GEOL 431 Vertebrate Paleobiology

Spring Semester 2023

The Archosauromorph Stem

Archosauromorpha: (Permian - Quaternary) Max ∇ (Passer (sparrow) ~ Lacerta (a squamate)). 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.

• Overlapping cervical ribs.
  
• Relatively long rhomboidal cervical vertebrae.

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:

• Long-necked fishers
• plant chopping herbivores
• plant-grinding herbivores
• aquatic predators
• terrestrial predators

Thus, they presented similar problems to those posed by Euryapsida. Improved sampling of basal taxa has shed increasing light. We primarily follow the comprehensive phylogeny of Ezcurra, 2016 with some illumination from Spiekman et al., 2021.

Choristodera:

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

• basal archosauromorphs
• basal lepidosauromorphs
• stem saurians.

Recent analyses (E.g. Simões et al., 2020) favor a basal archosauromorph placement.

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.

Morphological trends include:

• Posterior expansion of supratemporal fenestrae
• Contact of prefrontals on midline
• Nasals fused
• Nares confluent
• Neomorph (a novel ossification) closes off much of the craniotomy-quadrate passage.

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:

• Monjurosuchidae: (Early Cretaceous) Small terrestrial forms like Monjurosuchus from early Cretaceous.
• Hyphalosauridae: (Early Cretaceous) Small long-necked, small-headed fresh water aquatic forms like Hyphalosaurus from early Cretaceous.
• Simoedosauridae: (Late Cretaceous - Early Paleogene) Croc-sized robust fresh water aquatic forms like Simoedosaurus from early Paleogene.
• Chanmpsosauridae: (Late Cretaceous - Early Paleogene) Croc-sized long-snouted fresh water aquatic forms like Champsosaurus from early Paleogene.

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:

• Protorosaurus
• Prolacerta
• Macrocnemus
• Tanystropheus
• Dinocephalosaurus
• etc.

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.

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.

Dinocephalosauria:

(Middle Triassic) Takes the basic archosauromorph adaptation - a long stiff neck - to extremes through the multiplication of cervical vertebrae, analogous to eosauropterygians. Dinocephalosaurus, in fact, has 25. Dinocephalosaurians resemble other shallow marine long-necked archosauromorphs in general profile but differ in detail. Notably:

• The skull is characterized by nares set in long grooves and retracted from the end of the snout.
• A specimen preserves a late-term embryo in the abdomen. A radical departure for archosauromorphs, who seem generally to be strongly committed to oviparity.

Dinocephalosaurus was discovered in 2002, but first thought to belong to the similarly long-necked tanystropheids (below), but shown by Spiekman et al., 2021 to be a separate derivation of the long-necked morphotype.

Tanystropheidae:

(Latest Early Triassic - Late Triassic.) An independent derivation of the long-necked extreme.

• Earlier members like Macrocnemus were at most facultatively aquatic, and shows limb proportions similar to cursorial squamates, just with a longer neck. Even so, its remains are found in shallow marine environments, suggesting that it at least lived near water.
• but during the Middle Triassic we pick up fresh-water aquatic forms like Tanytrachelos or shallow marine forms like Tanystropheus. The best known example is Tanystropheus, but others are being discovered Li, et al., 2004. Which (if any) of these swam after prey and which lurked in ambush along the shoreline is debatable.
• Along with their transition to marine life came an elongation of the neck by the evolution of a small number of very long cervical vertebrae.
• The neck was also stiffened by long, slender cervical ribs. And yet, something about large tanystropheids brings out the crazy in illustrators.
  
  
  Significant synapomorphies of Tanystropheidae:
  • Elongation of cervical vertebrae
  • Shortening of metatarsal V, causing it to resemble a distal tarsal. (In some, the first phalanx of digit V becomes robust and metatarsal-like.)

Synapomorphy of Tanystropheidae + Archosauria:

• Premaxilla with a well-developed postnarial process that excludes the maxilla from the margin of the naris. (Link to skull of Macrocnemus.)

Were Dinocephalosaurus and Tanystropheus fully aquatic?

