Though once considered a contentious subject, most recent authors have agreed that dinosaurs represent a monophyletic group. However, a paper just released today (Parera et al., 2010) analyses the most extensive selection of reptilian taxa to date and turns the current scenario on its head, not only finding strong support for dinosaur (and particularly theropod) polyphyly but calling for significant changes to our images of Mesozoic reptiles. The failure of earlier studies such as Mortimer (2004) to recognise theropod polyphyly seems due simply to their failure to include enough taxa.
Animals traditionally regarded as 'dinosaurs' are placed by Parera et al. in three separate places in the reptile family tree. Ornithischians, sauropodomorphs and most theropods are in their usual place as sister to the clade including modern crocodiles and Parera et al. restrict the name 'Dinosauria' to this clade. However, dromaeosaurids and oviraptorosaurs clade with birds separately from the main dinosaur clade. Instead, this new clade (which Parera et al. simply refer to as 'Aves') appears as sister to the Triassic rhynchosaurs. While, to the best of my knowledge, such an arrangement has not been suggested before, it does not seem entirely incredible - for a start, both birds and rhynchosaurs have well-developed beaks.
The most surprising result of all, perhaps, is that compsognathids and therizinosaurs are also not dinosaurs. Instead, they form part of a large clade also including ichthyosaurs, plesiosaurs, pterosaurs and lepidosaurs for which Parera et al. resurrect the long-disused name Gryphi. This further corroborates the recent demonstration by Lingham-Soliar et al. (2007) that the supposed 'protofeathers' found in the compsognathid Sinosauropteryx are in fact collagen fibres from a thick insulating dermal layer. The supposed 'melanosomes' described from Sinosauropteryx by Zhang et al. (2010), cited as further evidence for a 'protofeather' interpretation of the fibres, are shown by Parera et al. (2010) to instead represent the eggs of skin-burrowing parasites that afflicted this poor individual.
The additional presence of preserved dermal fibres in ichthyosaurs and pterosaurs suggests that such a dermal layer was an ancestral feature of the Gryphi. The clade appears to have been ancestrally aquatic - aquatic lifestyles have been proposed previously for compsognathids by Bidar et al. (1972) and for pterosaurs by Wagler (1830), and Parera et al. further demonstrate the accuracy of these interpretations. The ancestors of lepidosaurs, found by Parera et al. to be sister to pterosaurs, must have at some point left the aquatic habitat and consequently lost their insulating dermal layer. However, perhaps the aquatic iguana of the Galapagos Islands represents the sole remnant of the Gryphi's long marine history?
Bidar, A., L. Demay & G. Thomel. 1972. Compsognathus corallestris, nouvelle espèce de dinosaurien theropode du Portlandien de Canjuers (Sud-est de la France). Annu. Mus. Hist. Nat., Nice 1: 1–34.
Lingham-Soliar, T., A. Feduccia & X. Wang. 2007. A new Chinese specimen indicates that ‘protofeathers’ in the Early Cretaceous theropod dinosaur Sinosauropteryx are degraded collagen fibres. Proceedings of the Royal Society of London Series B - Biological Sciences 274 (1620): 1823-1829.
Mortimer, M. D. 2004. The phylogeny of Neotetanurae (Theropoda, Dinosauria). Annals of the Museum of Natural History at the University of Ohio at
Springfield 2004: 1-369.
Parera, A. S., H. M. Whio & T. V. Pari. 2010. A comprehensive analysis of reptile phylogeny demonstrates theropod polyphyly, with notes on the life habits of compsognathids and pterosaurs. Transactions of the Royal Society of Hull 238 (1): 4-136.
Wagler, J. G. 1830. Natürliches System der Amphibien. München, Stuttgart, Tübingen.
Zhang, F., S. L. Kearns, P. J. Orr, M. J. Benton, Z. Zhou, D. Johnson, X. Xu & X. Wang. 2010. Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature 463: 1075-1078.
in The Biology Files