Geisterjagd in der späten Trias: Auf der Suche nach den letzten riesigen Ichthyosauriern

First of all, good morning everyone, and thank you for the nice introduction. And thank you all for being so numerous. My name is Tim Lamsus, I'm a master's student at the University of Bonn in the field of Organismic, Evolutionary and Paleobiology. And I'm very happy to be able to pick you up for a few minutes and take you on a journey into the late Triassic and to take you on a search for the spirits of the last giant Ichthyosaurs. Maybe a little bit about Ichthyosaurs in general. The Ichthyosaurs are a group of fully marine tetrapods. In other words, tetrapods that inhabited ancestral land and then made their way back into the water. Very similar to what the Plesiosauria and the Mosasauria later in the Mesozoic but also in the Cenozoic made our modern whales. The Ichthyosaurs were an extremely successful group that inhabited this land for more than 155 million years until they died out in the early Cretaceous 94 million years ago. The earliest remains of Ichthyopterygia, that is, Ichthyosauria, and basal relatives that we know come from the late sub-Triassic. A newspaper has just been published last week that refers to the origin of these animals and the earliest finds from 250 million years ago. In any case, these early representatives are still quite small forms that have already adapted to the marine environment at this point in time. For example, to fin-shaped extremities and a streamlined body. Now the Ichthyosauria do not necessarily remain as small for very long because already 3 to 5 million years after the appearance of this early Ichthyopterygia we already find the first giants of this group, such as the Musbongolus jangurum. This is an animal over 17 meters long from the sub-Triassic of Nevada. And 3 to 5 million years, this is geologically speaking, is really an incredibly short moment. These animals jump from being extremely small to an extremely large size in a very short time. The Zimboscondylidae are not the only and not the last Ichthyosauria that have followed this path of gigantism. In the Middle Triassic, another group of sometimes gigantic Ichthyosauria appears, the so-called Shastasauridae. The largest articulated skeleton of an Ichthyosauria belongs to this group. The Saurosicaniensis is an animal of about 21 meters in length, so already very large. And these animals then disappear at the end of the Triassic due to the massive mass extinction at the transition to the Jurassic. This mass extinction was a huge incident for the Ichthyosauria in general and only the smaller Ichthyosauria from the group of Parvipelvia survive this mass extinction. And they also remain relatively small afterwards. There is a certain exception to this in the genus Terminodontosaurus. In the lower Jurassic, they again show forms of 9 to a maximum of 10 meters in length. But especially compared to these early Shastasauridae from the Upper Triassic, this is really more of an echo of the gigantism among the Ichthyosauria. One problem that we now encounter as paleontologists when we want to deal with the research of these late Triassic Shastasauridae is that the fossil evidence of articulated Ichthyosauria remains is increasingly diluted over the course of the Triassic. And this can be very well illustrated by looking at how many Ichthyosauria taxa are known and named from the various sections of the Triassic. From the Middle Triassic, we still have 23 different named Ichthyosauria taxa. And this is already reduced in Caen, i.e. in the earliest section of the Upper Triassic, to 8, then in the North to 6. And from this last section of the Upper Triassic, the Rhete, which began 208 million years ago and was then ended by this mass extinction 201.5 million years ago, we do not have a single named Ichthyosauria taxa. And this section is very interesting for us paleontologists, especially in view of the following mass extinction events, to clarify questions, for example, whether the disappearance of this huge Ichthyosauria was the result of a gradual and slow search, as you could assume, for example, if you look at the numbers of these named taxa, or whether it was rather a catastrophic and very sudden event that these animals then extinct relatively short-term. Now, it is not that we do not know any fossils of Shastasauridae from the Rhete at all. But what we know of these spatial spirits are, above all, disarticulated and often also fragmentary clues. Here in Europe, for example, from France, some vertebral centers and jaw fragments. We know from the Swiss Alps further vertebral centers, a few rib fragments and a very large tooth. And from England there are two very large surangular fragments, i.e. fragments of two jaw bones. And I would like to highlight these bones from England. These are quite interesting, because comparisons with the more complete Shastasaurus sicaniensis indicate that these bones once belonged to an animal with an average size of over 26 meters in body length. And with that, these Rhetic Shastasaurids would suddenly move into a large spectrum, from which we have always assumed that it is due to the large modern bearded whale. So you can already see that the remains of these Rhetic Shastasaurids are quite sparse. And if we as paleontologists now try to close this gap and find new material, then this search begins, like any good ghost hunt, first with the search for the right cemetery. And with us, here in Westphalia, this cemetery comes from the district Höxter, near Warburg-Bodenburg. There are layers from Upper Trias to Unter Jura, including, of course, the very important Rhetic layer for us and the Trias-Jura border. And over 200 million years ago in the Rhete, the whole thing looked like this, that this area was flooded by a very large Mediterranean sea basin, which also included the already mentioned finds from France, England and Switzerland, shown here with red dots on the map. But now this locality from Warburg-Bodenburg also belongs to the Red Star