Monday, 31 August 2015

Life and death in the Wessex Formation

The Wessex Formation is a geological formation dating back to the early Cretaceous, 130 million years ago. It is a part of the Wealden Group, which in turn is part of the Wealden Supergroup. This Supergroup also houses the Weald Clay Group and Hastings Beds Group, which, along with the Wessex Formation, were home to some of the first dinosaurs discovered, such as Baryonyx and Iguanodon. During the early Cretaceous, the Wessex Formation, and Wealden Supergroup in general, was part of a large island in the Tethys, where Britain now is. It had a dry, fairly arid climate, with distinct wet and dry seasons, and where temperature differences between summer and winter were stark. Much of the Wessex lowlands during this period would have been mostly treeless, covered in shrubs and other low-lying plants. The uplands and floodplains on the other hand would have been much more verdant, at least for some parts of the year, and plausibly housed a greater diversity of species. Due to these differences in habitats, animals would probably have migrated across the island depending on the seasons, spending the fairly cool and wet winters in the lowlands, and the warm and dry summers in the uplands. In this post I will be covering several of the major groups present in this formation, from the smallest Crocodyliforms to the largest Sauropods.

Polacanthus at a creek, by John Sibbick

Like most British Fossil sites, the remains discovered in the Wessex Formation are numerous, spanning multiple lineages, but generally poorly preserved and fragmentary. Because of this, we have an excellent overaching view of the faunal diversity of the Formation, but lack information pertaining to the individual species and genera. Among these is Polacanthus, an early Ankylosaur that lived from approximately 130 to 125 million years ago. Restorations of this animal are typically based on Gastonia or other early Ankylosaurs, as they were probably closely related. Estimates put Polacanthus at around 5 meters in length, which would have made it a rather small herbivore for the time, but its thick plated and spiked armour would probably have been deterrent enough for most predators. Where exactly Polacanthus fits in Ankylosaur phylogeny is not certain, but some researchers have placed it as a basal Nodosaurid. Another Ankylosaur that may have coexisted with Polacanthus is Hylaeosaurus, a similarly sized animal from the same region, though it may have been extinct by the time of the Wessex Formation. 

Alongside Ankylosaurs, the largest Ornithischians in the formation would have been Iguanodonts, both of the genera Iguanodon and Mantellisaurus. These were both far larger than Polacanthus, with the biggest species of Iguanodon reaching upwards of 10 meters, possibly even 13. Iguanodonts were a group of basal Hadrosauroids, more primitive than the true Hadrosaurs of the late Cretaceous. Probably the most abundant medium to large sized herbivores in their habitat, the smaller species and juveniles would have been one of the main prey items for the predators of the time, though the largest adults would have been virtually immune to predation. A well noted feature of the Iguanodonts is their peculiar thumb spike, which was probably used for intra-specific combat more so than self defense, as was previously thought. Famously originally misinterpreted as a nose horn, this was later corrected when more complete remains of Iguanodon were discovered. 

A large herd of Iguanodon moving along the coast, from Walking With Dinosaurs

Five Sauropods are named from the Wessex Formation, but none are well known. Large animals generally do not fossilize well, and in the already poor conditions of the English fossil beds, Sauropods are typically only known from few and fragmentary remains, as is the case here. Three of the Wessex Sauropods, Eucamerotus, Luticosaurus, and Ornithopsis are classified as Titanosaurs, while of the other two, one is considered a Turiasaur, and the other not assigned to any specific group. The more well known species Pelorosaurus is known from the slightly earlier Hastings Beds, but some of the remains in the Wessex Formation may actually belong to this genus. The Wessex Sauropods were probably quite large, with the biggest possibly reaching 20 meters in length, as more complete fossils from earlier genera such as Cetiosaurus, a Jurassic animal, show that despite being an insular fauna, island dwarfism does not seem to have had a large affect on British dinosaurs, as while the species here are typically smaller, they are still often large in their own right. This is probably because Britain was part of one of the largest islands in the European archipelago, a so-called subcontinent. 

