Biseridens and Phthinosuchus – two misunderstood therapsids

Biseridens, according to Wikipedia,
“is the most basal genus of anomodont therapsid.”

Not so,
according to the large reptile tree (Fig. 3), which nests Biseridens (Fig. 1, Li and Cheng 1997; Liu, Rubidge and Li 2009) far from anomodonts, between Archaeosyoson and Jonkeria and kin among the Tapinocephalia.

Figure 1. Biseridens and Phthinosuchus, two related therapsids that have been giving paleontologists fits.

Figure 1. Biseridens and Phthinosuchus, two related therapsids that have been giving paleontologists fits.

Phthinosuchus, according to Wikipedia
“is the sole member of the the family Phthinosuchidae. It may have been one of the most primitive therapsids.” Not so, according to the large reptile tree (Fig. 2) where Phthinosuchus (Fig. 1, Efremov 1954) nests between Eotitanosuchus and ArchaeosyodonBiseridens at the base of the Dinocephalia.

So traditional nestings seem to be a little behind the times.
According to Liu, Rubidge and Li 2009, “Synapomorphies that distinguish Biseridens as an anomodont and not an eotitanosuchian as previously described: short snout (1); dorsally elevated zygomatic arch (2) and septomaxilla lacking elongated posterodorsal process between nasal and maxilla (3). The presence of a differentiated tooth row (4); denticles on vomer, palatine and pterygoid (5); contact between tabular and opisthotic (6); lateral process of transverse flange of pterygoid free of posterior ramus and absence of mandibular foramen exclude it from other anomodonts (7). Cladistic analysis indicates Biseridens to be the most basal anomodont (8).

Well, according to the large reptile tree…

  1. Eotitanosuchus has a long snout because it is basal to the clade of long snouted basal gorgonopsians and therocephalians. Biseridens ancestors, like Phthinosuchus, and Archaeosyodon, never had a long snout.
  2. the zygomatic arch (squamosal principally) is not dorsally elevated in the fossil (Fig. 1)
  3. sisters likewise lack this septomaxilla trait
  4. the dual rows of post-canine teeth and the large orbit in Biseridens are autapomorphies that distinguish it from sisters
  5. Denticlaes are also found on the palate of Phthinosuchus. I don’t have data for closer sisters.
  6. I don’t have comparable occipital data here
  7. I don’t have comparable palatal data here
  8. Be careful when a taxon nests as the ‘most basal’ to any clade without many more basal taxa on the inclusion list. As in another purported basal synapsid taxon, Caseasauria, it turns out that Biseridens actually nests elsewhere (Fig. 2).

Learn more about basal anomodonts here.

Figure 3. Basal therapsid tree.

Figure 2. Basal therapsid tree. Note the nestings of Phthinosuchus and Biseridens far from where tradition al paleontologists have been saying. I think more taxa near the base of the tree make tis tree distinct. Note the weak bootstrap scores at the nodes splitting Suminia from Venjukovia and splitting the basal dromasaurs.

 

References
Efremov IA 1954, The fauna of terrestrial vertebrates in the Permian copper sandstones of wester Cis-Urals: Travaux de I’institut Paleozoologique de l’Academie des Sciences de l’URSS, v. 54, 416pp.
Li J and Cheng Z 1997. First discovery of eotitanosuchian (Therapsida, Synapsida) of China. Vertebr. Palasiatica 35, 268–282.
Liu J, Rubidge B and Li J 2009. A new specimen of Biseridens qilianicus indicates its phylogenetic position as the most basal anomodont. Proceedings of the Royal Society B 277 (1679): 285–292. online

wiki/Biseridens
wiki/Jonkeria
wiki/Phthinosuchus

 

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