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.

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.

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.

• Dimorphodon: from the Early Jurassic represents the primitive condition. Like it, many other pterosaurs had
  • short necks
    
  • short wrists
    
  • long tails, with rudders at the ends.
  • tail-membrane stretched between legs and supported by specialized fifth digits (note: illustration is wrong). Soles of feet point medially.

  Although paraphyletic, it has sometimes been convenient to refer to these as "rhamphorhynchoids."

Pterodactyloids: Derived pterosaurs (Late Jurassic - Cretaceous)

Pterodaustro (Cretaceous)

• long neck
  
• short tail
  
• long wrists
  
• small - large size.
  
• 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:
  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.

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:

• Ancestral state: The earliest and most phylogenetically basal dinosaurs were bipedal and small - (turkey - beagle sized). E.G.:
  • Eocursor - an ornithischian
  • Saturnalia - a sauropodomorph saurischian
  • Eoraptor - 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. Thus, it has long been assumed that the last common ancestor of dinosaurs was a small biped. Imagine our surprise in 2003 when Silesaurus - a retriever-sized quadruped - was described and found to be represent the sister group of Dinosauria.
  

• Diversity: Major dinosaur groups include:
  
  
  • Ornithischia: Plant eating dinosaurs, including duckbills, armored dinosaurs (E.G. Huayangosaurus right), and horned dinosaurs large and small.
    
  • Saurischia which breaks down into:
    
    • Sauropodomorpha: Long necked pland eaters, such as Plateosaurus (right) and Supersaurus.
      
    • Theropoda: Bipedal predators such as Sinosauropteryx (right), Concovenator, Yutyrannus, Caudipteryx (note, feathers are real in all cases), Microraptor, and birds.
      

• Chronology:
  
  • Early dinosaurs appear near the middle of the Triassic, but only become common after the Late Triassic extinction. Indeed, prior to this, only th 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.)

• Of late, 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.

Monsters of homology - II: Theropoda

In discussing euryapsids we mentioned in passing the fact that when you start multiplying elements of the body 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.

The hands of theropod dinosaurs pose a similar challenge. 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.

Now the monstrosity:

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 joined 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.

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 mesenchymal condensations, birds and three fingered theropods seem to have elements corresponding to digits II, III, and IV.
• As fully formed bones, they have elements corresponding to digits I, II, and III.