The new Crato Formation enantiornithine

You blog readers are so fickle. Write several thousand words on a spectacular group of carnivorous mammals that have never previously been the subject of any sort of semi-popular review, and get bugger all attention. Write about 50 words and post two pictures, and - hey - the whole world goes nuts. Anyway, thanks to everyone who had a guess on the 'mystery' fossil. Some of you were pretty close to the mark, but evidently no-one who commented has checked Naish et al. (2007), the bird chapter of Martill et al.'s CUP volume The Crato Fossil Beds of Brazil (previously discussed in this article on tapejarid pterosaurs), as all is revealed therein...

As usual, I will admit that identifying this specimen from the photos I've posted is difficult as they are low-res and small, and many of the important details just aren't visible. To begin with, the specimen is from the Brazilian Crato Formation, and hence is probably late Aptian in age. What we appear to have is a partial skull (apparently preserved in palatal view), cervical, dorsal and caudal vertebrae (one with a chevron attached), a partial hindlimb with an articulated foot, and a partial ilium and possible ischium. The specimen includes slab and counterslab, and some parts missing from the main slab (like most of the dorsal vertebrae) are present on the counterslab. Feather impressions are visible, as is a large reversed hallux on what is clearly the foot. The specimen is small, with a total preserved length of 156 mm. Here's a labelled version...

These features all suggest that it's a bird: we now know that feathers and small size aren't unique to birds, but a large, reversed hallux is. Several detailed features of the vertebrae support this identification: the neural spines are longer at their apices than they are at mid-height, the centra of the dorsal vertebrae bear lateral excavations that extend for much of the centrum length, and the parapophyses on the dorsals are located mid-way along their length [close-up of the dorsals shown below, from the counterslab]. These characters are all present in enantiornithines, and specifically in euenantiornithines (Chiappe & Walker 2002, Naish et al. 2007). The enantiornithines were a widespread and apparently successful Mesozoic bird clade, closer to crown-group birds than were archaeopterygids or confuciusornithids. They included arboreal forms as well as terrestrial, flightless forms, gull-like generalists, and even long-footed waders and what might have been specialised auk-like divers [image at top shows a nice model of the Spanish Lower Cretaceous enantiornithine Iberomesornis romerali, from wikipedia].

While several feathers have been described from the Crato Formation, skeletal remains attributed to theropods hadn't been reported prior to this*. So this is almost certainly a new taxon, and in fact it possesses a character (two sub-rounded concavities on the lateral surface of one of the caudal vertebrae) that appears unique and is apparently diagnostic. Why then didn't we name it? Well, we have a problem. We never saw this specimen (the original is in a private Japanese collection), and based all our observations on photos. Publishing observations on photos alone is not good practise and, furthermore, some of us are strongly opposed to publishing observations on specimens not deposited within public museums. That sounds all very well and good, but it doesn't seem right to exclude significant data such as this, so long as permission to use it is provided. It's a tricky one however and people argue about this subject a lot.

* The younger Santana Formation (I follow Martill & Heimhofer (2007) in regarding the Santana and Crato as separate formations) has yielded several theropods, including the spinosaurid Irritator and the coelurosaurs Mirischia and Santanaraptor (Naish et al. 2004). Another Santana coelurosaur is in the process of being described.

There are a few other things worth saying while I'm here. The Crato bird seems to have long vaned feathers on its tarsometatarsus. That seems odd, but it now appears that other Mesozoic birds - including archaeopterygids (Christiansen & Bonde 2004) and some Chinese enantiornithines (Zhang & Zhou 2004) - had tarsometatarsal feathers, as did some non-avian maniraptorans like microraptorians and Pedopenna [shown here: from Xu & Zhang (2005)]. Also interesting is that the diversity of feather types in the Crato Formation hints at a mostly unknown theropod assemblage, with the most interesting specimen being a possible rectrix, 85 mm long, covered in tiny eggs. Originally suggested to be parasitic mite eggs (Martill & Davis 1998, 2001), they might have been laid by ostracods, as these apparently use floating feathers and similar objects as laying substrates.

UPDATE (added 7-7-2008): I hadn't realised when I wrote the above that Andrea Cau, at Theropoda, has also been writing about new Gondwanan enantiornithines and has just devoted two articles to the newly named Lebanese taxon Enantiophoenix electrophyla (this bird was first described in 2002 (Dalla Vecchia & Chiappe 2002) but went unnamed in the initial paper). For Andrea's articles go here and here.

The sea monsters are coming...

Refs - -

Chiappe, L. M. & Walker, C. A. 2002. Skeletal morphology and systematics of the Cretaceous Euenantiornithes (Ornithothoraces: Enantiornithes). In Chiappe, L. M. & Witmer, L. M. (eds) Mesozoic Birds: Above the Heads of Dinosaurs. University of California Press (Berkeley), pp. 240-267.

Christiansen, P. & Bonde, N. 2004. Body plumage in Archaeopteryx: a review, and new evidence from the Berlin specimen. C. R. Palevol 3, 99-118.

Dalla Vecchia, F. & Chiappe, L. M. 2002. First avian skeleton from the Mesozoic of northern Gondwana. Journal of Vertebrate Paleontology 22, 856-860.

Martill, D. M. & Davis, P. G. 1998. Did dinosaurs come up to scratch? Nature 396, 528-529.

- . & Davis, P. G. 2001. A feather with possible ectoparasite eggs from the Crato Formation (Lower Cretaceous, Aptian) of Brazil. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen 219, 241-259.

- . & Heimhofer, U. 2007. Stratigraphy of the Crato Formation. In Martill, D. M., Bechly, G. & Loveridge, R. F. (eds) The Crato Fossil Beds of Brazil: Window into an Ancient World. Cambridge University Press (Cambridge), pp. 25-43.

Naish, D., Martill, D. M. & Frey, E. 2004. Ecology, systematics and biogeographical relationships of dinosaurs, including a new theropod, from the Santana Formation (?Albian, Early Cretaceous) of Brazil. Historical Biology 16, 57-70.

- ., Martill, D. M. & Merrick, I. 2007. Birds of the Crato Formation. In Martill, D. M., Bechly, G. & Loveridge, R. F. (eds) The Crato Fossil Beds of Brazil: Window into an Ancient World. Cambridge University Press (Cambridge), pp. 525-533.

Xu, X. & Zhang, F. 2005. A new maniraptoran dinosaur from China with long feathers on the metatarsus. Naturwissenschaften 92, 173-177.

Zhang, F. & Zhou, Z. 2004. Leg feathers in an Early Cretaceous bird. Nature 431, 925.