Oxford’s exceptional fossils

Fossils always have a story to tell. Here are a number from the world-class collection of Oxford University Museum. Click on the photographs for a high-resolution image.

Phylum: Echinodermata Class: Asteroidea Species: Helianthaster rhenanus Most starfish have only 5 arms – this one had 16. Apparently the species was short-lived, for it occurs only in the world-famous Hunsrück Slate, Bundenbach, Germany (not far from the river Rhine, whence the species name rhenanus). Mud was deposited in an offshore basin and subsequently baked into fissile slate when the ancient continents Euro- america and Gondwana collided in the Variscan Orogeny. The formation is Early Devonian in age and has a total thickness of 3,750 metres.

In the earliest stages of the collision, sediment-laden turbidity currents buried communities on the sea-floor under beds a metre or more thick. Arthropod trails and crinoids with rooting structures demonstrate that the communities were living where they were fossilised, not buried by a global flood that began tens of kilometres lower in the stratigraphic column! Conditions for preservation were exceptional, not only rapid deposition but also the right sediment chemistry and bacteria to convert the organic matter into iron sulphide (pyrite). Occasionally even soft tissue, such as eyes, intestines and tentacles, were preserved. The 15-cm body of this unusual starfish is a fine example.

Phylum: Echinodermata Class: Crinoidea Species: Pentacrinites fossilis Although stemmed crinoids are often called ‘sea lilies’, they are actually animals. Some can even walk. Similarities in fundamental structure show that they are related (amazingly) to starfish, brittlestars, sea urchins and sea cucumbers. In addition to these 5 extant classes within the phylum, 15 others went extinct, all in the Palaeozoic, so there was far more diversity in the past than today. Nine of the classes had evolved by the Cambrian, when echinoderms first appear.

The tangled group above comes from a 2-metre interval within Black Ven Marls around Lyme Regis, dating to the Lower Jurassic. The unburrowed laminae and high amount of unoxidised organic matter of this interval show that the sea bottom was anoxic. Unlike most crinoids, however, Pentacrinites colonised floating logs, not the sea floor. The driftwood was also sometimes fossilised, with crinoids still attached, but only on the part that was under water. Tiny white discs on the upper surface mark places where larvae tried to get a foothold as the rest became submerged. In a matter of days the log sank completely, into the anoxic zone which brought the lives of the colony to an end. The same deadly conditions preserved them as fossils.

Driftwood-colonising crinoids are among the largest crinoids known. The first to evolve such a life-style were the genus Traumatocrinus, in the Triassic. Their stems grew to a length of over 11 metres. Similar lengths were reached by the closely related Seirocrinus, whose swirling arms grace what is arguably the most spectacular of all fossils, the 15-metre coalified log in the Urmuseum Hauff, at Holzmaden.

Phylum: Echinodermata Class: Echinoidea Species: Temnocidaris sceptifera Sea urchins, or echinoids, first appeared in the Ordovician. While their origins are obscure, they probably diverged from a long-since extinct echinoderm class called ophiocistioids. The sea urchin illustrated here is from much later, when most of Britain was submerged in warm shallow seas and chalk sediments were accumulating.

Sea urchins live either on the sea floor or within the sediment itself. Those on top of the sediment feed by scraping and grazing, usually on algae, though some also eat a range of invertebrate animals. Their skeletons, known as tests, are covered by protective spines which articulate with tubercles (visible here in the centre of the hexagonal plates), and muscles attached to the spines enable the animals to move. Sea floors may be very muddy, and long spines are adaptations to this kind of environment. Within hours of death the spines drop off. Thus, when they are fossilised alongside the skeleton, it is clear that the organism underwent only minimal transport. By the same token, sedimentation rates were fast enough for the hard parts of the animal to be buried almost intact.

Phylum: Arthropoda Class: Trilobita Species: Selenopeltis (order: Lichida) and Dalmanitina (order: Phacopida) Arthropods have an external skeleton, a segmented body and jointed appendages. They include trilobites, crustaceans, insects and spiders amongst others. Since these classes appear abruptly in the fossil record, unconnected by transitional forms, it is unlikely that they are genealogically related.

