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Thursday, 26 September 2013

The solution to everything: under the (Jurassic) sea, part 2

In the last post, I mentioned that I was currently working on a Oxford Clay Formation and ichthyosaur display for the University of Portsmouth. Most of that post was dedicated to the various graphics and text generated for the ichthyosaur end of things (specifically, Ophthalmosaurus), so we'll now turn attention to the other half of the display: the Oxford Clay Formation itself, its palaeoenvironment and fauna. The words below are a very brief introduction to one of Britain's most stellar fossil units, complete with some of the artwork and graphics which will soon be adorning the walls of UoP. If you want to know more about the Oxford Clay, you may also want to check out Mark Wildman's Saurian, which has discussed the Oxford Clay and its fossils at length across many posts. Baring a quick nod to Dave Martill for his help with shaping the words here, I'll hand you over to the display text.

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One-hundred and sixty million years ago, most of Europe - including the British Isles – was underwater, flooded by a warm, shallow sea populated by astonishing marine reptiles, gigantic fish and a diverse invertebrate fauna. The Oxford Clay Formation, one of the UK's most famous fossil-bearing rock units, provides an extensively researched window into this Jurassic marine ecosystem.
Extent of the Oxford Clay across the UK, with major localities. Whittlesey, the source of the marine reptile skeleton behind this and the preceding blog post, is highlighted in red. 
The Oxford Clay: geology, geography, economic geology
The Oxford Clay Formation is an extensive succession of dark mudrocks with intermittent limestones which crop out  almost continuously from Dorset to Yorkshire. Further exposures are found on the seabed of the English Channel and in Normandy, France. The entire Oxford Clay sequence is of late Middle Jurassic to lower Upper Jurassic age (164-159 Ma) and fossils occur throughout, although most vertebrate fossils occur in the Peterborough Member, a unit of organic-rich rocks which represent the lowest part of the formation. Considerable commercial interests in the Peterborough Member date to the 1870s when excavation of its clays began for brick making. The high organic content of these clays meant that they fired quickly at low temperatures, allowing for production of high-quality bricks at low cost. The Oxford Clay brick pits are now mostly closed, but the tremendous economic interest in the Oxford Clay has ensured that multitudes of fossils were continually excavated from quarries on an industrial scale for nearly 100 years, permitting a detailed view of the Oxford Clay palaeobiota.

Palaeoenvironment and palaeoecology
The Oxford Clay sea was a warm (water column temperatures of 20°C) and shallow (tens of metres) marine environment, with a rich supply of nutrients from local land sources. The abundance of light and nutrients supported a rich and complex ecosystem (below). Planktonic organisms, including numerous types of algae and zooplankton, were abundant in the Oxford Clay sea and likely formed the basis of its food web. Plankton was the food source for small invertebrates and juvenile fish, which in turn were preyed on by the larger fish, ammonites, belemnites, squid and reptiles that comprise the majority of Oxford Clay fossils. Ammonites are particularly common components of the Oxford Clay, being represented by some 78 species. The community of bony fishes and sharks was almost as rich as that of the ammonites, with 32 species adapted to exist in a variety of ecological niches. The Oxford Clay fauna contains one of the most spectacular bony fish to ever evolve, the 12-15 m long pachycormid Leedichthys problematicus. This animal was not a predator however, but instead filtered plankton from the water column using enormous gill apparatus.
Schematic reconstruction of the Peterborough Member fauna, palaeoenvironment and nutrient cycling. Animals are not to scale, unless you wish to invoke the Father Dougal sense of size. Based on data from Martill and Hudson (1991) and Martill et al. (1994).
The most famous Oxford Clay animals are the marine reptiles (below), which including ichthyosaurs (the 'fish lizards'), plesiosaurs (four-flippered reptiles with variably sized heads and necks, some of which – the pliosaurs - were the dominant predators of Jurassic seas) and thalassosuchians (marine crocodiles). Dinosaurs are also known from the Oxford Clay, likely representing animals washed in from the hinterland or individuals that died swimming between islands. Small flying reptiles, pterosaurs, were also present, but are very rare fossils.

