October 3, 2016:

by Dave Hone

A bizarre new extinct reptile with a domed skull of solid bone has been unearthed in Texas. If this sounds familiar, it could be because you have heard of a group of dinosaurs called the pachycephalosaurs that possessed very similar characteristics. One could almost call Triopticus a mimic were it not for the fact that it dates to 228-220 million years ago, meaning that it predates the pachycephalosaurs by over 100 million years. Moreover, Triopticus is one of numerous animals from this period (the Late Triassic) that were in some way copies of other reptiles that evolved later.

Triopticus is a small animal – the preserved dome of the skull is only around 5 cm long even though it is from an adult animal, but what there is of it is very unusual. There is a large pit in the skull that resembles the eye sockets of reptiles and gave rise to the animal’s name, as Triopticus means “three eyes”. This hole does not represent an extra eye, however, but may simply be a result of the surrounding bones having enlarged and expanded leaving this space behind, rather than there being a bit missing.

Aside from the difference of this divot, comparisons to the pachycephalosaurs are more than superficial. Both have greatly enlarged domes of solid bone that sat at the back of the head above the brain, both show some extra bumps and bosses, and both even show some similarities in the microstructure of the bone. Although the rest of Triopticus is missing, it is hard not to suggest that these animals may have bashed heads with one another as the pachycephalosaurs are thought to do (although this is not covered in the paper). Such similarities of form between only distantly related organisms is termed “convergent evolution” and there are numerous examples of this in the fossil record and alive today (think of the hydrodynamic shapes of fish, dolphins and penguins).

However, Triopticus and a number of the reptiles that lived alongside it show some remarkable convergences with other reptiles that came later, and most notably the dinosaurs. In the Late Triassic there were various animals showing adaptations and body plans that will be familiar to those who have browsed even children’s books on dinosaurs. There were bipedal plant eating reptiles similar to the ornithomimosaurs, herbivorous forms with leaf-shaped teeth covered in armour like the later ankylosaurs, large-headed reptiles with sharp teeth that looked like predatory dinosaurs (if on four legs rather than two), and even long-snouted semi-aquatic animals that resembled extinct and even living crocodilians.

These “pairs” are already known to palaeontologists, but an analysis of skull and body shapes shows how similar animals were to each other in the ages before the dinosaurs and others diverged, and then later how similar these various different forms became, despite their fairly distant relatedness. It’s notable that Triopticus is a particular outlier, being even more distant from the ancestral form that the pachycephalosaurs – it has quite an extreme set of anatomical features.

That convergent evolution is rampant within some groups is not big news, but the sheer range of extinct reptile species that ended up taking on similar forms (and often more than once) is a reminder of the selective pressures that evolution can bring to some lineages. Even so these are typically limited to classic ecological features like specialised teeth for eating or claws for digging, so modify the skull in such a shape more than once as seen here is quite a surprise and one hopes that more will come to light in the future. It will certainly be interesting to see if the rest of Triopticus matches the thick-headed dinosaurs in any other areas.