New Dinosaur Takes Part in an Island Evolutionary Dance

Dinosaurs come into view only slowly. Complete, or even near-complete, skeletons are incredibly rare, and our understanding of what a given dinosaur was like in life is based on a shifting bed of new fossil finds and analyses. And while they might not be as likely to make headlines, scrappy dinosaurs are still vitally important to ongoing investigations into the diversity and evolution of the Mesozoic celebrities. The partial remains test ideas and raise new questions about what we thought we already understood. The latest of these fractional dinosaurs, described by Andrew Farke and Joe Sertich this week in PLoS One, is Dahalokely tokana.

In terms of dinosaur superlatives, Dahalokely isn’t in the running for the classic titles of biggest, fiercest, or weirdest. Based upon the scattered vertebrae and rib pieces recovered from this animal, Farke and Sertich the bipedal dinosaur was modestly-sized at about eleven and a half feet long. And while no skull material has yet been found, Dahalokely probably chased after prey smaller than itself. The details of the Dahalokely bones indicate that this creature was an abelisauroid – a widespread group of carnivorous dinosaurs.

But the importance of Dahalokely isn’t so much in the dinosaur’s anatomy – of which we know relatively little as yet – but in the new geographical and evolutionary context the dinosaur provides.

As Farke recounts in his post on the new dinosaur, the discovery of Dahalokely was the result of a targeted effort to find dinosaurs on the island of Madagascar that are geologically older than those that have been discovered before. Previous efforts on the island have turned up a rich 70 million year old dinosaur fauna – including the big abelisaurid Majungasaurus, the armored sauropod Rapetosaurus, the snaggletoothed Masiakasaurus, and the small, bird-like Rahonavis – but more ancient dinosaurs are mostly known from hard-to-identify fragments. Fortunately, Sertich found the first bones of what would eventually be named Dahalokely in 2007, and helped pick up the rest of what remained in 2010. And as Farke and Sertich have now described, the bare bones pulled from that 94-89 million year old rock came from a critical time that set the stage of later dinosaur evolution on Madagascar and India.

Madagascar is a tatter of a landmass that has split and split again since the Cretaceous. About 130 million years ago, what is now mainland Africa began to split from adjoining landmasses, with what is now Madagascar and India still fused to prehistoric Antarctica. Thirty million years later, the Madagascar-India chunk broke from Antarctica, with Madagascar and India ultimately being ripped apart by about 88 million years ago.

Within the choreography of this continental dance, Dahalokely took the stage about 90 million years ago – while Madagascar and India were together, before the isolation of those pieces created living laboratories where dinosaur evolution diverged. And according to the analysis of Farke and Sertich, Dahalokely was more archaic than Majungasaurus, yet still had features shared among later abelisaurid dinosaurs on both Madagascar and India.

There’s no evidence that Dahalokely was directly ancestral to later dinosaurs on either landmass. And more than the current smattering of bones will be needed to further test and investigate exactly where the dinosaur fits among other abelisauroids. Nevertheless, Dahalokely might represent the general type of abelisauroid that existed on the joined Madagascar-India island before the two split.

If Farke and Sertich are right, Dahalokely – or something like it – set the stage for the later evolution of these blunt-nosed, stubby-armed predators as their traveling refuge split and the pieces continued to drift during the latter part of the Cretaceous. Dahalokely has become another reference point for how abelisauroid evolution unfolded over the course of millions of years, outlining how the “splendid isolation” of islands spurs evolution to bloom in fantastic ways.

For more on this dinosaur, and the journey from the field to publication, see Farke’s companion post on DahalokelyFarke’s companion post on DahalokelyFarke’s companion post on Dahalokely.

References:

Farke, A., Sertich, J. 2013. An Abelisauroid Theropod Dinosaur from the Turonian of Madagascar. PLoS ONE. 8, 4: e62047. doi:10.1371/journal.pone.0062047