The Hidden Benefits of Having a Beak

4 December 2013 by Travis Park, posted in Dinosaurs, FEA, Therizinosaurs

When you utter the phrase “theropod dinosaur” most people’s response would be “Tyrannosaurus rex” or “meat-eaters” or even perhaps slightly less succinctly “the ones with the big sharp pointy teeth”. But not all theropods reinforced this stereotypical picture of the group, indeed several groups of theropods didn’t even eat meat, but were herbivorous, consuming plants for sustenance instead of other animals! Even more peculiar, some of these plant eating theropods have partially or completely lost their teeth (known as edentulism) and also possessed beaks, a feature that we would traditionally associate with modern birds.

One such group of theropods is the therizinosaurs, a strange and enigmatic clade that lived in Asia and North America during the Cretaceous period. The morphology of these unusual animals has continued to perplex palaeontologists ever since the first therizinosaur was described in 1954 as a turtle-like lizard! Some researchers have previously interpreted them to be piscivorous (fish-eating), or insectivorous (insect-eating) but they are now generally regarded to be having been plant eaters. Traditional explanations for the evolution of the beaks have been as a response to weight-saving demands for the evolution of flight. But therizinosaurs were far too large to fly, so could there perhaps be another explanation for the presence of a beak in these animals?

An examplage of a therizinosaur, Nothronychus. They really were strange creatures. Image source nhm.ac.uk.

A study published this week in the journal PNAS (Proceedings of the National Academy of Sciences) has explored just this question. The team, led by University of Bristol PhD student Stephan Lautenschlager, created a highly detailed biomechanical model of Erlikosaurus andrewsi, a therizinosaurid from the Upper Cretaceous of Mongolia and tested how the model would react when placed under various stresses and strains using an analytical technique known as finite element analysis (FEA). The study found that the beak actually had a function that had not been appreciated until now, namely, playing an important role in enhancing cranial stability by mitigating stress and strain during feeding. As Lautenschlager explains, “It has classically been assumed that beaks evolved to replace teeth and thus save weight, as a requirement for the evolution of flight. Our results, however, indicate that keratin beaks were in fact beneficial to enhance the stability of the skull during biting and feeding.”

A reconstruction of Erlikosaurus andrewsi with a beak. Image by S. Lautenschlager.

The study compared models of the E. andrewsi skull with and without the beak present and also simulated six different biting positions along the jaw and several different combinations of muscle interactions between the skull and the neck muscles. Co-author Dr Emily Rayfield, also from University of Bristol said “we were able to deduce very accurately how bite and muscle forces affected the skull of Erlikosaurus during the feeding process. This further allowed us to identify the importance of soft-tissue structures, such as the keratinous beak, which are normally not preserved in fossils.”

Comparisons of (A–L) Von Mises stress and (M–X)
displacement distribution in the different skull configurations
of E. andrewsi subjected to different bite positions.
Each of the four columns presents a different modeled
state, from left, restored skull, with small rhamphotheca,
with large rhamphotheca, with teeth added to the premaxillae
and maxillae. The rows depict different bite positions
(indicated by red arrows). Image from Lautenschlager et al 2013.

Edentulism and beaks have also evolved in other several other tetrapod lineages (e.g. ceratopsians, turtles) and, like feathers, it appears that birds have exapted them rather than them evolving as response to gaining the ability to fly. Co-author Lawrence Witmer, from Ohio University Heritage College of Osteopathic Medicine also said “Beaks evolved several times during the transitions from dinosaurs to modern birds, usually accompanied by the partial or complete loss of teeth and our study now shows that keratin-covered beaks represent a functional innovation during dinosaur evolution.”

So there you have it, plant eating theropods with beaks. Go home evolution, you’re drunk. Only joking, you’re awesome. Keep up the good work.

Also, the paper is open access, so you can get a copy of the PDF by clicking on the link below.

 

Reference

Lautenschlager S, Witmer LM, Altangerel P, Rayfield EJ (2013) Edentulism, beaks and biomechanical innovations in the evolution of theropod dinosaurs. PNAS: 1310711110v1-201310711.

 


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