The DAWNDINOS team has published a new study, about how the thigh bone (femur) of Triassic archosaurs varied according to locomotor habits (quadrupedal/bipedal) and body size (from smaller to larger). We found striking convergent evolution between pseudosuchians (= crocodile lineage) and avemetatarsalians (= dinosaur lineage) in the Late Triassic period!
The study, published in Journal of Anatomy, was led by Romain Pintore, PhD student at CNRS/MNHN in Paris, FR and former Research Assistant on the DAWNDINOS project, along with Alexandra Houssaye from the GRAVIBONE project (CNRS/MNHN), Sterling J. Nesbitt (Virginia Tech, USA) and John R. Hutchinson (RVC, UK). We used 3D imaging, high precision measurements and statistics to analyse the shapes of archosaur femora with a level of precision that was never reached before. The full article from Journal of Anatomy can be freely accessed here: https://doi.org/10.1111/joa.13598
Archosaurs were always diverse in their locomotor abilities, to the point where this diversity was even suggested to be pivotal in the evolutionary success of dinosaurs vs. pseudosuchians during the Triassic-Jurassic transition. In modern animals, locomotor diversity usually leads to various limb specializations that directly correlate with their locomotor habit and body mass. For example, in living mammals from mice to elephants we see limb bone shapes that are more suited to running or to supporting weight. We say that bones get functionally constrained by locomotor abilities through the process of natural selection; their shapes must be properly suited to what the animals do in their environment. You can read further about this here (https://dawndinos.com/leg-bone-differences-between-lightly-and-heavily-built-archosaurs/) and here (https://dawndinos.com/new-dawndinos-studies-of-archosaur-locomotion/).
We applied this concept to study 72 femora from 36 species of Triassic archosaur(iform)s and identify what femoral features (of 3D shape) varied the most accordingly to locomotor habit and body size. Our focus on these factors is important because they were central in the evolutionary history of archosaurs.
The first step of this project was to digitise 3D scans of fossil femora. Romain Pintore travelled in the southern part of the USA to scan more than 100 specimens using a photogrammetric approach. You can read about this very fun step of the project here (https://dawndinos.com/my-american-adventure-by-romain-pintore/). We then located hundreds of anatomical points (i.e., “landmarks”) on every femur to characterise their 3D shapes, which we then compared using statistical approaches.
We found that body size was the main biological factor related to the shapes of femora of early archosaurs (horizontal axis on Figure 1). Small archosaurs had long and thin (i.e., “gracile”) femora whereas large archosaurs had wide ends in relate to the total length of the bone (A on Figure 2). Locomotor habit was the second most important factor (vertical axis on Figure 1). Bipeds had a small angle between the two bone ends (i.e., “offset”) and a more curved femur, whereas quadrupeds had a large offset between the two ends and a straight femur (B on Figure 2).
Figure 1: Shape variation of all the femur bones, with increasing body size along the horizontal axis (“PC1”) and locomotor habits (bipedal = negative; quadrupedal = positive) along the vertical axis (“PC2”).
Figure 2: Femur shape variation with A) increasing body size (left: smaller to right: larger); B) locomotor habit (left: bipedal to right: quadrupedal) in side and bottom views.
Interestingly, we found that the larger archosaurs were, the more evident their bipedal or quadrupedal locomotor habit was (Figure 1), as judged by the 3D shape of their femur. So body size and locomotor habit were closely related in archosaurs. This relationship was even clearer when looking at the feature that varied the most: the fourth trochanter (i.e., an attachment point for a major leg-retracting muscle coming from the tail). The fourth trochanter’s shape (“F.t” on Figure 2 and 3) changed from A) flatter to completely rounded with smaller to larger archosaurs (A on Figure 3); and B) symmetrical to asymmetrical with quadrupedal to bipedal locomotor habit (B on Figure 3).
Figure 3: Red vectors indicate the greatest variations with A) increasing body size and B) locomotor habit (quadruped to biped). In both cases, the fourth trochanter (“F.t”) is always the feature varying the most, but in different ways.
However, we found that specialization to body size occurred similarly in both lineages of archosaurs (pseudosuchians and avemetatarsalians), without first evolving in their common ancestor (Figure 1). This phenomenon is known as convergent evolution and represents valuable knowledge to scientists because it can obscure the evolutionary history of lineages, giving false impressions that they are closely related, when actually (as we show here) the similarity is due to specialisations to supporting smaller/larger body size against gravity. Our findings also agree with other recent findings that the morphological diversity of Triassic avemetatarsalians was not as low as was previously thought (Figure 1), which is relevant to re-evaluating the Locomotor Superiority Hypothesis.
Finally, we used the 3D shape variations linked to known locomotor habits to estimate the most probable locomotor habit in archosaurs whose locomotor habit was uncertain. We found that our estimations gave a 93% success rate when performed on fossils with already known locomotor mode! For example, the shape of the femur of Silesaurus suggests that it was bipedal (https://dawndinos.com/silesaurus-opelensis/), which is surprising and deserves further investigation. However, some mysteries remained, such as for the enigmatic Postosuchus (https://dawndinos.com/postosuchus-2/)and Riojasuchus (https://dawndinos.com/riojasuchus-tenuisceps/).
Our study should help the DAWNDINOS team, and hopefully other scientists, to better understand the patterns of extinction and survival among pseudosuchians and avemetatarsalians during the Triassic-Jurassic transition and whether locomotion played a role in the success of dinosaurs.