Tanytrachelos in New Mexico – taxon exclusion problems

Modified June 09, 2015 with the addition of clades named by Peters 2000 overlooked by Pritchard et al. 2015. 

Pritchard et al. (2015)
report on 3D Tanytrachelos (Fig. 1) individual bones from New Mexico (Late Triassic, Chinle Formation). And I think they’re spot on with regard to bone identification.


Figure 1. Tanytrachelos – close to Tanystropheus, but tiny with a distinct skull. The New Mexico material matches both in shape and size this North Carolina and Virginia material from the Late Triassic.

The problem comes from their phylogenetic analysis.
From the Pritchard et al. text: “Our analysis incorporated a range of fossil taxa that have traditionally been allied with Tanystropheus and Macrocnemus.” Unfortunately that tradition is ‘bogus’ based on the larger taxon list of the large reptile tree in which macronemids and tanystropheids are lepidosauromorphs, not archosauromorphs. In the Pritchard et al. cladogram (Fig. 2) note the separation of Prolacerta and Protorosaurus to make room for a “by default clade” of tanystropheids that should nest within the Lepidosauromorpha when more taxa are added. This abbreviated taxon list and “by default clade” actually separates the two prorotorosaurs from each other.

Figure 2. Cladogram from Pritchard et al. nesting tanystropheids between two protorosaurs, which should have nested together.

Figure 2. Cladogram from Pritchard et al. nesting tanystropheids between two protorosaurs, which should have nested together (Fig. 3 and large reptile tree). Lepidosauromorphs are in yellow. Archosaurmorphs are in white.

A subset
of the large reptile tree taxon list (Fig. 3) matched (as closely as possible) to the Pritchard et al taxon list demonstrates the problems with such a short taxon list using these taxa in which archosauromorphs and lepidosauromorphs are shuffled like a deck of cards. And sister taxa do not resemble one another at each color-shift clade. For instance, in figure 2 Macrocnemus does not resemble Mesosuchus and in figure 3 Macrocnemus does not resemble Petrolacosaurus.

Figure 3. Subset of the large reptile tree matched to the Pritchard et al. taxon list. Here Protorosaurus and Prolacerta nest together, but the other clades interweave archosauromorphs and lepidosauromorphs.

Figure 3. Subset of the large reptile tree matched to the Pritchard et al. taxon list. Here Protorosaurus and Prolacerta nest together, but the other clades interweave archosauromorphs (white) and lepidosauromorphs (yellow). If you think this cladogram needs more taxa, you are right. And you can find them at the large reptile tree. And this cladogram proves there is a limit to taxon exclusion past which the tree topology breaks down. But then, we are cherry-picking here from two widely separated clades.

Traditions need to be tested
The Pritchard et al. studied relied on a traditional taxon list that was falsified four years ago here. So why did the referees let this manuscript get published? (Answer: tradition, status quo, established paradigm, plus shunning, marginalizing, ignoring the larger gamut study).

The sternum issue
Tanystropheids, like all tritosaurs and most squamates (but not Adirosaurus through snakes because the forelimbs are shrinking), have a sternum not found in protorosaurs and other archosauromorphs. I know I just pulled a “Larry Martin” by noting one and only one trait…

below is the list of all the other traits from the large reptile tree that unambiguously separate tanystropheids (T) from protorosaurs (P). There are 30. Many of these traits extend to other tritosaurs (a subset of the Lepidosauria) and are not found in other archosauromorphs or vice versa.

  1. ventral naris: T = chiefly mx; P = chiefly pmx
  2. dorsal nasal shape: T = pmx invasion; P = narrows toward naris
  3. pmx orientation: T = horizontal; P = down
  4. naris placement: T = displaced or elongated; P = snout tip
  5. posterolateral pmx: T = absent; P = narrower than naris
  6. frontal/parietal suture: T = straight and > than nasal suture; P = not
  7. frontal shape: T = wider posteriorly; P = not
  8. frontal posterior process: T = absent; P = present
  9. postparietals: T = absent; P= present
  10. tabulars: T = absent; P = present
  11. friontal/nasal suture: T = anteriorly oriented; P = zigzag
  12. quadratojugal presence: T = jugal ramus only; P = quadrate ramus only
  13. squamosal/quadratojugal indent: T = no qj ascending process; P = semicircle
  14. parietal and frontal fusion: T = both fused; P = no fusion
  15. pterygoid lateral edge: T = ectopterygoid continues margin laterally; P = sharp angle
  16. pterygoid shape: T = narrow; P = broad triangular
  17. procumbent pmx teeth: T = present; P = absent
  18. posterior mandible shape: T = deeper anteriorly; P = mid rise
  19. caudal transverse processes: T = absent beyond 8th caudal; P = present beyond
  20. short lumbar ribs: T = present; P = not short
  21. second sacral rib: T = not bifurcate; P = bifurcate
  22. chevron shape: T = parallel to centra; P = descends, distal wider
  23. anterior caudal spines: T = shorter than centra; P = taller than centra
  24. sternum: T = present; P = absent
  25. scapula shape: T = not robust; P = robust
  26. pubic apron: T = not present; P = present and wide
  27. tarsus: T = not fenestrated; P = fenestrated
  28. calcaneal tuber: T = no tuber: P = lateral tuber
  29. metacarpal 5: T = straighter or twisted: P = hooked
  30. pedal 3.1 longer than p2.1: T = present; P = not

This reference probably snuck under the radar
Pritchard et al. noted several unnamed clades that were actually named in Peters 2000, some 15 years ago. Further work with the large reptile tree has shown that these clades are all lepidosaurian, not archosaurian or protorosaurian.

Clades named by Peters 2000

Macrocnemus + Characiopoda

Tanystropheidae + Langobardisaurus + Fenestrasauria

Cosesaurus + Sharovipteryx + Longisquama + Pterosauria

Peters D 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Pritchard AC, Turner AH, Nesbitt SJ, Irmis RB and Smith ND 2015. Late Triassic tanystropheids (Reptilia, Archosauromorpha) from northern New Mexico (Petrified Forest Member, Chinle Formation) and the biogeography, functional morphology, and evolution of Tanystropheidae. Journal of Vertebrate Paleontology 25(2):e911186 (20pp).

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