithson TR 1994. Eldeceeon rolfei, a new reptiliomorph from the Viséan of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 84 (3-4): 377–382.

wiki/Eldeceeon

Figure 1. The complete skull of Anthracosaurus greatly resembles its relative, Neopteroplax.

Figure 3. The complete skull of Anthracosaurus greatly resembles its relative, Neopteroplax. These are basal flathead taxa with orbits high on the skull, distinct from reptilomorphs with smaller skulls and lateral orbits.

https://en.wikipedia.org/wiki/Anthracosauria
“Anthracosauria” is sometimes used to refer to all tetrapods more closely related to amniotes such as reptilesmammals, and birds, rather than lissamphibians such as frogs and salamanders. An equivalent term to this definition would be Reptiliomorpha. Anthracosauria has also been used to refer to a smaller group of large, crocodilian-like aquatic tetrapods also known as embolomeres.

Gauthier, Kluge and Rowe (1988) defined Anthracosauria as a clade including “Amniota plus all other tetrapods that are more closely related to amniotes than they are to amphibians” (Amphibia in turn was defined by these authors as a clade including Lissamphibia and those tetrapods that are more closely related to lissamphibians than they are to amniotes).

Similarly, Michel Laurin (1996) uses the term in a cladistic sense to refer to only the most advanced reptile-like amphibians. Thus his definition includes DiadectomorphaSolenodonsauridae and the amniotes.

Laurin (2001) created a different phylogenetic definition of Anthracosauria, defining it as “the largest clade that includes Anthracosaurus russelli but not Ascaphus true“. [Ascaphus is the extant tailed frog.]

Michael Benton (2000, 2004) makes the anthracosaurs a paraphyletic order within the superorder Reptiliomorpha, along with the orders Seymouriamorpha and Diadectomorpha, thus making the Anthracosaurians the “lower” reptile-like amphibians. In his definition, the group encompass the EmbolomeriChroniosuchia and possibly the family Gephyrostegidae.

None of these apply to Anthrosaurus in the LRT.

Distinct from prior authors, the LRT recovers Limnoscelis, Diadectes and other diadectomorphs deep with the Lepidosauromorpha branch of the Reptilia. More taxa solved this problem, too.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.