BPI 2871 has a new sister: Elachistosuchus huenei

Earlier we looked at a tiny basal choristodere, BPI 2871, which was derived from a line of much larger proterosuchids, according to the large reptile tree.

Recently a new PlosOne online paper (Sobral et al. 2015) reintroduces Elachistosuchus huenei (Janensch 1949, Late Triassicm, Norian, Germany; MB.R. 4520 (Museum für Naturkunde Berlin, Berlin, Germany)) with CT scans.

Figure 1. Elachistosuchus (Janensch 1949, Sobral et al. 2015) is a sister to BPI 2871, a basal choristodere.

Figure 1. Elachistosuchus (Janensch 1949, Sobral et al. 2015) is a sister to BPI 2871, a basal choristodere, has been misidentified for over fifty years.The left upper temporal fenestra has been largely closed by crushing here. Like BPI 2871, the nares were located on top of the skull, close to the snout tip. Note the vestige of the antorbital fenestra.

And they don’t know what it is. 
According to Sobral et al, Elachistosuchus could be “an archosauromorph, a lepidosauromorph or a more basal, non-saurian diapsid.” That confusion arises from using outdated matrices with too few generic taxa and too many suprageneric taxa.

Sobral et al. used the matrix from Chen et al. 2014, which nested Elachistosuchus in a polygamy with Choristodera, Prolacerta + Tanystropheus + Macrocnemus, and Trilophosaurus + Rhynchosauria + Archosauriformes. As readers know the large reptile tree found many of these taxa on opposite sides of the reptile cladogram.

Sobral et al. also used the matrix from Ezcurra et al. 2014, which nested Elachistosuchus with the gliding Permian lepidosauriform, Coelurosauravus.

Hmmmm…

Sobral et al. report: 
“These different placements highlight the need of a thorough revision of critical taxa and new character sets used for inferring neodiapsid relationships.” 

Exactly. 
That’s why large reptile tree and reptileevolution.com are here. It’s good to have hundreds of specimen-based taxa for new taxa to nest with. More choice. More accuracy. Complete resolution.

To their credit,
a Sobral et al. analysis nested Elachistosuchus with choristoderes.

Figure 2. Dorsal, lateral and palatal views of BPI 2871 with bones colorized above. Below, reconstructed images of BPI 2871 tracings. It is more complete than illustrated by Gow 1975. Click to enlarge. Note the tiny remnant of the antorbital fenestra. The squamosal has been broken into several parts.

Figure 2. Dorsal, lateral and palatal views of BPI 2871 with bones colorized above. Below, reconstructed images of BPI 2871 tracings. It is more complete than illustrated by Gow 1975. Click to enlarge. Note the tiny remnant of the antorbital fenestra. The squamosal has been broken into several parts. This is a sister to Elachistosuchus.

Among earlier workers
Janensch (1949) considered Elachistosuchus a pseudosuchian archosaur with an antorbital fenestra. Walker (1966 ) considered  Elachistosuchus a rhynchocephalian lepidosaur.

The large reptile tree (now 575 taxa)
finds Elachistosuchus nests firmly as a sister to the BPI 2871 specimen (Fig. 3) that Gow mistakenly attributed to Youngina, but it nests far from Youngina at the base of the large and small choristoderes. And these two taxa are both derived from much larger proterosuchids in yet another case of phylogenetic miniaturization at the genesis of a new clade, in this case the Choristodera.

Elachistosuchus has a larger orbit and a maxilla with a straight, not convex, ventral margin of the maxilla than the BPI 2871 specimen. The former extends the geographic range of the latter, from southern Africa to Germany.

Both probably look like juvenile proterosuchids (whenever they are discovered, we can compare them). Phylogenetic miniaturization often takes juvenile traits and sizes and makes them adult traits and sizes to start new clades.

Janensch thought Elachistosuchus had an antorbital fenestra. As in BPI 2871, that is the vestige of the antorbital fenestra found in ancestors and lost in descendants.

Contra the title of the Sobral et al. paper
Elachistosuchus huenei has nothing to do with the origin of ‘Sauria.’

Sauria definition: “.Any of various vertebrates of the group Sauria, which includes most of the diapsids, such as the dinosaurs, lizards, snakes, crocodilians, and birds. Sauria was formerly a suborder consisting ofthe lizards” Rather, Elachistosuchus is a basal choristodere and a derived proterosuchid according to the large reptile tree. Based on the current definition of ‘Sauria’ ‘Sauria’ is synonymous with ‘Amniota’ which is a junior synonym for ‘Reptilia’ because the last common ancestor of lizards and dinosaurs is the basalmost reptile/amniote, Gephyrostegus bohemicus.

The reason why Sobral et al. were confused
with regard to their blurred nesting of Elachistosuchus is due to taxon exclusion. BPI 2871 is a rarely studied taxon and was not included in their analyses. Moreover, traditional paleontologists are not sure what choristoderes are. They don’t recognize them as being derived proterosuchids. And to make matters worse, traditional paleontologists prefer to think of Proterosuchus specimens as members of an ontogenetic series, when they should consider them as a phylogenetic series.

Figure 4. This is where Elachistosuchus nests, next to BPI 2871, at the base of the Choristodera.

Figure 3. This is where Elachistosuchus nests, next to BPI 2871, at the base of the Choristodera. Click to see the complete reptile cladogram.

The large reptile tree (Fig. 3) has proven itself time and again to solve paleontological problems in the reptile family tree. It is unfortunate that it has been rejected for publication so many times. If published, it can be use.

A MacClade file is available on request.

References
Chen X, Motani R, Cheng L, Jiang D, Rieppel O. 2014. The enigmatic marine reptile Nanchangosaurus from the Lower Triassic of Hubei, China and the phylogenetic affinities of Hupehsuchia. PLoS ONE. 2014; 9: e102361. doi: 10.1371/journal.pone.0102361 PMID: 25014493
Ezcurra MD, Scheyer TM, Butler RJ 2014. The origin and early evolution of Sauria: reassessing the Permian saurian fossil record and the timing of the crocodile-lizard divergence. PLoS ONE. 2014; 9: e89165. doi: 10.1371/journal.pone.0089165 PMID: 24586565
Gow CE 1975. The morphology and relationships of Youngina capensis Broom and Prolacerta broomi Parrington. Palaeontologia Africana, 18:89-131.
Janensch W 1949. Ein neues Reptil aus dem Keuper von Halberstadt. N Jb Mineral Geol Palaeont B. 1949:225–242.
Sobral G, Sues H-D & Müller J 2015. Anatomy of the Enigmatic Reptile Elachistosuchus huenei Janensch, 1949 (Reptilia: Diapsida) from the Upper Triassic of Germany and Its Relevance for the Origin of Sauria. PLoS ONE 10(9): e0135114. doi:10.1371/journal.pone.0135114
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0135114
Walker AD 1966. Elachistosuchus, a Triassic rhynchocephalian from Germany. Nature. 1966; 211: 583–585.

wiki/Elachistosuchus

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