Maquette. So this locality became really known for the first time in 2013 through the discovery of Reticosaurus metensii, a very early Plesiosauria. Incidentally, this fossil is also here in Münster in the Natural History Museum, so it is certainly worth a visit. And in 2015, through this first discovery, it came to the beginning of the excavations, which are now taking place annually, through the Museum of Natural History Münster and the University of Bonn. With the material I want to introduce here, it is a number of Reticosaurus vertebrae from the bone beds, i.e. the bone beds of Bodenburg. As with the other European finds, this is an exceptionally isolated, sometimes fragmentary find. But due to the storage in these bone beds, which are then partly characterized by many small bones, sharp teeth and similar, very abrasive material, unfortunately many of these bones suffered under strong abrasion. And that means that we have vertebrae, for example, with anatomical features, which you can see in the upper picture, which, for example, damaged these facets for the neural and hematopoietic arches in posterior tail vertebrae, or were completely lost, by which they are practically sanded off or even sanded out. In extreme cases, we even have vertebrae where the whole sides of a vertebra are almost completely sanded down. And as a result, unfortunately, we can hardly assign some vertebrae, or where it is at least clear that this is not a vertebra of these large Chastisaurids, but a smaller Reticosaurus, which has not been described more closely so far. But we can assign most of the vertebrae to the Chastisaurids, and this assignment is primarily due to three typical features and aspects. That would be, on the one hand, quite simply, the size. We are talking about large animals here, so we also expect large bones. And with the Spetriastic Ichthyosaurs, it is the case that non-Chastisaurid Ichthyosaurs hardly exceed vertebrae heights of 50 mm. The vertebrae here from Bohnenböck are much larger and sometimes exceed this limit by more than double that. So this is a good indication that these are large Chastisaurids. And another aspect is the anatomy, which then helps us to further anchor this in this family. Because for Chastisaurids, elongated rib facets are typical in the rump area, and these then continue to round off to more rounded facets in the front tail area. And this is an anatomy that we also find here in the Bohnenböcker vertebrae. Finally, there is still the morphology of the vertebrae to be mentioned, because these are extremely short. The relative length of a vertebra in Ichthyosaurs is typically made comparable by the calculation of the ratio of vertebral height by vertebral length, whereby a higher value corresponds to a shorter vertebra. And although short rib morphologies are very typical for this family of Chastisaurids, these height-length ratios in the Bohnenböcker Chastisauridae even exceed the highest values, i.e. the shortest vertebrae, that we know so far from the record holders of Chastisaurids. More specifically, it would be the Articunisaurus popularis. And this extremely short morphology also brings these vertebrae from Bohnenböck even closer to the already known Rhaetian finds, because they also show a very similar vertebral morphology. In summary, these vertebrae, despite their relatively poor maintenance, can certainly be classified as a family of Chastisaurids, although they now resemble the other Rhaetian finds from Europe. But even now, when compared to the former, but sometimes better preserved, Ichthyosaurs, they show a high affinity to the Taxon Chonysaurus popularis, a 15-meter-long representative from the north of Nevada, whose skeletons are now also seen here as a reconstruction, together with some vertebrae from Bohnenböck and their approximate location within the vertebral column. Yes, now the question is, what can we actually start with now? What conclusions can we draw from these finds? First of all, these isolated and sometimes fragmentary bones are evidence of at least two different Ichthyosauridae taxa. First of all, the large vertebrae that point to these large Chastisaurids, but also these almost indescribable vertebrae so far tell us that there must have been another Ichthyosauridae taxa here, even if we don't know exactly which one, but this one was much smaller than that of these large Chastisaurids. Oh, excuse me. These rhetorical finds from Bohnenböck and from these other European sites help us now to see that these animals have not only survived up to the Rhete and have also reached great body sizes there, but in some cases these animals have become even more colossal, despite this now slowly approaching mass extinction at the Trias-Jura border. And we can certainly draw some conclusions from this on the environment of these animals, because this suggests that in the case of the Central European seabed, even in the Rhete, shortly before this mass extinction, it must have been an extremely intact and very stable ecosystem, simply to be able to carry animals of such dimensions at all. And these two findings together help us to support the view that the decline of these gigantic Chastisaurids and the ecosystem in the Central European seabed should not have been, as may have been assumed, a slow process, which reduced the size and diversity of these animals over a very long period of time, but rather a very sudden catastrophe, which gave these animals a very abrupt end at their peak. With this, I would like to thank you for your attention and my presentation. At this point, I would also like to thank Jelle Heine from the University of Bonn and Dr. Achim Schwermann from the Museum of Natural History in Münster for their effective help and advice. And of course, I would also like to thank the institutions that made this scientific work possible, namely the Ruhr-Universität in Bochum, especially there, Dr. Claudia Distler, and the University of Bonn, especially there, Prof. Dr. Martin Sander from the Biological Institute.