Monsters of Homology II: Archosauria and the folly of counting on fingers

Who are the archosaurs? There are two major node-based groups.

As their names indicate, synapomorphies of these groups have to do with features of their ankles and feet. Interesting because these features predispose members of both groups to evolve toward erect posture. At right: Crurotarsan (left) and avemetatarsalian (right) ankle schematics highlighting two proximal tarsals, the astragalus (A) and calcaneum (C).

In the case of Crurotarsi the calcaneum (heel) takes the form of a lever for the rotation of the foot on the shin. That action works most efficiently when the stance is more nearly erect.

In avemetatarsalians, the ankle joint is linear and there is no calcanea lever.

Crurotarsi took over land ecosystems during the Middle and Late Triassic only to suffer greatly in the Late Triassic extinction event, with only the lineage that gave rise to Crocodylia surviving.

Crurotarsan superlatives:

Phytosauria: (Late Triassic)

Fresh-water aquatic predators, superficially similar to living crocodilians, but different in detail. Locate the nares here, for instance.

Ornithosuchus by D Bogdanov from Wikipedia
Ornithosuchidae: (Late Triassic)

Big predators.

Typothorax by ahless28
Aetosauria: (Late Triassic)

Fasolasuchus by Kahless28
"Rauisuchian-grade" crurotarsans: (Middle - Late Triassic) A paraphyletic group of top terrestrial predators of the Late Triassic.

Extinction The terminal Triassic really was the end of an era. For the last 30 million years, rauisuchians, aetosaurs, and various non-archosaurian archosauromorphs had ruled the roost, (accompanied by occasional small mammal-like cynodonts, dinosaurs, pterosaurs, and the first turtles.) Then came a significant extinction event. The precise cause is not known, although the Central Atlantic Magmatic Province (CAMP), a first-class flood basalt associated with the opening of the Atlantic Ocean dates from that time.

From Crurotarsi, only Crocodylomorpha survived:

Crocodylomorpha: (Late Triassic - Recent) Animals more closely related to Crocodylia than to any rauisuchian-grade crurotarsan. Although living members are large freshwater predators, the earliest forms were small, long-legged, and terrestrial like Pseudhesperosuchus (Late Triassic).
Avemetatarsalia (Triassic - Recent): Archosaurs more closely related to birds than to crocodilians. The primary avemetatarsalian groups are speciose and enduring, with great significance for the history of science: Outside of these groups, the fossil record of Avemetatarsalia is limited to a small handful of revealing creatures.
Synapomorphy - the naive 20th century view: The ankle joint runs between the proximal and distal tarsals (right). (In contrast to the "crocodile-normal" setup in Crurotarsi (left) in which the calcaneum rotates around a peg in the astragalus.)

Sigh. Life used to be simple, then facts intruded in the form of new animals like Teleocrater. It shares subtle technical characters with more derived members of the bird stem, but it's ankle is crocodile-normal! That, apparently, is the plesiomorphic arrangement for Archosauria.

But that doesn't alter the central biomechanical theme of this group's evolution - the functional decoupling of fore and hindlimb function for bipedalism or powered flight. This functional decoupling enabled the fore and hindlimbs to evolve into different forms.

Sceromochlus taylori by T-PEKC from DeviantArt
A handful of small (mockingbird - bluejay sized) basal avemetatarsalians are Late Triassic (but pre-extinction event) age and give a general picture of the ancestral state for members of this group:

Pterosauria: Flying reptiles of the Mesozoic. NOT BIRDS. NOT DINOSAURS.


Both synapomorphies were for support of the wing membrane.

The membrane stretched from tip of finger either to torso or to hind limb. The wing was radically unlike that of either a bat or a bird in terms of skeletal structure.

Histology of wing surface: Unlike the bat wing, which is essentially normal skin, the pterosaur wing is covered with dense parallel fibers (probably collagen). Rather than stretching like the membrane of a bat wing, this surface more likely folded like a collapsable fan. The wing membrane was also invested with this slips of muscle.

Pterosaurs are known from Late Triassic through the end of the Cretaceous and on all continents except Antarctica (they are probably there too). Best finds from are from:

"Note on the Pterodactyle Tribe
Considered as Marsupial Bats" 1843,
by E. Newman from Strange Science
Since there is no living analog to pterosaurs, and since in many cases their anatomy is poorly known, it is difficult to know how they lived. Indeed, mid-twentieth century paleontologist Alfred Romer declared them to be the most enigmatic of fossil vertebrates. Certain issues, however, are coming into focus:

Evolutionary trends: During pterosaur evolution, various groups have developed many interesting features. In this review, we sample a very primitive and a rather derived pterosaur.

Sordes pilosus by D. Bogdanov from Wikipedia
Sordes: (Late Jurasic) represents the primitive condition. Like it, many other basal pterosaurs had

Although paraphyletic, it has sometimes been convenient to refer to this grade-group as "rhamphorhynchoids."

Pterodaustro by m Shiraishi from Jurassic Gallery
Pterodactyloids: (Late Jurassic - Cretaceous)

Pterodaustro (Cretaceous): Has many characters acquired in the general course of pterosaur evolution that characterized the monophyletic Pterodactyloidea:

Link to a composite comparison of the "rhamphorhynchoid" and pterodactyloid conditions.

Eudimorphodon, a "rhamphorhynchoid" from Dinopedia (left) Tropeognathus, a pterodactyloid from (right).
Pterodactyloid skulls are also highly derived with respect to "rhamphorhynchoids."