Composition and abundances of the Peterborough Member reptile fauna. Based on Martill and Hudson 1991.
The sea floor was not as vibrant with life as the water column. Because the sea floor sediments and bottom waters had relatively low oxygen levels, the diversity of benthic species was restricted compared to the waters above. Bivalves, gastropods, arthropods and foraminifera comprise the majority of fossils from these communities, as well as the burrows of organisms which lived within the soft sea floor sediments. Sediment stability was an issue for some benthic organisms, leading to colonisation of decaying animal skeletons as substitutes for firm substrates by some species..

Micro- and macroconchs (male and female, respectively) of the ammonite Erymnoceras coronatum, hanging around the Oxford Clay seaway. The macroconch is 40-50 cm across, while the microconch, as is typical of ammonites, is about 20-25% of that size.
Ammonites: floating clocks and palaeontological enigmas
Ammonites, nektonic cephalopods with chambered external shells, form the backbone of biostratigraphy for Mesozoic rocks. Ammonite faunas evolved rapidly enough to permit identification of one million year intervals of Mesozoic time, allowing for very precise dating of ammonite-bearing rocks. The Erymnoceras coronatum ammonites shown above are one of the index fossils for the Peterborough Member, placing it firmly in the middle Callovian stage of the Jurassic.

Oxford Clay ammonites provide key data on the evolution of ammonites, and were integral in identifying male (small, elaborately ornamented ‘microconchs’) and female (much larger, less ornamented ‘macroconchs’) morphs. Despite the abundance and familiarity of ammonite fossils however, many aspects of their anatomy and lifestyles remain mysterious. Questions such as what they ate, where they lived in the water column, their floating orientation, as well as the exact nature of the squid-like creature living within the shell, remain unanswered.

Bonus fun: the assembled board
As a way of signing off these two linked posts, I thought it might be fun to show off the entire display board itself, just so anyone interested can see how all the text and images here will hang together. The entire thing is well over 3 m long, so should look fairly imposing when it's finally printed.
UoP's Oxford Clay and Ophthalmosaurus display text, coming soon to a display cabinet near me.
And that's our time in the Oxford Clay seaway done for the time being, folks. I'm hoping to get back to fairly regular posting over the next few weeks, because things have been a bit quiet about here of late thanks to a particularly busy conference season. Coming soon, hopefully: some comments on the All Yesterday's sequel, All Your Yesterdays.

References

  • Martill, D. M. and Hudson, J. D. (1991). Fossils of the Oxford Clay (Field Guides to Fossils) (No. 4). The Palaeontological Association, London.
  • Martill, D. M., Taylor, M. A., Duff, K. L., Riding, J. B., & Bown, P. R. (1994). The trophic structure of the biota of the Peterborough Member, Oxford Clay Formation (Jurassic), UK. Journal of the Geological Society, 151(1), 173-194.

Saturday, 14 September 2013

The solution to everything: under the (Jurassic) sea, part 1

It's been very quiet around these parts of late as my August and September transformed into a minor tour around Western Europe for talks and conferences. SVPCA in Edinburgh, the VIth International Symposium of Dinosaurs and their Environment in Burgos, Spain, a talk about my book in London and - next week - the Jehol/Wealden biotas conference in Southampton. Busy times indeed, leaving little room for blogging, painting or, well, anything at all, really.

In the interests of posting something, I thought I'd share two halves of a project I've was working on before I set off on my travels. Some months ago I was asked by the University of Portsmouth to spruce up a display featuring a partial skeleton of the ichthyosaur Ophthalmosaurus icenicus from the Oxford Clay Formation, a famous unit of Jurassic sediments deposited 162 - 158 Ma. Being the well organised professional that I am, I can't show you any photos of the specimen or display here*, but I can share some of the artwork and text which will, in the coming weeks, be plastered all up in our geology department. The display is divided into two broad components, one part being about the rich palaeontology of the Oxford Clay Formation itself - depositional setting, palaeobiota and the like - and the other dedicated to Ophthalmosaurus. It's worked out that the ichthyosaur section is far more complete than the other, so we'll start with that today, and have the sister portion following shortly. Maybe I'll even get my act together and show photographs of the specimen itself, because it's pretty neat.

*Is this the result of another batch of sticky palaeontological politics? Heck no: I just haven't taken any photos yet.

