As far as archives go, the fossil record is a flawed one. There’s plenty that we’re missing and can never be replaced. That’s simply the nature of how prehistoric life comes down to us, pointed out by Charles Lyell and chorused by Charles Darwin back in the 19th century. And even if we had every fossil and mapped every formation, we’d still only have snippets of the story of life - enough to learn some amazing things, but always blind to all those individuals and species that just didn’t happen to be fossilized. But it’s not all a downer. Within the fossil-bearing rocks actually laid out before us, paleontologists can simultaneously detect gaps and also point out where new finds might be made.

Take plesiosaurs, for example. This is the group that paleontologists Samuel Tutin and Richard Butler chose for their new analysis of just how complete the record of these marine reptiles really is. By compiling data from 178 specimens of 114 plesiosaur species throughout their temporal range in the fossil record, focusing on the completeness of individual skeletons and how that differs, the researchers were able to outline our knowledge of these quad-paddled swimmers.

Just speaking from the standpoint of discovery, this is an excellent time for plesiosaur finds. While the first plesiosaur discoveries added up relatively slowly – and the fragmentary nature of early skeletons led to the naming of species no longer considered valid – the late 1870s saw a boost from the Bone Wars, Tutin and Butler write, with additional finds happening here and there through the 20th century. In the 1990s, however, exploration of new localities in China, South America, and elsewhere led to an explosion of novel discoveries, and the greater interest in the group turned up additional species in classic spots, too. The fossil boom isn’t just restricted to dinosaurs – plesiosaurs are also being discovered and described at a breakneck pace, too.

The two forms of plesiosaur analyzed in this study. Credit: Tutin and Butler 2017

But what about the fossil record itself? For example, previous research on ichthyosaurs – the shark-like “fish lizards” that evolved and thrived alongside plesiosaurs – detected that there’s a negative correlation between the completeness of ichthyosaur skeletons and sea level. That is, the best and most complete ichthyosaur skeletons seemed to come from times of relatively low sea levels. Tutin and Butler suspect this might be the case for plesiosaurs, as well, which not only will influence comparisons between plesiosaurs but may help highlight places where finding relatively complete plesiosaurs skeletons is more likely.

Even so, the plesiosaur fossil record looks pretty good compared to animals that trod about on land or flew through the air at the same time. “Median completeness values for plesiosaur and ichthyosaurs were significantly higher than in contemporary terrestrial groups,” Tutin and Butler write, which means that we have a better picture of what marine reptiles of the Mesozoic were like than the many dinosaurs who come to us as scrappy skeletons. 

How many more plesiosaurs are out there to be discovered, no one can really say. This is true both for individual specimens or for species, given the plesiosaur species count depends on the scientific philosophy of how taxa are distinguished from each other. But the record presented to us, though they are snapshots of different times and places, has given us plesiosaurs in wonderful detail, and this time of intense exploration shows no sign of slowing down. If ever plesiosaurs had a scientific heyday, this is it. 

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Tutin, S., Butler, R. 2017. The completeness of the fossil record of plesiosaurs, marine reptiles from the Mesozoic. Acta Palaeontologica Polonica. doi: 10.4202/app.00355.2017