The Archosauromorph Stem

John Merck

Archosauromorpha: (Permian - Quaternary) All organisms more closely related to Passer (sparrow) than to Lacerta (a squamate). A total-group definition. The node-based group on which Archosauromorpha is anchored is Archosauria, the last common ancestor of crocodilians and birds and all of its descendants - a crown group. As a total-group, Archosauromorpha has no formal synapomorphies.
Nevertheless, known archosauromorphs display evolutionary trends that make them recognizable.

Thus, ancestrally archosauromorphs had comparatively long and stiff necks, and could to some degree elevate their heads off the ground (although this could be reversed secondarily.)

Basal stem archosauromorphs:

Unlike the primitive relatives of lepidosaurs, whose fossil record is sparse, the basal (i.e. non-archosaurian) archosauromorphs present an embarrassment of riches, being common and ecologically diverse during the Triassic, but having their roots in the Permian. They were among the first targets of cladists. (See Gauthier, 1984, Benton, 1985.)

Although there is almost complete agreement on what animals belong on the archosaurian stem, well-studied taxa in the early age of cladistics were typically divergent and ecologically specialized, including:

Thus, they presented similar problems to those posed by Euryapsida. Improved sampling of basal taxa has shed increasing light. We follow the comprehensive phylogeny of Ezcurra, 2016.

Champsosaurus by Polygone Studio


(Late Triassic - Neogene) Before we start, an intractable phylogenetic headache: Members of Choristodera - a well-diagnosed group - has been found to be:

Originally only known to paleontologists as large long-snouted and medium-snouted forms from the Late Cretaceous and Early Paleogene. These were superficially convergent on crocodilians but, when considered in detail, extremely different as well. Eventually, more basal forms were identified going as far back as the late Triassic from fragmentary specimens of nondescript smaller reptiles already housed in museums. Only recently have well-preserved remains of earlier choristoderes become known.

Assorted choristoderes including Simoedosaurus (a) and Champsosaurus (d)
Morphological trends include:
Choristodere phylogeny: Is poorly resolved but for one pattern: Ancestrally, choristoderes seem to have been small, but members of Neochoristodera (Late Cretaceous - Paleogene) became crocodilian sized (and in some ways must have competed with them ecologically.) Well-known groups within Choristodera include:

Protorosaurus speneri, from Wikipedia

The Base of Archosauromorpha and the Clade that Wasn't:

For the first decade of cladistics, workers like Gauthier, 1984 and Benton, 1985 recovered a clade of longish to long-necked creatures called either Protorosauria or Prolacertiformes that included both terrestrial and somewhat aquatic forms like:

Beginning in the late 1990s, this clade began to fall apart, with forms like Prolacerta appearing in the crown of the tree (Dilkes, 1998), and others like Protorosaurus (right) falling to the base (Ezcurra, 2016). It now seems that "protorosaurians" represent the basal morphotype for archosauromorphs, but not a monophyletic group. Indeed, Protorosaurus (Late Permian) is a well-studied close approximation of the ancestral archosauromorph form. It's sister taxon, the long-necked Early Triassic Czatkowiella hare (Borsuk-Bialynicka and Evans, 2009) is similar. (Ezcurra, 2016's basal member, Aenigmatosaurus is, well, enigmatic.)

Protorosaurus was fully terrestrial and probably ecologically similar to living varanid lizards. This comparison invites speculation that, like modern varanids, it might have used gular pumping of the hyoid skeleton to ventilate the lungs more efficiently.

Tanstropheus langobardicus, from Wikipedia


(Latest Early Triassic - Late Triassic.) These take the basic archosauromorph adaptation - a long stiff neck - to extremes.
  • Were they fully aquatic?

    Synapomorphy of Tanystropheidae + Archosauria:

    Trilophosaurus buettneri skull from Paleofile


    Trilophosaurus, not Tanystropheus. (Later Permian - Quaternary.) Named by Ezcurra, 2016, this group encompasses the major terrestrial radiation of archosauromorph herbivores and predators. Significant synapomorphies include:

    Crocopoda contains:

    Trilophosaurus buettneri by FunkMonk fromWikipedia


    (Middle - Late Triassic) a herbivorous radiation that was, at first, puzzling because its first studied member was the extremely derived Trilophosaurus buettneri (right) prior to cladistics considered a "euryapsid" because of its temporal fenestration.

    Boring on the outside, interesting on the inside. Vaguely resembles a large lizard with a turtle's head, complete with beak in front. Yank its jaws open to admire its wonderfully strange teeth, like a battery of three-cusped parallel razor-blades. Strangest of all, these are maxillary teeth that don't line up with the margin of the skull. Freaky. In the last twelve years, related creatures have come to light including:

    Synapomorphies of Allokotosauria include:

    Mesosuchus browni from ResearchGate


    (Middle - Later Triassic) Triassic ambulatory scissors for processing tough plant material.

    Prolacerta broomi from Online Biolog Library - SUNY Orange


    (Early Triassic) Originally considered a protorosaurian, now deemed closer to Archosauria. Well known from cranial and postcranial material. Prolacertids, themselves, are diagnosed by:

    Euparkeria capensis from Berkeley Museum of Paleontology

    Potential synapomorphies with archosaurs involve the transformation of the palate:


    Archosauriformes: (Late Permian - Quaternary)

    Crown-group Archosauria is nested inside Archosauriformes, a clade containing both Archosauria and critters that would, in the in the pre-Phylogenetic Systematics days, have been called archosaurs based on general similarity, but today are seen to lack some synapomorphy of Archosauria.

    Major taxa within Archosauriformes include:

    Proterosuchus vanhoepeni From Paleofile

    Significant synapomorphies of Archosauriformes are not subtle:

    Beyond this, we see some general tendencies:

    Proterosuchus fergusi from Wikipedia


    (Late Permian to Early Triassic) Arguably the most basal: Proterosuchidae, including Proterosuchus and Chasmatosaurus. Present prior to the Permo-Triassic extinction and common immediately after it - a classic "disaster taxon." (Cf. the dicynodont Lystrosaurus.) Probably similar in life style to crocodilians, but without strong specializations for aquatic life.

    Synapomorphies of Proterosuchidae:

    Shansisuchus shansisuchus by Dmitri Bogdanov from Wikipedia


    (Early to Middle Triassic) The first archosauromorph apex predators. During the Middle Triassic they held the role that would be taken by proper archosaurs during the Late Triassic, and parallel some of their derived features.

    Synapomorphies of Erythrosuchidae:

    Synapomorphy with Archosauria:

    Euparkeria capensis from Biodiversity Explorer


    (Late Early Triassic to Quaternary) Passer not Erythrosuchus. Derived features are concentrated in the femur, which has: Of many basal members, the iguana-sized Euparkeria has played an important historic role in our understanding of Archosauriform evolution, being argued to be:

    Pes and ankle joint of Euparkeria capensis from Demuth et al., 2020
    A quantitative analysis of Euparkeria's hip joint mobility by Demuth et al., 2020 suggests that it is capable of a wide range of postures ranging from an intermediate semi-upright to fully upright. It's obliquely angled ankle joint, however, suggests that it didn't typically assume a full upright stance.

    Proterochampsa nodosa from ResearchGate


    (Middle Triassic to Late Triassic) An ecologically diverse assortment of smallish eucrocopods. Synapomorphies include: Proterchampsia is comprised of two clades:

    Proterochampsia and Archosauria are united by subtle synapomorphies of the vertebral column and coracoid.

    Of the groups referred to in this lecture, only two:

    ..survived past the end of the Triassic. We address Archosaurian evolution and diversity in the coming lectures.
    Additional reading: