Archosauromorph Misinformation in the Encyclopedia of Geology, 2nd edition

In the 2nd edition of Encyclopedia of Geology,
Ezcurra, Jones, Gentil and Butler 2020 provide their guide to:

“How to Recognize Fossils of Archosauromorpha and Archosauriformes
The earliest archosauromorph lineages (e.g., Protorosaurus speneri, early tanystropheids) have a general body plan that resembles that of large modern lizards (e.g., varanids, teiids), but they are characterized by a series of unique evolutionary novelties in their skeleton that appeared in the common ancestor of the group during the Permian.

These diagnostic anatomical features include:

  1. snout representing around half or more of the skull length,
  2. posterior margin of skull roof defined by a low vertical lamina, 
  3. absence of a notochordal canal in the vertebral centra (with the exception of Aenigmastropheus, the earliest diverging archosauromorph), 
  4. neck with at least a slightly sigmoid profile,
  5. third cervical vertebra longer than the second one,
  6. anterior cervical vertebrae with rib facets on the centrum,
  7. last cervical and trunk vertebrae with bony buttresses (laminae) reinforcing rib articulations,
  8. absence of intercentra (ossifications that lie between the vertebral centra) posterior to the second cervical vertebra,
  9. very long cervical ribs extend parallel to the neck and possess an anterior process,
  10. long transverse processes in trunk vertebrae,
  11. humerus with low degree of torsion between the ends of the bone,
  12. and absence of an ossified distal carpal 5 (small wrist bone above the lateral-most digit).

“Most of the diagnostic features that characterize the archosauromorph body plan are concentrated in the vertebral column and are related to the elevation of the head above the level of the trunk and possible reduction of the mass of the vertebrae without a loss in strength. However, the functional significance of these changes and potential paleoecological implications remain mostly unexplored.”

Figure 2. From Ezcurra et al. 2020 with an overlay based on LRT results.

Figure 1.  From Ezcurra et al. 2020 with an overlay based on LRT results. The authors have not done their own testing, but are relying on popular consensus. That’s not good science.

Despite their sincere attempts, this is misinformation at its core. 

  1. Due to taxon exclusion Ezcurra et al. have no idea that the validated split between Lepidosauromorpha and Archosauromorpha occurred following Silvanerpeton in the Viséan (Early Carboniferous. The authors report: “middle-late Permian” for that split.
  2. No one can make a list of traits that all Archosauromorpha have in common and determine clade membership on that basis. In this respect the authors are attempting to  “Pull a Larry Martin“. You can only determine clade membership by a cladogram and look for that last common ancestor.
  3. Thus the only way to recognize an archosauromorph (definition: all taxa closer to archosaurs than to lepidosaurs) is to see where a taxon nests on a wide gamut cladogram like the large reptile tree (LRT, 1655+ taxa), where mammals and their synapsids ancestors are also members of the new Archosauromorpha. 

Ezcurra et al. consider the following members of the Lepidosauromorpha
to be basal members of the Archosauromorpha.

  1. Tanystropheidae. (Tanystropheus + Macrocnemus and kin) The LRT nests that clade in the Tritosauria and that clade in the Lepidosauria. Huehuecuetzpalli is in that lineage.
  2. Allokotosauria (Azendohsaurus + Trilophosaurus and kin) The LRT nests that clade in between Sphenodontia (= Rhynchocephalia) and Rhynchosauria within the Lepidosauria.
  3. Rhynchosauria (see above).

Ezcurra et al. consider the following clades
to be basal members of the Archosauromorpha.

  1. Prolacertidae (Prolacerta and kin, but not Protorosaurus) The LRT supports this assignment.
  2. Proterosuchidae (Proterosuchus and kin).The LRT supports this assignment.
  3. Erythrosuchidae (Erythrosuchus and kin). The LRT supports this assignment.
  4. Euparkeriidae (Euparkeria and kin). The LRT supports this assignment.
  5. Proterochampsidae (Proterohampsa and kin). The LRT supports this assignment.
  6. Doswelliidae (Doswellia and kin).The LRT supports this assignment except Vancleavea is a thalattosaur, an archosauromorph not related to Doswellia.

Ezcurra et al. consider the following clades
to be arguable basal members of the Archosauromorpha. Arguable? Test them! Based on their text and cladogram (Fig. 1) it is clear that Ezcurra’s team is following the authority of previous authors, not sure where some clades nest, rather than running the analysis themselves.

  1. Choristodera (Champsosaurus and kin) The LRT supports this assignment.
  2. Testudines (turtles, traditionally). The LRT nests this clade within the Lepidosauromorpha. Purported ancestors: Pappochelys is a sauropterygian archosauromorph in the LRT. Eunotosaurus is a lepidosauromorph not related to turtles, which arise from pareiasaurs in the LRT.
  3. Sharovipterygidae (Sharovipteryx and kin). The LRT nests this clade between tanystropheids and pterosaurs in the Tritosauria in the Lepidosauria. Ozimek is a long-limbed protorosaur, not related.
  4. Kuehneosauridae (Kuehneosaurus and kin. The LRT nests this clade among basalmost lepidosauriformes.
  5. Phytosauria (Phytosaurus and kin). The LRT supports this assignment.

It should be noted that Ezcurra et al. consider the clade
Pterosauria to be a part of the Avemetarsalia within the Archosauria. This myth was proven wrong twenty years ago. The LRT nests pterosaurs with tanystropheids and sharovipterygids within the Tritosauria and Lepidosauria. The authors have not done their own testing, but are relying on popular consensus. That’s not good science. Now, sadly, that misinformation is set in stone in the pages of the Encyclopedia of Geology.


References
Ezcurra MD, Jones AS, Gentil AR and Butler RJ 2020. Early Archosauromorphs: The crocodile and dinosaur precursors. Chapter in Encyclopedia of Geology, 2nd edition. Elsevier Inc. https://doi.org/10.1016/B978-0-12-409548-9.12439-X

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