It’s a Hard-Knock Life for an Ichthyosaur
Bone is a fascinating thing. It holds our bodies up, supports our organs, allows us to move, gesture, and talk, and even provides our muscles with the necessary calcium and phosphate to function and obtain energy. But bones can be brittle; they can’t withstand everything we throw at them (sometimes literally), and they can break. For anyone, like me, who has broken a bone or two (or five!), we count ourselves lucky that bones can also heal themselves. And this is not just through the miracles of modern medicine; bone has the capacity to repair itself, and has done so for millions of years.
We know this because we see the evidence written all over dozens and dozens of fossils. Sue the T. rex is a quintessential example, with numerous broken and subsequently healed ribs, vertebrae, and limbs, as well as evidence of infection and disease. Paleontologists love examples like Sue, because they illuminate things we can’t otherwise see in the fossil record (outside of trace fossils)—they give us evidence of a life lived. Pathologies can tell us about competition among organisms, they can tell us who was prey (or a predator), and ultimately they tell us who survived a traumatic event and lived to tell the tale through their bones. It’s like showing off your scars and bragging about the crazy events in your life, but through the fossil record instead.
I’ve talked about paleopathology on the blog before (looking at you, Dilophosaurus!), and now I want to highlight a new study published last week in PLOS ONE, by authors Judith M. Pardo-Pérez, Benjamin P. Kear, Heinrich Mallison, Marcelo Gómez, Manuel Moroni, and Erin E. Maxwell. Their study focuses on a more understudied group, when it comes to paleopathology—ichthyosaurs.
Speficially, Pardo-Pérez et al (2018) examined 39 specimens of Temnodontosaurus, an Early Jurassic ichthyosaur from southern Germany. The goal of this study was to provide an atlas of pathological evidence in large ichthyosaurs that can be used and translated across other taxa.
The study is thorough, even going through specific examples that are not pathologies; for example, some broken bones do not show evidence of healing; the authors contribute these to post-mortem taphonomic processes such as scavenging, erosion, compression, etc.
However, the key evidence of pathological bone modification is fiber remodeling or callus development; clear evidence that the organism survived the traumatic event and healed.
Of the 39 specimens the team examined in collections throughout Germany, 21% showed osteological pathologies. Some individuals contained multiple pathologies within different regions of the body. When breaking down the anatomical placement of pathologies among specimens included in this study, 23% were found in the skull region, followed by dorsal ribs (21%) and pectoral girdle and forefins (11%). Most of the pathologies observed are attributable to simple trauma with evidence of healing, rather than infectious or articular disease.
This study highlighted a reporting bias in previous studies of Temnodontosaurus. Previous studies have mostly noted only broken ribs, whereas according to Pardo-Pérez et al (2018), pathologies are present throughout the body and skull.
Pardo-Pérez et al. (2018) gives insight into what might have caused the pathologies observed in Temnodontosaurus. For example, one specimen, UM-O no. 4 shows some evidence of bone trauma and healing in the premaxilla and dentary, something that has been observed in other marine animals, such as the plesiosaur Pliosaurus. The authors suggest that a misalignment of the upper and lower jaws could have caused some occlusal stress that lead to this injury. Another specimen, UM-O no. 14, shows a similar pathology but with evidence of infection, leading the authors to believe that this ichthyosaur may have suffered from an abscess.
Other specimens, however, indicate that they may have been victim to an attempted attack. One specimen (SMNH 15950) shows ten circular areas separated by a few centimeters on its snout, clearly indicating that another large marine reptile, possibly another ichthyosaur or a crocodylomorph like Steneosaurus, attempted to take a bite out of this fella.
But what about the fractured ribs? Well, Pardo-Pérez et al. (2018) suggests several possible explanations. Obviously, one could assume that ichthyosaurs might have aggressive confrontations with other ichthyosaurs for a number of reasons—mating, niche competition, territory, etc. But other, less considered, options have been suggested by other studies. It could be possible that the ichthyosaur breached itself on a reef, or collided with a reef or rock. It’s also been suggested by previous studies that changes in atmospheric pressure as the ichthyosaur dove deep into the sea could lead to broken ribs. However, Pardo-Pérez et al. (2018) reject this last idea, noting that the lack of physiological evidence (avascular necrosis) suggests that these ichthyosaurs were not partaking in any abyssal adventures.
What is confirmed to be low in number are pathologies attributable to joint disease, which are more common in other aquatic organisms, particularly in the vertebral column. Plesiosaurs, mosasaurs, and even cetaceans all indicate a higher rate of infections in the vertebral column compared to ichthyosaurs like Temnodontosaurus. The difference in distributions of this type of pathology may have other implications with regard to types of movement among these animals. Likewise, avascular necrosis, a type of pathology that would indicate these organisms were moving into deeper depths, are also absent in this study. The authors note only one example of avascular necrosis that was reported in a specimen of Temnodontosaurus from England, so if this is truly the case, the geographic differences may indicate preservational differences, or more significantly may indicate different lifestyles in geographically distinct populations of Temnodontosaurus.
Whatever the case, ichthyosaurs had their own shares of battles in the Jurassic seas, and thankfully, we have their bones that bear the scars and tell the tales.
Reference: Pardo-Pérez JM, Kear BP, Mallison H, Gómez M, Moroni M, Maxwell EE (2018) Pathological survey on Temnodontosaurus from the Early Jurassic of southern Germany. PLoS ONE 13(10): e0204951. https://doi.org/10.1371/journal.pone.0204951
Featured image: Temnodontosaurus trigonodon. (GPIT/RE/1491/13), Complete skeleton in dorsal (skull) and ventral view (postcranium). From Pard0-Pérez et al. (2018), CC-BY.