Ophthalmosaurus icenicus skeleton in lateral view. From McGowan and Motani (2003).
The painting at the top of this post is of O. icenicus and, as may be expected, is one component of the new display. It's one of my first efforts at a detailed painting of a marine animal and my first ever real attempt at rendering an ichthyosaur. Both were a lot of fun to do, and I wouldn't be surprised if we don't see more ichthyosaurs around these parts in future. The reconstruction benefited enormously from conversations with University of Bristol PhD student Ben Moon who, among other things, is redescribing O. icenicus for his thesis. Ben not only provided suggestions and comments about an earlier version of the image but also supplied me with a heap of literature concerning Ophthalmosaurus and ichthyosaurs in general. Ben blogs about his work and ichthyosaur science over at Ichthyosaurs: a day in the life…, so be sure to head over there if fish lizards float your boat.

Before I hand you over to the other components of our display, I'll say a few things about the reconstruction which, for reasons of space, couldn't be included in the exhibit. I set the scene in a shallow, coastal setting rather than the infinite blue seas we often see marine reptiles in. I completely understand why such compositions dominate marine reptile art, but I figured it would be nice to try something a little different. Plus, setting the animal closer to the shore meant I could make the water a little stiller, as if this chap had swum into a quiet, shallow lagoon or bay. Having relatively still water was important here because of the point of view. Again, just to be different, I thought a somewhat dorsal view of the animal may be interesting, but choppy waters would mean having to obscure or distort its proportions with waves and ripples, which didn't seem like a sensible thing to do in an educational display piece.

A dorsal view also allows for showcasing the dimensions of this animal. Rather than lithe and slender, as we often imagine aquatic animals are, Ophthalmosaurus was a broad and rotund animal with powerful shoulders, a barrel-shaped body, and a wide posterior skull region (below). Scale is always difficult to convey in images with no familiar objects to relate to (the seagull-sized floating pterosaur is the best I've got for scale here), but I tried to give an impression of the large size of this animal, too. Ichthyosaurs are often depicted resembling small dolphins or porpoises, but even mid-sized, 4-5 m long ichthyosaurs like Ophthalmosaurus were a lot bigger. I wondered if this size, not to mention the jaws brimming with 160 conical teeth (the original Walking with Dinosaurs, which likely introduced many of us to O. icenicus, erred on this front: see below for details), would allow O. icenicus to predate fairly large squid along with smaller fish and cephalopods. Reflecting this, I riddled it's hide with scars from battles with relatively mighty teuthids. Not all these scars may have been made by big squid, however, as ichthyosaurs were not above inflicting serious injury on each other, either. The colours of the animal were, again, an attempt at injecting a little originality into depictions of this animal. Although a lot of oceanic creatures are undeniably shades of grey, black and white, the superb visual acuity of Ophthalmosaurus suggests that visual signalling and recognition of individuals may have been important to these ichthyosaurs (Humphries and Ruxton 2001). I thought a complex pattern of ocean-penetrating reds, browns and whites may reflect this idea nicely.

Ophthalmosaurus icenicus in anterior view. Far from being lithe and slim, O. icenicus was almost as wide as it was tall. This is one of the many adaptations O. icenicus bears to fast swimming, and has also prompted the controversial hypothesis that the Antrhopocene joke 'yo' momma so fat...' had origins in Upper Jurassic marine settings. Image from McGowan and Motani (2003).

I'll stop there - this was meant to be a short, 'picture of the day' type post - and hand you over to the display text about this species. A lot of the information is quite basic, but it may still prove somewhat interesting. We've yet to print any of these images and text out for our display by the way, so be sure to leave any constructive comments you may have in the comment field below. Tune in soon for some details of the Oxford Clay seaway which housed O. icenicus, not to mention a plethora of other fascinating animals. Over to the display text...

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Only one species of ichthyosaur is currently recognised from the Oxford Clay, Ophthalmosaurus icenicus. O. icenicus has a wide distribution across Europe and Asia, a 20 million year stratigraphic range, and is famous for bearing some of the largest eyes of any animal known. Ophthalmosaurus and its opthalmosaurid relatives were a diverse and important group of Jurassic and Cretaceous ichthyosaurs, dominating the Cretaceous chapter of ichthyosaur evolution until the group became extinct at the end of the Cenomanian (Late Cretaceous, 94 Ma).

