A very general visual depiction of the clade Diapsida, which includes such well-known groups as dinosaurs (including birds), pterosaurs, crocodilians, snakes, lizards, plesiosaurs, and mosasaurs, among many others.
Coelurosauravus (meaning “hollow lizard grandfather”) by Alain Bénéteau:
“This is the cover illustration of the book I’m finishing. Coelurosauravus is a perfect critter to illustrate the diversity of time before dinosaurs. For more info about the book on sale in march 2009 please see my website paleospot”
Definition: The clade of the most recent common ancestor of Ceratosaurus and Carnotaurus, and all of that most recent common ancestor’s descendants.
Organisms Within: The clades Ceratosauridae and Abelisauroidea
Time Range: Given this is another node-based clade, we can only guess at when the earliest member of this group evolved. Since earliest known Abelisauroids are from the earliest portion of the Middle Jurassic, it stands to reason that the earliest Neoceratosaur had to evolve before this; the best guess at such is shown below.
Characteristics: This group contains all the more derived members of Ceratosauria, and they remained as weird and strangely diverse as their less derived relatives. The bulk of this group included the Abelisauroids, which ranged from the huge and tiny-armed Carnotaurus to the small and fast Noasaurids.
Neoceratosaurs were relatively medium-sized predators or smaller during the Jurassic; however, the later Abelisaurids that would inhabit mostly the Southern Hemisphere got very large and often were at the top of the food chain in their environments.
The beginning members of this group would probably have had some feathers, though as Abelisauroids evolved, the derived Abelisaurids primarily lost their fluffy covering. Furthermore, Ceratosaurids probably had osteoderms along their back, though this says nothing about the Noasaurid group, which were primarily small and thus probably retained their feathers.
Biogeography: It is very uncertain where Neocreatosauria originated, given that Ceratosaurs were very widespread and Neoceratosaurs were also; early members of Abelisauroidea are not helpful, either, as they are fairly widespread. As such, it is unlikely it will ever be determined where this group first evolved.
Posts about Ceratosauridae and Abelisauroidea to come soon.
Definition: The group including the most recent common ancestor of Ceolophysis and modern birds, and all of that animal’s descendants
Organisms Within: Zupaysaurus, Tachiraptor, Dilophosauridae (not examined here), Coelophysoidea (not examined here), Averostra (not examined here), & two miscellaneous genera without further placement.
Time Range: Shown below, numbers on the left in millions of years. Though the only basal Neotheropods known are from the Triassic-Jurassic boundary, members of other groups are present much earlier than that, about 220 million years ago, implying that they must have evolved at around that time or older.
Neotheropods differ from basal theropods mainly on a few skeletal points. The ilium bone in the pelvis, expanded towards the top of the animal (dorsally). The snout is more narrow than in basal theropods, there are more vertebrae fused to the hips, there is a complete loss of the fifth digit of the foot (as shown below), and a general stiffening of the legs overall.
Like all early dinosaurs, Neotheropods were probably covered in a layer of primitive feathers - though some theropods groups would later lose feathers, the general line leading up to birds must have kept them, given that birds have feathers. They were bipedal, fast predators, and basal members of Neotheropoda were probably outcompeted by later animals, and even during their existence Coelophysoids were far more common.
Fossil Locations: It is fairly likely that Neotheropods also originated in Argentina, during that very rapid early diversification of dinosaurs in the region. This assessment is based mainly on the presence of basal theropods mainly in the region, though its possible that it also occurred in North America, where many Coelophysoids lived. Coelophysoids have also been found in Europe and Asia, and so given this widespread range of this group and its early evolution, as well as the locations of basal theropods, its reasonable to suppose that Argentina is where they started, unless evidence to the contrary comes to light.
Biogeography: Neotheropods spread throughout the globe, as described above; this was relatively easy due to Panagaea, however, most Coelophysoids congregated around North America and Europe. The two basal members of the group stayed in South America. Maps from Dr. Christopher Scotese.
Definition: All dinosaurs more closely related to Megalosaurus than to Apatosaurus
Organisms Within: Daemonosaurus, Eodromaeus, Tawa, Herrrasauridae (not examined here), Neotheropoda (not examined here), & miscellaneous nomen dubium & not formally described genera (not examined here)
Time Range: Shown below; numbers on the left are in millions of years. It is clear that Theropods and Sauropodomorphs diverged from one another early in Saurischian evolution, and basal Theropods lasted longer in the Triassic than basal Saurischians did, though they probably went extinct (with more derived theropod lineages surviving) at the end of the Triassic.
Characteristics: Small carnivorous Saurischians, one of the two major lineages of Eusaurischia, the other being Sauropodmorphs. Theropoda comprises a wide range of dinosaurs from Coelophysis to Tyrannosaurus to Velociraptor to the modern house sparrow and all modern birds. The early ones, however, were all small gracile bipedal carnivores, with long necks and tails.
They were probably covered with protofeathers based on phylogenetic bracketing; and though they look similar to the basal Saurischians of last week, they were distinct in being a part of only the Theropoda line, and having diverged from the Sauropodomorphs. Tawa, however, may be a Coelophysoid, an even more derived Theropod group.
Fossil Locations: It appears that Theropods originated where dinosaurs did, in Argentina, which makes sense given their apparent early divergence from other Saurischians. They then migrated up through to North America.
(Colored = country has fossil sites; line points to the approximate fossil location).
Biogeography: At the time of the Triassic, all the land masses were combined into one supercontinent, the famous Pangaea. Though the major locations of the continents remained fairly unchanged over the time period these three organisms lived, each individual age is given its own map below. All maps taken from Dr. Christopher Scotese.
It appears that basal Theropods migrated up to the modern American Southwest, though later Theropod distribution covers all continents.