Dinosaurs have been extinct for millions of years, yet we continue to be enamoured of them.

The legacy of dinosaurs isn’t just found at the movies, however, as there are several descendant species that currently inhabit the earth. Present-day lizards and birds inherited scaly skin and modified beaks from dinosaurs.

However, missing among these remnants of dinosaur ancestry are their magnificent weaponized tails. This caught the attention of Dr. Victoria Arbour, a postdoctoral fellow at U of T and the Royal Ontario Museum, and Dr. Lindsay Zanno, an Assistant Research Professor at North Carolina State University.

The pair recently published a study that investigated why certain amniotes — tetrapod vertebrates like reptiles and birds — no longer have bony spikes, focusing on turtles in particular.

Although turtles are one of the closest relatives to dinosaurs, they lack the rear defences that their ancestors once had. According to the study, this is partly because of various morphological and ecological constraints that might limit the bony tail weaponry from being passed down in amniote evolution.

The researchers found that this loss was a result of sexual and natural selection, specifically pertaining to locomotive, feeding, or defensive traits.

Bone formations of tail weaponry involve modifications to the stiffness of the tail toward the end of its attachment, its width toward the centre, and the presence or absence of spikes at the end of it.

Arbour and Zanno classified tail weaponry into two categories: tail clubs and tail flails. Instead of these structures, turtles hide their tail ends in dermal armour.

Amniote species like lizards and pangolins are known to use tails as an antipredator defence. For most of these amniotes, the use of tails as weapons is a last resort and usually used for vigilance or escape. The Agama lizard species are the only living creatures that use the tail for male-to-male combat.

Due to the high cost and risk of direct combat, the selective pressures favouring this behaviour were found to be weaker than other antipredator strategies. In addition, tail lashing as a defensive strategy requires such traits as having a large body size and thoracic stiffness, which are largely absent in present-day amniotes.

Arbour and Zanno made use of several museum collections to gather data needed for the study and stressed the importance of using fossils in this type of research. Of the species they analyzed, 164 of 286 were extinct species known only from fossil records.

“Including fossils into big picture studies of evolutionary patterns is important because there are many cases where animals known only from fossils have anatomical features that aren’t present in modern day animals,” said Arbour in reference to the weaponized tails.

The pair hopes to continue to use fossils to study the descendants of dinosaurs. One avenue for further research, for example, is to study the evolutionary connection between glyptodonts, an extinct family of armadillos, and ankylosaurs, a heavily armoured dinosaur.

“Some of the next steps that Lindsay Zanno… and I are taking are to look at how similar glyptodonts and ankylosaurs really are – trying to measure the strength of convergent evolution in these two groups, and see if glyptodonts evolved tail clubs in the same stepwise pattern that ankylosaurs did,” wrote Arbour.

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