Although a complete skeleton of O. icenicus has never been found in the Oxford Clay, a full knowledge of its skeletal anatomy has been assembled from multiple incomplete skeletons. Unlike many other ichthyosaur specimens, Oxford Clay O. icenicus material is frequently preserved in three dimensions, making it an important species for understanding the anatomical complexities and functional anatomy of these reptiles. Since its recognition in 1874, O. icenicus has become one of the most completely known of all ichthyosaurs and a common component of studies into ichthyosaur taxonomy and functionality. Ophthalmosaurus perhaps attained the pinnacle of its fame in 1999 when it featured prominently in the classic BBC documentary Walking with Dinosaurs.

Anatomy
Like all ichthyosaurs, O. icenicus is supremely adapted to life in the marine realm. It possesses a full complement of ‘thunniform’ (Greek, ‘tuna-like’) features common to Jurassic and Cretaceous ichthyosaurs including reduced hindlimbs, a well-developed caudal (tail) fin, and a short, inflexible trunk region. O. icenicus was a moderately sized ichthyosaur, attaining body lengths of 4-5 m when fully grown.

Not so toothless after all: the fierce jaws of Ophthalmosaurus icenicus. From Kirton (1983).
The skull of O. icenicus has attracted much research interest because of its peculiar anatomy. The bones supporting the eyeball, the sclerotic rings, are enormous at 220 mm across. Among living animals, only giant squids have larger eyes but, for its body size, O. icenicus has the largest eyes known of any animal, alive or extinct. These eyes sit above a long set of jaws which have long been considered entirely, or almost entirely toothless. This interpretation is erroneous, however, as well-preserved O. icenicus and closely related ophthalmosaurid species clearly show small, slender and pointed teeth in each jaw. It seems that these teeth were weakly anchored into their dental grooves (like many ichthyosaurs, O. icenicus mostly lacks individual tooth sockets), and fell away readily once their owners began decomposing.

Lifestyle
The enormous eyes of O. icenicus have prompted much discussion among palaeontologists. It is generally considered that these eyes allowed O. icenicus to dive to great depths to find food, with their 90 mm wide pupils able to gather light beyond the perception of most other marine animals. Despite their size however, the eyes of Ophthalmosaurus would only permit vision at 40 m greater depth than those of marine animals with 'typically-sized' eyes, and only 50 m more than our own. The giant eyes of O. icenicus were considerably more capable of detecting shape and other visual details in low light conditions however. In environments where we could only see grainy outlines of other animals, Ophthalmosaurus could see in high definition. Possible confirmation that O. icenicus dived to great depth stems from evidence of decompression trauma (‘the bends’) in several specimens, a harmful condition caused by development of gas bubbles in the bloodstream of animals rapidly ascending from deep water.

The slender jaws and tightly packed, simple teeth of O. icenicus suggest it primarily ate squid and small fish, a diet confirmed in part by preserved stomach content of closely related, North American ophthalmosaurids. Propulsion for swimming was generated by the large, lunate caudal fin. Like other advanced ichthyosaurs, O. icenicus swam like a modern shark or whale, with a largely immobile trunk skeleton minimising undulations along the body when swimming, maximising the propulsive effects of the tail fin. This made O. icenicus one of the fastest reptiles, for its body size, in the Oxford Clay palaeoenvironment. The large, powerfully muscled shoulder girdle and forelimb paddle of O. icenicus betray an ability to rapidly steer and manoeuvre during pursuit of its prey. It is likely that O. icenicus used its powerful swimming ability to roam across several Jurassic seas, a habit which may explain its occurrence in numerous, geographically distant locations.

References
  • Humphries, S., & Ruxton, G. D. (2002). Why did some ichthyosaurs have such large eyes?  Journal of Experimental Biology, 205, 439-441.
  • Kirton, A. M. (1983). A review of British Upper Jurassic ichthyosaurs. Unpublished PhD Thesis, University of Newcastle-upon-Tyne. 239 pp.
  • McGowan, C. & Motani, R. (2003). Part 8 Ichthyopterygia. Sues H–D (ed.) Handbook of Paleoherpetology. Munchen: Verlag Dr. Friedrich Pfeil. 175 p.