Birds have paedomorphic dinosaur skulls

A new paper by Bhullar et al. (2012) proposes that birds had paedomorphic dinosaur skulls.  They retained juvenile traits as adults. This seems quite reasonable. They also had paedomorphic bodies, the smallest of all dinosaurs. Not sure if that’s mentioned in the paper or not. I haven’t read it.

Pairs of archosaur skulls demonstrating ontogenetic changes.

Figure 1. Pairs of archosaur skulls demonstrating ontogenetic changes. Young ones on the left. Adults on the right. The greatest disparity appears to be in crocs, but the youngest Coelophysis and Archaeopteryx are unknown and not shown. Compare the juvenile Coelophysis to the adult Archaeopteryx for rostral length and eye size. This is allometric growth. From. Bhullar et al. 2012.

Allometric vs Isometric Growth
Crocs and dinos experience(d) allometric growth. The proportions of the skull change(d) during ontogeny (growth, maturity). Mammals, with their cute features as juveniles experienced the same sort of growth. All three are new archosauromorphs.

Move over to the other side of the reptile tree and you have pterosaurs. The embryos and juveniles are virtual copies of adults, only smaller. This is termed isometric growth. However, among living new lepidosauromorphs, turtles and lizards, allometric growth still rules. What spurred the development of isometric growth? It’s hard to say. It has nothing to do with maternal care, because cute baby turtles are on their own.

Juvenile fossils are rare.
Immature specimens are rarities, but close to pterosaurs we have Huehuecuetzpalli, a tritosaur lizard, known from a single adult and juvenile, both proportionally identical according to Reynoso (1989). Other smaller vs. larger tritosaurs, including freaky, long-necked Tanystropheus purports to have distinct juveniles with a shorter rostrum and multi-cusped teeth, but these traits distinguish species and genera, not adult and juvenile. The closest sisters of the smaller Tanystropheus are smaller still and have a shorter rostrum. One of these is tiny Cosesaurus. Others include a variety of Langobardisaurus and Tanytrachelos, all considered adults.

Tiny Pterosaurs
Many, but not all, of the tiniest pterosaurs have a short rostrum and large eyes. Traditionally these traits have labeled them juveniles, but no attempts have been made to match juveniles to adults phylogenetically — except here. And here is where tiny pterosaurs with a short rostrum have been linked to larger pterosaurs also with a short rostrum. Note: not all tiny pterosaurs had a short rostrum!

No doubt the tiny scaphognathid descendants of larger scaphognathids had a shorter rostrum and traditional juvenile features. Since hatchling and embryo pteros were identical to adults, these paedomorphic traits must be retentions of embryonic, not juvenile traits. Just a genetic time shift is all that is necessary to make this happen.

Tiny Birds
The Smithsonian blog reports, “the skulls of young birds are anatomically almost identical to those of adults.” Hmmm. How interesting with regard to arboreal vertebrates finding convergent ways to go — if true. But let’s picture a baby chick, eaglet or ostrich compared to an adult and immediately this fantasy of isometric growth vanishes. The Archaeopteryx teenager (Fig. 1) is far from being a hatchling. The blog reports, “As a new Nature study by Bhart-Anjan Bhullar and collaborators suggests, this feature of bird life can be traced back to ancient transformations that effectively locked bird skulls into a permanent juvenile anatomy.” 

Let’s test this:
As a test Google: “hatchling stork,” “hatchling hummingbird,” and “hatchling pelican.” They ALL have a short rostrum — unlike the embryo Pterodaustro and the JZMP embryo ornithocheirid. So, not all birds experience isometric growth. These were very much like the alligator (Fig. 1) in their allometric growth.

Speaking of Dinosaurs…
There are some adult dinos with a short rostrum and big eyes. I direct your attention to MassospondylusDaemonosaurus and their plant-eating descendants. All adults and I’ll wager their rostrum was not appreciably shorter as a hatchling.

Nature carves out its own rules. We have to find them.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Bhullar B, Marugán-Lobón J, Racimo F, Bever G, Rowe T, Norell M and Abzhanov A 2012. Birds have paedomorphic dinosaur skulls Nature DOI: 10.1038/nature11146
Reynoso V-H 1998. Huehuecuetzpalli mixtecus gen. et sp. nov: a basal squamate (Reptilia) from the Early Cretaceous of Tepexi de Rodríguez, Central México. Philosophical Transactions of the Royal Society, London B 353:477-500.

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