Pros:

• Taphonomy and gut contents indicate they lived in or near marine environments and ate nektonic prey. (Spiekman et al., 2020)
• Their hindlimb were biomechanically poorly suited for striding on land.
• In Tanystropheus hydroides, the nares were retracted posteriorly and positioned on the dorsal surface of the snout.

Cons:

• The long stiff neck presents a real obstacle to agile swimming.
• Their post cranial skeletons lack strong adaptation for swimming.

For Tanystropheus, at least, two biomechanical hypotheses are current:

• They lurked near the shore and ambushed aquatic prey, similarly to a heron.
• Biomechanical analysis of hindlimb suggests they might have swam by simultaneous kicking of hindlimb like a frog, enabling them to lurk on the bottom and ambush prey from below. (Renesto and Saller, 2018)

Of course, these speculations aren't mutually exclusive.

Crocopoda:

(Later Permian - Quaternary.) Max ∇ (Trilophosaurus ~ Tanystropheus)

Named by Ezcurra, 2016, this group encompasses the major terrestrial radiation of archosauromorph herbivores and predators. Significant synapomorphies include:

• Changes in the region around the supratemporal fenestra, including:
  • development of a supratemporal fossa around anterior and lateral margins of fenestra. (See Trilophosaurus - right.)
  • transformation of posterolateral parietal into a dorsoventrally deep plate.
• Retroarticular process of jaw upturned.
• Modification of the ankle through the development of a projecting tuber of the calcaneum.

Crocopoda contains:

Allokotosauria:

(Middle (Early?) - Late Triassic) Max ∇ (Trilophosaurus, Azendohsaurus ~ other archosauromorphs) (Nesbitt et al., 2015) 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. Well known from the Late Triassic of North America, but Middle (and possibly Early) Triassic forms are known from Europe (Sues and Schoch, 2023) Freaky. In the last twelve years, related creatures have come to light including:

• The long snouted Teraterpeton Sues, 2003. A long-snouted and slightly less specialized version of Trilophosaurus that retains mediolateraly expanded teeth and a closed infratemporal region.
• Azendohsaurus: First described as a sauropodomorph dinosaur, but later revealed to be a stem archosauromorph. Shares adaptations to herbivory with Trilophosaurus (Flynn et al., 2010) although not as extreme, including multi-cusped leaf-shaped teeth and downward deflection of the posterior maxilla.
• Shringasaurus: Cow-sized horned allokotosaur of Middle Triassic India (Sengupta et al., 2017)

Synapomorphies of Allokotosauria include:

• Enlargement of the dorsal process of the jugal
• Development of a rugose lateral border of the prefrontal
• Expansion of the supratemporal fossa onto the posterolateral squamosal

Rhynchosauria:

(Middle - Later Triassic) Min ∇ (Mesosuchus, Howesia) (Dilkes, 1998)

Triassic ambulatory scissors for processing tough plant material.

• Ancestral Middle Triassic rhynchosaurs like Mesosuchus seemed normal enough.
• During a brief interval in the Late Triassic, these blossomed into a speciose radiation of the most common plant eating amniotes.

  
  

  
• Proper synapomorphies are very technical, but anatomical trends include:
  • Loss of premaxillary and anterior dentary teeth
  • Teeth present are permanently fused to bone
  • Development of maxillary tooth batteries
  • Nares confluent
  • Secondary closure of the infratemporal bar
  • Reorientation of the iliac blade to project anterodorsally.

Rhynchosauridae: Min ∇ (Rhynchosaurus, Stenaulorhynchus, Hyperodapedon) (Dilkes, 1998). Skulls of all members were extremely modified for cutting and grinding tough plant material. Cranial specializations ranged from significant in Rhynchosaurus (a stenaulorhynchine) to profound in Hyperodapedon (a hyperodapedontine.) See lateral and ventral view.

Synapomorphies of Rhynchosauria + Archosauria include:

• Subnarial foramen that is not bordered by premaxilla.
• Premaxilla slightly downturned with respect to maxillary tooth row.
• Palate reinforced by interlaced overlap of ectopterygoid and pterygoid, and ectopterygoid contact with jugal.