Pelorosaurus, by Nobu Tamura

Of the Wessex Formation's non-Dinosaurian fauna several species of Pterosaurs are known, among these the large Ornithocheirus. While some species of this genus had wingspans of up to 5 meters, the remains known from Britain indicate a smaller animal, only reaching around 2.5 meters from wing tip to wing tip. While Ornithocheirus, like many Pterosaur genera, was previously speculated to be a piscivorous species, later research on Pterosaurs in general has made this unlikely. More plausibly, it would have hunted small terrestrial animals such as Mammals and lizards, though it would probably also have been an opportunistic scavenger. Another Pterosaur found in the Wessex Formation was Istiodactylus, a relatively large species, which with a wingspan of up to 4 meters may have been the biggest Pterosaur in its habitat, barring any yet undiscovered species. Sometimes called "Duck-billed" due to the shape of their jaws, though these were filled with small sharp teeth, adapted for slicing through flesh. 

Another common non-Dinosaurian part of the Wessex fauna was the Goniopholidids, Primitive and superficially Crocodile-like Neosuchians that died out at the end of the Mesozoic. They ranged from the diminutive Vectisuchus, which was only 1.2 meters long, to the large Anteophthalmosuchus, which could grow to up to 3.5 meters long. These would have filled essentially the same niches as modern Crocodiles do, hunting small Dinosaurs such as Hypsilophodon. One interesting thing to note however is that due to the extreme aridity of early Cretaceous Britain, many of the island's wetlands would almost entirely dry out during the summer. Due to this it is plausible that many of the larger Crurotarsans would have had to essentially hibernate through this period, subsisting on very low amounts of food for months at a time, while the smaller species would probably be capable of surviving in the muddy riverbeds and dried up lakes during these periods.

Ornithocheirus, by Julius Csotonyi

The last large part of the Wessex Formation's fauna that I have yet to talk about is the Theropods. While some of the larger Goniopholidids may have been keystone predators of smaller species, the Theropods were the undisputed top predators of their environment. Several famous Theropods are known from the Wessex Formation, including the Spinosaurid Baryonyx, a gracile and piscivorous animal that was nevertheless the largest predator in its habitat. Another significant animal from the area was Eotyrannus, a genus with a particularly convoluted history of classification. Originally thought to be a basal Tyrannosauroid, hence its name, it was later reclassified as a Megaraptorid. However, since Megaraptors have now also been reclassified as a group of derived Tyrannosauroids, this means that Eotyrannus is technically still a Tyrannosaur, albeit not a basal one. Eotyrannus itself was a fairly small animal, at only 4 meters in length, and would most likely have hunted small Mammals and dinosaurs. By far the largest predator(aside from Baryonyx) in the Wessex Formation would have been Neovenator, a genus of large Allosauroids. These impressive animals could reach almost 8 meters in length, and as such would have been more or less undisputed. Interesting to note is that some cladograms actually find it to be close to the Megaraptorans. However, the aforementioned and more recent findings placing the Megaraptorans within Tyrannosauroidae make this unlikely.

The Wessex Formation was also home to several small and diminutive Theropods, but most of these are still very poorly understood, and cannot be confidently confirmed to belong to any group. An exception to this is Aristosuchus, which despite its name is actually a Compsognathid, along with Calasmosaurus, another Compsognathid, which is estimated to possibly have reached upwards of 5 meters, making it one of the largest members of its group. The other, more poorly known small Theropods include Yaverlandia, originally identified as an Ornithischian but now known to be some form of Maniraptoran, Ornithodesmus, a small Dromaeosaurid, originally thought to be a Pterosaur, and Thecocoelurus, which may in fact be one of the earliest Ornithomimosaurs.

Eotyrannus hunting Hypsilophodons, by Luis V. Rey

The Wessex Formation is fascinating for the same reason that so many other British fossil beds are, namely the incredible diversity in species preserved there. Groups of many lineages, some newly emerged, others near the end of their existence, all seem to be present in these faunas. While this could perhaps be argued to be a result of the insular nature of the British faunas, it seems more likely that these Formations are merely more complete than most others, and that in reality, this diversity was present across most of the world during much of the Mesozoic. In this regard, the British faunas, the Wessex Formation included, give us a particularly clear view of just how lively and biodiverse Mesozoic ecosystems would have been, allowing us to more easily reconstruct the sort of interactions that would surely have taken place all those millions of years ago. 

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