Darwinopterus by from Impact Lab
Throughout the history of pterosaur research, the morphological gulf between rhamphorhynchoid and pterodactyloid pterosaurs seemed unbridgeable. Suddenly in 2009 we learned of a true intermediate: Darwinopterus, a creature with the head and neck of a pterodactyloid and the legs and tail of a rhamphorhynchoid. A good example of mosaic evolution, in which one part of the body evolves faster than others.

Monsters of homology: There is a problem with our understanding of pterosaurs, however. The actual identity of the elements of the hand and wrist are debated. The debate focuses on the question of what happened to digit V?

In any scenario, pterosaurs are missing a manual digit. For a review, see Unwin et al. 1996. This problem foreshadows an even bigger problem with digit homology in dinosaurs.

And now for what you have yearned for...Dinosaurs!

Dinosauria: (Triassic - Recent) The most recent common ancestor of Megalosaurus and Iguanadon, the first known dinosaurs, and all of its descendants. In this course, we will not address dinosaurs in any depth, but we do note the following:

Eoraptor lunensis by Charlie McGrady from CM Studio
Ancestral state: The earliest and most phylogenetically basal dinosaurs were bipedal and small - (turkey - beagle sized). E.G.:

Indeed, Dinosauromorpha (Triassic - Recent) - the clade containing critters closer to dinosaurs than pterosaurs - is populated at its base by roadrunner-sized bipeds like Marasuchus and Lagerpeton (Late Triassic). Thus, it whas long assumed that the last common ancestor of dinosaurs was a small biped. Imagine our surprise in 2003 when Silesaurus (Late Triassic) - a retriever-sized quadruped - was described and found to be represent the sister group of Dinosauria. After this description, many unidentifiable remains in museum drawers were found to belong to related animals, such that Silesauridae is now a speciose group. But of dinosauromorph diversity, only proper Dinosauria survived the Terminal Triassic extinction event.

Huayangosaurus from
  • Ornithischia: (Latest Triassic/Earliest Jurassic (?) - Recent) Plant eating dinosaurs, including: Saurischia (Triassic - Recent) which breaks down into:


    For about forty years, dinosaur biology has been the subject of contentious, unusually public, and often irrational debate centering on activity levels and thermal metabolism; but often conflating this with such issues as origins of flight or the position of birds within Dinosauria. We bypass it for the moment, noting that similar debates have raged (at saner volume-levels) about the other major avemetatarsalian group: Pterosaurs. One observation: We see both fully developed feathers and tubular feather-like fuzz in bird-like theropods, but also stiff tubular bristles in small ornithischians. Maybe the last common ancestor of dinosaurs had some kind of insulating body coating. Could feathers be homologous to pterosaur pycnofibers? Stay tuned!

    Phylogeny headache:

    It used to be so simple. From the 19th Century forward, dinosaur workers acknowledged two major clades:

    Saurischia had two components:

    People debated whether Dinosauria was monophyletic, but no one questioned Ornithischia and Saurischia. But the problem: There are no unambiguous Triassic ornithischians. Why, if they are the basal branch?

    Then came 2017, a year of confusion.

    Yikes. Imagine a strict consensus of these three hypotheses.

    Monsters of homology: Counting theropod fingers

    This is a fundamentally different type of monstrosity. To compare:

    Duplication: In discussing euryapsids we mentioned in passing that when you start multiplying elements like phalanges (right) or cervical vertebrae, keeping track of traditional homology becomes meaningless. For example, the various phalanges of digit I in the derived plesiosaur D each, in a way, partake of the identity of the single digit I phalanx of the ancestral eosauropterygian A, even though they all fail the conjunction test. What remains homologous is the developmental process by which phalanges are generated.

    Identification: In discussing snakes, ichthyosaurs (right), and pterosaurs we encountered the problem of there being no test of homology, even similarity, that we could use effectively to get at the identity of certain elements.

    Shifting identities: The hands of theropod dinosaurs pose a new and different challenge. Background: Generations of paleontologists have recognized a trend in theropod evolution toward the reduction of fingers. Consider:

    It seems so clear: Theropods go from a five fingered hand to a three fingered hand by the sequential loss of digits V and IV, whereas digit I is specialized and readily recognizible as a thumb.

    Now the monstrosity:

    Remember the developmental sequence of endochondral bone:

    Now consider:

  • Cleared and stained specimen A shows the condensation sequence of normal tetrapod fingers in an alligator. We note: The first condensation is digit IV, followed by digits V, III, II, and I.

  • The ostrich (B) shows four digits. Digit V never chondrifies, but digit I never condenses at all, leaving digits II, III, and IV as ossified fingers in the adult. And yet the fossil record has been telling us that birds have digits I, II, and III. WTF?

    Can it be that the identity of digits is totally different in birds and theropod dinosaurs?

    The discovery of this discrepancy in the late 1990s gave rise to two general responses:

    Vindication: Limusaurus inextricabilis, 2009: A moderately basal theropod (more derived than Coelophysis, less derived than Velociraptor) came to light with what almost looks like a three-fingered hand, complete with a specialized thumb, except that a small metacarpal I is also present, but not part of the "thumb" digit. This is entirely consistent with what we would expect from an animal at an evolutionary stage at which the "frame shift" was in progress. The identity of the thumb has been transferred to mesenchymal condensation II, but condensation I still becomes slightly ossified.

    The strange lesson: You can't assume that the homologies you identify at one developmental stage of an element will be transferred to the next developmental stage. The pattern-generating mechanisms of development might actually shift!

    The issue remains controversial, but it appears that homologies are developmentally fluid!