Thursday, 5 October 2017

Old Fossils and the Crocodiles of Old England

The Melksham Monster closely resembled the species shown in this artist's impression (Plesiosuchus manselii), which also belongs to the Geosaurini group.

Credit: Fabio Manucci
Fossil points to early rise of ancient crocodiles | The University of Edinburgh

In a vindication of the way science constantly re-examines and re-assesses itself in the light of new information, some of the most rewarding fossil hunting is now being done in museums.

The museums of the world contain millions of fossils, very often meticulously catalogued with details of when and where they were discovered, but filed away in draws and cupboards and almost as effectively hidden from general view, and often scientific view, as they originally were when buried in rocks awaiting discovery by some intrepid fossil hunter a century or more ago. At the time of their discovery they may well have been mere curiosities. With little information to go on, no-one would have been aware of their significance or where they fitted in the evolutionary history of life on Earth.

The jigsaw puzzle was then far too fragmentary to know where this particular piece fitted.

One such fossil was recently discovered in the archives of the Natural History Museum, London, where it had been since 1875. It had originally been discovered about 150 years ago at Melksham, Wiltshire, England, in a formation known as Oxford Clay. When examined in detail by a team from Edinburgh University, Scotland and the Natural History Museum, London, it was found to be that of the earliest known crocodylomorph of the Geosaurini lineage, putting the evolutionary origins of this group back millions of years before the 152-157 million years previously thought.

Abstract
Metriorhynchids are an extinct group of Jurassic–Cretaceous crocodylomorphs secondarily adapted to a marine lifestyle. A new metriorhynchid crocodylomorph from the Oxford Clay Formation (Callovian, Middle Jurassic) of England is described. The specimen is a large, fragmentary skull and associated single ramus of a lower jaw uniquely preserved in a septarian concretion. The description of the specimen reveals a series of autapomorphies (apicobasal flutings on the middle labial surface of the tooth crowns, greatly enlarged basoccipital tuberosities) and a unique combination of characters that warrant the creation of a new genus and species: Ieldraan melkshamensis gen. et sp. nov. This taxon shares numerous characters with the Late Jurassic–Early Cretaceous genus Geosaurus: tooth crowns that have three apicobasal facets on their labial surface, subtly ornamented skull and lower jaws elements, and reception pits along the lateral margin of the dentary (maxillary overbite). Phylogenetic analysis places this new species as the sister taxon to Geosaurus. The new taxon adds valuable information on the time of origin of the macrophagous subclade Geosaurini, which was initially thought to have evolved and radiated during the Late Jurassic. The presence of Ieldraan melkshamensis, the phylogenetic re-evaluation of Suchodus durobrivensis as a Plesiosuchus sister taxon and recently identified Callovian Dakosaurus-like specimens in the Oxford Clay Formation, indicate that all major Geosaurini lineages originated earlier than previously supposed. This has major implications for the evolution of macropredation in the group. Specifically, we can now demonstrate that the four different forms of true ziphodonty observed in derived geosaurins independently evolved from a single non-functional microziphodont common ancestor.


One of the challenges faced by the palaeontologists was that the fossil itself was embedded in a nodule of super-hard rock with veins of calcite running through, formed durign the fossilisation process. It took many hours of work over several weeks to expose the skull and teeth without damaging them. Preserving the detail of the teeth especially was important as they are key to classification. With this specimen it is now known that four forms of ziphodonty evolved independently from a common ancestor which had apparently functionless small serrations on its teeth.

It’s not the prettiest fossil in the world, but the Melksham Monster tells us a very important story about the evolution of these ancient crocodiles and how they became the apex predators in their ecosystem. Without the amazing preparation work done by our collaborators at the Natural History Museum, it would not have been possible to work out the anatomy of this challenging specimen.

Davide Foffa, lead author
School of GeoSciences, Edinburgh University
This ten-foot long marine early crocodile once swam in the warm sea that then covered most of Europe, including southern England where it was found. It would have been a top predator when dinosaurs roamed the land.

One of the fascinating things about the terrestrial tetrapods is that soon after they had evolved a fully terrestrial life-style, not needing to return to water to breed like the amphibians, some of these early reptiles returned to the water to become the crocodylians and turtles. Like the fully aquatic mammals, the whales and the manatees, they never evolves the ability to breath under water, as their fish ancestors had.

Evolution is incapable of going into reverse. Once the structures such as gills with their associated supporting arches, gill slits and circulation had been adapted or had atrophied they could not be easily reconstructed by an unplanned, utilitarian evolutionary process. This would not be so for a system designed by an intelligent designer which had already designed the respiratory system of fish, hence these adapted terrestrial tetrapods have been forever tied to the surface of the water and the need to breath air.

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