#averostra

Note: Since there are no basal members of this clade, I am using basal members of it’s branches for images. Source: http://scienceblogs.com/tetrapodzoology/2009/06/19/limusaurus-is-awesome/ 

Group: Averostra 

Classification: Cellular Life, Archaea, Proteoarchaeota, Eukaryota, Unikota, Opisthokonta, Holozoa, Filozoa, Metazoa, Eumetazoa, Planulozoa, Bilatera, Nephrozoa, Deuterostomia, Chordata, Craniata, Vertebrata, Gnathostomata, Eugnathostomata, Teleostomi, Euteleostomi, Sarcopterygii, Rhipidistia, Tetrapodomorpha, Eotetrapodiforms, Elpistostegalia, Stegocephalia, Tetrapoda, Reptiliomorpha, Anthracosauria, Batrachosauria, Cotylosauria, Amniota, Sauropsida, Eureptilia, Romeriida, Diapsida, Neodiapsida, Sauria, Archosauromorpha, Archelosauria, Archosauriformes, Crurotarsi, Archosauria, Avemetatarsalia, Ornithodira, Dinosauromorpha, Dinosauriformes, Dinosauria, Saurischia, Eusaurischia, Theropoda, Neotheropoda  

Definition: All of the descendants of the most recent common ancestor of Ceratosaurus and Allosaurus 

Organisms Within: Ceratosauria, Tetanurae - note that by its very definition, there are no Averostrans that are in neither Ceratosauria or Tetanurae other than the very first one. So this is a different sort of Clade of the Week post where I talk about the features that both groups share and include examples from both of these clades, but there are no individual genera really to speak of. I don’t want to skip valid clades like Averostra and talking about their features just because they have no basal genera. 

Time Range: The hypothetical first Averostran shown below, though they may have evolved earlier than that; no earlier than around 220 MYA.  

Characteristics: Averostrans are the group of theropods that include both birds and things like Abelisaurids, but despite looking quite different (for example, most later members of Ceratosauria were primarily scaly, while Tetanurae was a very feathery clade indeed, even when you forget about birds), they do have some shared features. 

Source: https://en.wikipedia.org/wiki/Elaphrosaurus 

They have a promaxillary fenestra - meaning, an extra opening in the front outer side of the upper jaw. This made the skull lighter, which was definitely helpful in strengthening it further - all of the very large theropod predators were in this group. The lightening of the skull would also help birds in flight. 

Source: http://www.paleospot.com/illustrations/

The basal member of the group would have been primarily feathered, though many of the Ceratosaurians lost that fluffy covering. Though Ceratosaurs were also less diverse, they did last until the end of the Cretaceous, and included the weird almost armless Abelisaurs. Tetanurans, on the other hand, stiffened their tails and became very diverse, even when you don’t count the birds. 

Source: http://expeditions.fieldmuseum.org/antarctic-dinosaurs/antarctic-fossils 

Biogeography: It is very uncertain where Averostra actually originated, given that Neotheropods were very widespread by its evolution and no basal members of the clade are known; furthermore, members of Ceratosauria and Tetanurae are also very widespread. 

Posts on Ceratosauria and Tetanurae to come soon! 

Sources: 

https://en.wikipedia.org/wiki/Averostra

https://en.wikipedia.org/wiki/Ceratosauria

https://en.wikipedia.org/wiki/Tetanurae