The specimens exhibited come from Mount Boutschrafin in Morocco and are of Ordovician age. The larger trilobites, with long genal and pleural spines, are Selenopeltis. They had a blunt cephalon (head), holochroal eyes and only a tiny pygidium (tail section). The small Dalmanitina (left middle) had long genal spines and tail spine, schizochroal eyes and a relatively large pygidium. Each of its eyes (compound, as in all trilobites) consisted of an upper and lower unit, separated by an undulating surface that corrected for spherical aberration. Unique to this genus, the lens was bifocal, enabling the trilobite to see simultaneously both very near and very far.

The trilobites lie on top of a colony of brittlestars.

Phylum: Arthropoda Class: Trilobita Species: Walliserops trifurcatus (order: Phacopida) This fossil dates to the Devonian period, a time when fishes were increasing in size, in abundance and in predatory capability. Lungfish, for example, had evolved massive teeth and palates for crushing shells and exoskeletons. To cope with such threats, trilobites had to develop new defences: more powerful eyes, stronger and thicker exoskeletons, and more flexible rolling up of their bodies (like hedgehogs).

This is partly what evolution is about: an arms race where predators evolve ever more effective strategies for catching their prey, and animals lower down the chain evolve ever more effective counter-measures. The immediate ancestor of Walliserops responded by growing a trident, which, although too delicate a structure to function as a weapon, made the animal look bigger and spinier. It also grew spines above each eye. How does ‘Nature’ come up with such marvels? (Provenance: Djebel Issoumour, Morocco).

Phylum: Mollusca Class: Bivalvia Species: Inoceramus cuvieri Molluscs include cephalopods (such as squid and octopus) and gastropods (such as snails), as well as shellfish (bivalves). Amongst other ingenuities, cephalopods, unlike bivalves, have camera-type eyes. The interrelatedness of the various molluscan classes is thus not obvious. Common ancestry may be inferred on grounds of comparative anatomy and is quite probable, but only if we postulate supernaturally powerful genetic programming.

This specimen is around 30 cm long and the annual growth bands show it was over 200 years old. With lengths of 2-3 metres the largest inoceramids would have reached even greater ages. Note the chalk matrix enclosing much of the shell. Inoceramus cuvieri lived during the middle Turonian stage of the Late Cretaceous, when sea-levels were high and chalk-producing seas flooded vast areas of the continents. Refer here for evidence that the repetitive beds of chalk sequences were also annual.

Phylum: Chordata Class: Dinosauria Species: Tyrannosaurus rex Dinosaurs first appeared in the late Triassic, apparently at the same time in various parts of the world, having evolved not long before from certain bipedal archosaurs. One can speak about ‘dinosaurs’ as a distinct group because, despite their amazing diversity as time went on, they had distinctive features in common, and the transformation of their ancestors into the dinosaur form was relatively sudden.

The first dinosaur to be described and named was Megalosaurus, by William Buckland, professor of geology at Oxford. The fossil was found at a village six miles away. The animal pictured is a cast of Tyrannosaurus from Hell Creek Formation, South Dakota (informally known as ‘Stan’, after its discoverer). The formation straddles the Cretaceous-Tertiary boundary, making Stan one of the last dinosaurs ever to prowl the Earth. Healed wounds in the ribs, the neck and the back of the skull show that he had had a number of fights with others of his kind. Tyrannosaurs were theropods, a carnivorous lineage that can be traced back to the earliest dinosaurs. From the beginning they had a distinctive ‘saurischian’ hip, where the pubis pointed forward and the ischium backward in a reptile-like arrangement. Many other dinosaur lineages had an ‘ornithischian’ arrangement, where the ischium was oriented more like a bird’s. Oddly, birds are all thought to be descended from saurischians rather than ornithischians, a belief that is probably not entirely correct.