Saturday, June 20, 2009
A Jurassic bird from China helps clarify theropod digital homologies
A new article printed in Nature on Thursday (Xu, et al. 2009) describes a Jurassic bird, pushing back the origin of birds almost 80 million years, Protoavis notwithstanding. More importantly, the new animal demonstrates that avian digital homologies have existed for just as long, and that, therefore, theropod dinosaurs cannot have been the ancestors of birds. The digits of theropod dinosaurs—especially maniraptorans, who are most often cited as likely avian ancestors—display a pattern of lateral digit reduction (LDR). In the most basal theropods, all five digits were present, though digit V was reduced to a bony splint. Later in theropod evolution, digit IV was lost as well. In all known tetanurine theropods, the remaining digits are, unquestionably, digits I-III (thumb, index, and middle fingers). This pattern of digit loss is at odds with most other animals, especially mammals, who tend to reduce their digits based on bilateral digit reduction (BDR). Were this the case in theropods, we would expect to see tetanurine theropods with digits II-IV (digits I and V would be lost).
Paleontologists have been at a loss to explain how theropods with digits I-III evolved into birds, which retain digits II-IV (based on embryology studies). The Hox genes are usually blamed for the switch, but genetic correlates do not fossilize well. Paleontologists have further attempted to discredit the studies which demonstrate that birds develop digits II-IV, insisting that the microscopic limb and digit buds in bird embryos cannot be accurately seen, and thus, perhaps birds really do develop digits I-III. This author thinks they doth protest too much, and that it is far more parsimonious to believe that, in fact, the ancestors of birds must have gone through a period of BDR, while theropods, who exhibit LDR, are not suitable ancestors for birds.
The new fossil is unfortunately named Limusaurus by the authors (who wear their bias on their sleeves), who consider it a ceratosaur. Ceratosaursia is a primitive group of theropod dinosaurs including such familiar forms as Ceratosaurus and Carnotaurus. These are big, toothy predators with robust features and powerful jaws. Limusaurus, by contrast, is a very small bird with long legs built for fast running; extremely reduced, splint-like arms; a toothless mouth covered in a rhamphotheca (beak); a single, fused sternal plate; and a gastric mill implying herbivory. These are all features indicative of an avian classification for Limusaurus. What’s more, the presence of a large singular sternal plate suggests that Limusaurus’ ancestors were flighted. Given its suite of basal and advanced features, Limusaurus may, in fact, be a basal paleognathid, obviously more derived than the tinamous, but could be an extremely basal ratite. For these reasons, I believe a name change is in order. To reflect the true phylogenetic nature of the animal, I recommend that Limusaurus be changed to Limuornis.
The authors correctly note that Limuornis retains a stub of digit I in the form of a single rounded metacarpal. This is not surprising in basal bird, and it is even less surprising to see that digit IV is also extremely reduced, though not the same extent, in a flightless animal. Digits II and III, though small, are fully-formed and possess small, blunt claws. The entire arm is very short, reflecting the condition seen in modern ratites. In some ways, the arms are vestigal, but they were probably used in courtship or aggression displays. The long legs and small, raised hallux suggest that Limuornis was strongly cursorial and did not fly or use its arms for prey capture. In fact, prey capture was not probable anyway—a large gastric mill of stones suggests that Limuornis was an herbivorous animal or, at the very most, omnivorous as in modern ratites. As stated above, Limuornis’ finger formula is II-III-IV, like modern birds but unlike theropod dinosaurs. More interestingly, the particular phalangeal formula of Limuornis’ fingers are incompatible with theropod hands. Whereas the tetanurine theropod hand formula is 1-2-3-x-x (metacarpals not included), the formula of Limuornis is x-3-3-x. Even if theropods did have fingers II-III-IV, their phalangeal formula is unlike Limuornis, so hand morphology is again incompatible. A reduction in manual phalanges is not unexpected given Limuornis’ flightless condition.
Despite its many derived features, Limuornis retains a few strikingly basal characters, including the presence of a long tail and a triradiate pelvis. Though incomplete, the tail is probably as short or shorter than in Archaeopteryx. While it may seem strange that a basal bird would retain its tail, keep in mind that a variety of basal flying birds, including Archaeopteryx and Jeholornis have long tails, and almost all flightless Mesozoic birds, including dromaeosaurs and troodontids, also have long tails—in some cases longer than Archaeopteryx. A long tail is obviously primitive for Aves, and it may have been lost multiple times within the group, including here, in paleognathes. The triradiate pelvis is a bit more difficult to explain, but, like the long tail, is probably primitive for Aves. Modern ratites have a pelvis that is not unlike Cretaceous flightless birds, though the pubis is more backswept. If a long tail was lost multiple times among birds, can we not say the same for the triradiate pelvis? This structure is potentially primitive for paleognathes as well, and the pubis retrovated to make room for a larger gut as herbivory became more important to the ratite diet. This would explain the differences between ratite pelvic girdles and those of flighted birds—the quadraradiate pelvis is merely a case of parallelism. Notably, the back half of the iliac blade in Limuornis is surprisingly similar to the same structure in emus and ostriches.
Limuornis is an exciting discovery which suggests that crown-group birds originated sometime in the Triassic period or, at the latest, the Early Jurassic. Incompatible finger formula notwithstanding, it is clear that the theropod dinosaurs that lived during the Late Triassic and Early Jurassic are horrible candidates for avian ancestry. They are either too large and ungainly (Dilophosaurus, Herrerasaurus) or too unspecialized (Eoraptor). While it remains possible that birds and dinosaurs share an arboreal common ancestor, I believe that Limuornis demonstrates that birds evolved prior to theropod dinosaurs, and that the two must have evolved from highly disparate groups of tetrapods.