Monsters of Homology III: Avemetatarsalia and the folly of counting on fingers
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:
- Pterosauria: (Triassic - Cretaceous) The flying reptiles of the Mesozoic
- Dinosauria: (Triassic - Rec) The dominant land vertebrates of the Jurassic and Cretaceous, but which includes Aves, the birds, who continue to the present.
Synapomorphy: The ankle joint runs between the proximal and distal tarsals (right). (In contrast to the setup in Crurotarsi (left))
The central biomechanical theme of this group's evolution, however, is 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
- Scleromochlus: (right) Very small with long limbs, disparate fore and hind limb length, and a very large head, either the basal member of Avemetatarsalia or the sister taxon of pterosaurs.
Marasuchus by Paleoaeolos from Deviant Art
- Marasuchus (Lagosuchus of some authors) Also tiny (right), represents a handful of creatures that were closer to dinosaurs than to pterosaurs.
Pterosauria: Flying reptiles of the Mesozoic. NOT BIRDS. NOT DINOSAURS.
- Elongate finger - generally thought to be the fourth
- Pteroid bone - generally thought to be a specialized carpal
Both synapomorphies were for support of the wing membrane.
- In birds, the distal caprals, metacarpals and phalanges of digits I, II, and III fuse to form a support for flight feathers.
- In bats, digits II through V are elongate and support a flexible membranous wing.
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:
- The Late Jurassic of Germany (same time and place as Archaeopteryx, the first known bird-like theropod dinosaur)
- The Early Cretaceous of China
- The Middle Cretaceous of Brazil.
"Note on the Pterodactyle Tribe
Considered as Marsupial Bats" 1843,
by E. Newman from Strange Science
- Active metabolism and flight: The body proportions of pterosaurs are generally similar to those of actively flying birds. Although some pterosaurs are very large and probably adapted for soaring, in the manner of modern pelagic birds, most are small, E.G. the wide-mouthed nightjar mimic Anurognathus, well within the size range of modern birds.
The flight apparatus of pterosaurs indicates the presence of powerful, bird-like flight muscles. Indeed, the pterosaur sternum (breast-bone) was similar to that of a bird in size.
Sordes pilosus from Paleocritti
- Fur? We now have pterosaurs including Sordes and Jeholopterus whose fossils seem to be covered with a fur-like integument. Note: Just because it looks superficially like fur does not mean that it was histologically identical. The congruence test definitely tells us that it could not be homologous with the fur of mammals. Because they are non-homologous, these structures are termed pycnofibers. Nevertheless:
- If its function is analogous, then it would appear that pterosaurs were actively seeking to prevent heat exchange with the environment, not promote it, as cold-blooded creatures do.
- Could pycnofibers be homologous to feathers? That is more ambiguous.
Anurognathus from Wikipedia
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
- short necks
- short wrists
- long tails, with rudders at the ends.
- rear membrane stretched between legs and supported by specialized fifth digits. Soles of feet point medially. The tail is free of the membrane.
Pterodaustro (Cretaceous): Has many characters acquired in the general course of pterosaur evolution that characterized the monophyletic Pterodactyloidea:
- long neck
- short tail
- long wrists
- small to extremely large size. Indeed, Quetzalcoatlus, the largest known flying animal (or was it?) was a pterodactyloid.
- Unique scapular attachment to the axial skeleton. In large pterosaurs, whose wings bore all of their body weight while flying, an energy saving strategy was developed: The Notarium. This allowed the axial skeleton to be supported effortlessly by the pectoral girdle.
- Conquest of the ground: No known pterodactyloid has a uropatagium stretched between its hindlimbs. Moreover, all known pterosaur trackways seem to have been made by pterodactyloids. Perhaps, "rhamphorhynchoids" didn't get around well on land. From trackways and biomechanical models, we have a good idea of the stance that pterodactyloids would assume on the ground. (Earlier notions of sprawling or bipedal pterodactyloids are obsolete.)
- Taking off: How pterosaurs launched themselves from a flat surface is a hot topic currently. Arguably, "rhamphorhynchoids" couldn't, but instead dropped from a high perch, bat-style. Pterodactyloids are thought to have used their forelimbs to vault into the air.
Eudimorphodon, a "rhamphorhynchoid" from Dinopedia (left) Tropeognathus, a pterodactyloid from Paleofile.com (right).
- Nares become confluent with antorbital fenestrae.
- In many, teeth are reduced
- Many develop elaborate crests or cranial ornamentation. (E.G. Pteranodon, Tapejara, and the extreme Nyctosaurus.)
Darwinopterus by from Impact Lab
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?
- Scenario I: The long wing-bearing digit is digit IV, and the pteroid bone is a modified carpal.
- Scenario II: The long wing-bearing digit is actually digit V, and the pteriod bone is metacarpal I - the metacarpal of the first digit.
- Scenario III: The pteroid bone is a new derived element and has no bearing on the identity of the fingers.
And now for what you have yearned for...Dinosaurs!
Dinosauria: 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
- Eocursor (Late Triassic) - an ornithischian
- Saturnalia (Late Triassic) - a sauropodomorph saurischian
- Eoraptor (Late Triassic) - a theropod saurischian (right)
Indeed, Dinosauromorpha - 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 Terakoshi.com
- Armored dinosaurs (E.G. Huayangosaurus right and Edmontonia)
- Horned dinosaurs large and small.
- Sauropodomorpha: Long necked plant eaters, such as Plateosaurus (right) and Supersaurus.
Sinosauropteryx by Julius Csotonyi
- Theropoda: Bipedal predators large and small such as:
- Sinosauropteryx (right)
- Caudipteryx (note, feathers are real in all cases)
Plateosaurus by Raul Martin
- Early dinosaurs appear near the middle of the Triassic, but only become common after the Late Triassic extinction. Indeed, prior to this, only the herbivorous sauropodomorphs achieved any great size. They were the only herbivores of the time to be able to browse well above the ground. Their competition, rhynchosaurs, dicynodonts, and aetosaurs were short-necked.
- By the Early Jurassic, they had become both the dominant land predators and herbivores.
- Bird-like theropods such as the well-preserved Archaeopteryx appear in the late Jurassic.
- The K-T extinction exterminates all dinosaurs except proper crown group birds. (I.e. some VERY birdlike flying theropods bought it.)
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:
- Alligator: a crurotarsan.
- Herrerasaurus: an ancestral theropod.
- Coelophysis: an early theropod.
- Velociraptor: a derived theropod.
- Columba: a bird.
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.
Remember the developmental sequence of endochondral bone:
- Mesenchyme cells clump together as condensations
- Cartilage is laid down at the sites of the condensations
- The cartilage is then ossified into bone.
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:
- To opponents of the idea that birds are theropods, it was a vindication, suggesting that the similarity between the bird and theropod hand (including the specialized thumb) was convergent and not synapomorphic. (Ignoring the fact that phylogenetic analyses place birds within theropoda even if you remove hand characters.) Creationists have jumped on this band-wagon.
- Paleontologists (notably Wagner and Gauthier, 1999) and developmental biologists reacted with a more far-reaching idea: That a "frame shift" had occurred in which the developmental identity of digit I had been switched to the mesenchymal condensation that would typically become digit II. This frame shift, they argued, was analogous to the issues of identity surrounding euryapsid phalanges or snake vertebrae. In the absence of positive evidence, that idea was a big stretch that met with much skepticism, but not crazy.
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!
- As mesenchyme, birds and three fingered theropods seem to have elements homologous to condensations for digits II, III, and IV.
- As fully formed bones, these elements are homologous to digits I, II, and III.