Prolacertidae:

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

• Parietal-frontal contact that is deeply embayed, with parietals invading the interorbital skull roof.

Potential synapomorphies with archosaurs involve the transformation of the palate:

• Premaxillae have medial shelf
• Vomers are extremely narrow
• Pterygoids remain separate for entire length
• Pterygoids have a row of fang-like teeth on anterior ramus.
• Two rows of foramina on dentary.

Also:

• Dorso-ventral reorientation of the braincase.

Archosauriformes: (Late Permian - Quaternary) Min ∇ (Proterosuchus, Archosauria)

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:

• Proterosuchidae - medium-sized terrestrial predators
• Erythrosuchidae - terrifyingly large terrestrial predators
• Proterochampsia - an odd assortment of medium-small terrestrial and aquatic predators
• Archosauria - The last common ancestor of birds and crocs, and all of its descendants.

Significant synapomorphies of Archosauriformes are not subtle:

• Antorbital fenestra present
• Pineal foramen absent
• Midline postparietal dermal element present. Link to image. See red element at left.
• Teeth serrated
• Premaxilla strongly downturned
• Reestablishment of infratemporal bar
• Mandibular fenestra present
• Paired osteoderms down the midline of the back (See illustration of Euparkeria) provide mechanical strength to trunk.

Beyond this, we see some general tendencies:

• Increased size
• Fore and hindlimbs that assume a semi-sprawling posture, in contrast to other sauropsids considered.

Proterosuchidae:

(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:

• Diastema between premaxillary and maxillary tooth rows.
• Maxilla extends past posterior orbit border

Erythrosuchidae:

(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. If you're thinking that the head is improbably large, you are not wrong. Uniquely among archosauromorphs, the heads of erythrosuchids have been shown by Bestwick et al., 2022 to scale with positive allometry.

Synapomorphies of Erythrosuchidae:

• Anterior third of maxillary tooth row abruptly upturned.
• Maxilla extends past posterior orbit border
• Loss of palatal teeth

Synapomorphy with Archosauria:

• A "triradiate" pelvis in which the pubes and ischia project anteroventrally and posteroventrally.

Eucrocopoda:

(Late Early Triassic to Quaternary) Max ∇ (Passer ~ Erythrosuchus). Derived features are concentrated in the femur, which has:

• Inturned head
• Fourth trochanter - a crest for insertion of caudofemoralis muscles.

Of many basal members, the iguana-sized Euparkeria has played an important historic role in our understanding of Archosauriform evolution, being argued to be:

• A facultative biped
• capable of an "improved" stance.

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.

Proterochampsia:

(Middle Triassic to Late Triassic) Max ∇ (Proterochampsa ~ Euparkeria, Erythrosuchus, Passer, Crocodylus) (Nesbitt, 2011)

An ecologically diverse assortment of smallish eucrocopods. Synapomorphies include:

• Nares face dorsally
• Transverse profile of snout between nares and orbits is gently rounded (as opposed to the oreinorhinal configuration of other archosauriformes.)
• Maxilla contacts prefrontal
• Tooth-bearing part of dentary ventrally curved.
• Metatarsal V lacks a hooked proximal end (reversal).

Proterchampsia is comprised of two clades:

• Proterochampsidae: (Early Late Triassic) Medium-sized fresh-water aquatic predators with flattened skulls that arguably lived as modern crocodilians do. (See Proterochampsa right)

• Dosweliidae: (Late Middle Triassic to Late Triassic) Medium-sized predators occupying a range of ecologies. Although united by technical synapomorphies like:
  • Maxilla extends posteriorly past orbit
  • Squamosal form posterior border of supratemporal fenestra,
  • Extensive square scutes, including on limbs.
  they come across as a assortment of weirdos:
  • Doswellia: A gracile terrestrial predator with an oddly-configured, boxy trunk region and long slender limbs.
  • Vancleavea: An angilliform fresh water aquatic predator with predator whose body is covered with imbricating scutes. Its skull is strongly laterally compressed and it has secondarily closed its antorbital and supratemporal fenestrae.

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

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

• Choristodera
• Archosauria

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