The bones of dinosaurs have told countless tales about their origins and behaviour, but dinosaurs left behind more than just their skeletons. As they walked about, they made tracks, and some of these also fossilised over time. They too are very informative and a new set, made by some of the dinosaurs’ closest relatives, reveals how these ruling reptiles rose to power at a leisurely pace.

Dinosaurs evolved during the Triassic period from among a broader group called the dinosauromorphs. These include all dinosaurs as well as their closest relatives, species like Lagerpeton and Lagosuchus that only just miss out on membership in the dinosaur club. Fossils of these latter animals are extremely rare and only ten or so species are well documented.

Their tracks, on the other hand, are more common. To Stephen Brusatte­­ from the American Museum of Natural History, they presented a great opportunity for understanding how early dinosaurs strode into power. Brusatte writes that “footprints are an often ignored source of data” because they are often poorly preserved and difficult to assign to a species or time. However, they can also turn up in environments that haven’t been so kind to bone, and they are often found more frequently.

Dinosauromorph tracks have been uncovered all over the world, from Argentina to Poland, and it’s the latter country where Brusatte has found the oldest set so far. Together with Grzegorz Niedzwiedzki and Richard Butler, he uncovered three sets of prints in Poland’s Holy Cross Mountains.

It has been difficult to study dinosauromorphs because there haven’t been enough fossils and the lack of fossils has made it hard to work out which tracks were theirs. On top of that, many tracks haven’t been preserved very well, fossil tracks look different depending on the surfaces that their maker walked in, and working out when tracks were made is very tricky.

But according to Brusatte, the Polish tracks rise above those problems thanks to a combination of fortunate location and perfect timing. Because they’re in central Europe, they can be easily compared to a sequence of Triassic rocks, whose ages are very accurately known. That told Brusatte when the tracks were made. And in recent years, enough dinosauromorph fossils have been discovered to tell us what features define the bodies, feet and prints of this group. That told Brusatte that these tracks were clearly made by walking dinosauromorphs.

According to their shape and spacing, their makers’ legs were held underneath their bodies rather than sprawled to the side.  They weren’t flat-footed but walked on their toes. Their metatarsal bones were bunched together rather than splayed out. The outer two toes were much smaller and the middle three, which were almost parallel to one another. These features are common to dinosauromorphs and absent in other reptiles. Best of all, they’re found in all three sets of tracks, even though they came from areas with different surfaces,  suggesting that their distinctive shape reflects the feet of their makers, rather than the features of their environment.

Mike Benton (who once supervised Brusatte but wasn’t involved in this study) says, “I think the argument is sound.  Many might argue you can never be absolutely sure about the identity of a track-maker, and so it’s a bit of a leap to go from tracks to makers to rewriting dinosaurian history.” However, he points out that Brusatte has identified “only derived (advanced) characters clearly seen in the prints” rather than just going on “general resemblance”.

The track-makers clearly walked on four legs, but their front limbs were much shorter than the hind ones. And at least one set of dinosauromorph tracks was made by a two-legged walker. It seems that many of the species were on the cusp of developing the two-legged stance of the first dinosaurs.

More importantly, the tracks suggest that the dinosauromorphs arose much earlier than previously thought. The earliest skeletons hail from around 242 and 244 million years ago, but the oldest of Brusatte’s tracks data back to between 249 and 251 million years ago, at the earliest part of the Triassic.

Benton says, “If this is all true, then we don’t have to look for the origin of dinosaurs in the late Triassic, say 230 million years ago, but in the early Triassic, say 245+ million years ago… But more important is that this would put the initial origin of dinosaur-like beasts into the immediate aftermath of the devastating end-Permian mass extinction, and so make them part of the great recovery of life from that devastation – not before suspected.”

Indeed, Brusatte’s data suggest that the dinosauromorphs evolved in a geological heartbeat after the greatest mass extinction of all time, a cataclysmic event “when life nearly died”. It seems that the dinosauromorphs were among the first pioneers of life’s greatest comeback although, as Brusatte writes, “This hypothesis, however, clearly demands further study.”

However, it would take many millions of years for these pioneers to make their presence truly felt. The areas around Brusatte’s new tracks are littered with footprints, and only two or three out of every hundred were made by dinosauromorphs. The rest were largely made by crurotarsans, another lineage of reptiles that dominated the Triassic.

In an earlier study, Brusatte showed that crurotarsans were outcompeting the ascendant dinosaurs for several 30 million years and were only supplanted after a big extinction event at the end of the Triassic. His new tracks support that idea. Triassic Poland was crurotarsan country, with dinosaur ancestors in the minority. After the first dinosauromorphs appeared, it took some 40 to 50 million years for their dinosaur descendants to dominate the land. As Brusatte writes, “The dinosaur radiation was a drawn-out affair.”

Reference: Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2010.1746

Image by Donna Braginetz

More on dinosaur evolution:

• Do new discoveries ever “rewrite evolutionary history”?
• Rise of dinosaurs down to luck not superiority
• Tianyulong – a fuzzy dinosaur that makes the origin of feathers fuzzier
• Raptorex shows that T.rex body plan evolved at 100th the size
• First birds were poor fliers – flaps would have buckled Archaeopteryx feathers

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October 5th, 2010 Tags: Brusatte, dinosauromorph, footprints, Permian, tracks, Triassic
by Ed Yong in Dinosaurs, Evolution, Palaeontology | 4 comments | RSS feed | Trackback >