Monday, November 15, 2010

Averostra and Avepoda

It's been a while since my last post.  Things have been busy around here with my new computer and the holiday season starting up, butb I'll try to post some entries I recently finished.  Today we look at two of Paul's clades from DoA.

Avepoda Paul, 2002
Definition- (metatarsal I does not contact distal tarsals homologous with Allosaurus fragilis) (modified from Paul, 2002)
= Paleotheropoda Paul, 1988

Comments- This clade was first suggested by Paul (1988) as a more appropriate name for Theropoda (as their feet are birdlike as opposed to beastlike), though he recognized Theropoda could not be renamed. In his later 2002 book, Paul actually proposed Avepoda as a subgroup of Theropoda, for those "that either possessed a foot in which metatarsal I did not contact the distal tarsals, or descended from such theropods, and belonged to a clade which includes Neotheropoda." The definition is here modified by using Allosaurus fragilis as the internal specifier, as that taxon is here used as the internal specifier of Theropoda. Avepoda has almost exclusively been used by Paul, with other authors using Neotheropoda (sensu Sereno) for the group since the two usually have the same known content (though Avepoda is near certainly more inclusive than Neotheropoda sensu Sereno, unless coelophysoids were the first theropods to develop the tridactyl pes). However, Procompsognathus was an avepod possibly outside the Coelophysis+Passer clade in Paul's (1988) phylogeny and Gauthier's (1986) analysis, as was Liliensternus in the latter. More recently, Liliensternus was found to be outside the Coelophysis+Passer clade in Bittencourt Rodrigues' unpublished thesis. Avepoda also has an advantage over Neotheropoda in only having one definition, whereas the latter can also apply to the more exclusive Ceratosaurus+Passer clade. Note it is not the same as Avipoda Novas, 1992, which was proposed for a clade similar in extent to Tetanurae.

Paleotheropoda was proposed as a paraphyletic order of theropods by Paul (1988), including all theropods which were not avetheropods. This consisted of what are generally recognized today as coelophysoids, ceratosaurs and megalosauroids, as well as a few other taxa such as Piatnitzkysaurus and sinraptorids. No other reference used the term due to the general dislike of paraphyletic groups, and in 2002 Paul referred to the same grade as baso-avepods. As Paul's (1988) Theropoda was equivalent to Avepoda, Paleotheropoda is catalogued here as a synonym of that group.

Averostra Paul, 2002
Definition- (promaxillary fenestra homologous with Dromaeosaurus albertensis) (modified from Paul, 2002)
Other definitions- (Ceratosaurus nasicornis + Allosaurus fragilis) (Ezcurra and Cuny, 2007)

Comments- Paul (2002) proposed Averostra for a clade of "avepods that either possessed at least one accessory maxillary opening in the lateral wall of the antorbital fossa that led into a bony mediorostral maxillary sinus, or descended from such avepods, and are members of the clade that includes the Dromaeosauridae." This included taxa generally recognized as ceratosaurs and tetanurines, but excluded coelophysoids. The definition is here modified to use Dromaeosaurus albertensis (as the eponymous species for Dromaeosauridae) and to specify the promaxillary fenestra (as it is the first accessory maxillary opening to evolve, and the only one present in taxa Paul considers basal averostrans such as Ceratosaurus). Ezcurra has used Averostra for the ceratosaur+tetanurine clade in several papers, and in 2007 with Cuny gave it a new node-based phylogenetic definition with that extent- "Ceratosaurus nasicornis, Allosaurus fragilis, and all the descendants of their common ancestor." However, Paul's apomorphy-based definition may not be limited to that clade. Promaxillary fenestrae have since been identified in Dilophosaurus, Zupaysaurus, "Megapnosaurus" kayentakatae, herrerasaurids and Heterodontosaurus. Unfortunately, the phylogenetic position of most of these taxa is controversial, making it difficult to determine which clade an apomorphy-based Averostra designates. In a more traditional topology where coelophysoids are ceratosaurs and/or Dilophosaurus and/or Zupaysaurus are basal coelophysoids, Averostra encompasses at least Neotheropoda sensu Sereno. In alternative phylogenies where Zupaysaurus and/or Dilophosaurus are closer to birds than to Coelophysis, Averostra encompasses at least the former two taxa and perhaps Coelophysoidea (ambiguous since Megapnosaurus and Coelophysis lack promaxillary fenestrae). As derived ornithischians, sauropodomorphs, Eoraptor and Tawa all lack promaxillary fenestrae, it's not certain whether those of herrerasaurids and/or Heterodontosaurus are homologous to theropods'. In a phylogeny like Sereno's where herrerasaurids are sister to avepods (and presumably Tawa), Averostra may encompass the herrerasaurid+avepod clade. No standard topology (where sauropodomorphs and Eoraptor are closer to avepods than ornithischians are) results in Heterodontosaurus' promaxillary fenestra being homologous to theropods' though. With these caveats in mind, Averostra is here provisionally placed at the level of Avepoda.

References- Gauthier, 1986. Saurischian monophyly and the origin of birds. Memoirs of the Californian Academy of Sciences 8, 1-55.
Paul, 1988. Predatory Dinosaurs of the World. Simon and Schuster, New York. 464 pp
Paul, 2002. Dinosaurs of the Air. The John Hopkins University Press, Baltimore and London. 460 pp.
Ezcurra and Cuny, 2007. The coelophysoid Lophostropheus airelensis, gen. nov.: A review of the systematics of "Liliensternus" airelensis from the Triassic-Jurassic outcrops of Normandy (France). Journal of Vertebrate Paleontology. 27(1), 73-86.
Bittencourt Rodrigues, 2010. Revisao filogenetica dos dinossauriformes basais: Implicacoes para a origem dod dinossauros. Unpublished Doctoral Thesis. Universidade de Sao Paulo. 288 pp.


  1. Interesting discussion, Mickey. Didn't realize Averostra (which was clearly intended as an apomorphy-based alternative to something like Bakker's Neotheropoda) had such an ambiguous definition.

  2. Avepoda: "This clade was first suggested by Paul (1988) as a more appropriate name for Theropoda (as their feet are birdlike as opposed to beastlike), though he recognized Theropoda could not be renamed."

    I had thought that Avepoda was first mentioned in PDW too. I don't have PDW handy though.

  3. Paul (1988) wrote- "...the bird foot- in which the inner cannon bone does not reach the ankle- is especially distinctive. It is not proven to appear anywhere else, and may have evolved only once, so it is a good minimum definition of the Theropoda. (In this regard, the name Theropoda, which means beast-footed, is most inappropriate. Avepoda would be much better, but it is too late for that.)"

    So he mentioned it, but didn't actually propose it as a name until he decided to try it for a theropod subgroup in 2002.

  4. Thanks. Mind if I post this on the DML? I can leave your name out if you like. :-)

  5. No problem, I've been following the DML thread.