A Humerus Tale: Investigating and recording a new species of Ichthyosaurus from Pleinsbachian deposits at Charmouth, England.

Abstract

In the palaeontology collection of Doncaster Museum and Art Gallery an almost complete ichthyosaur skeleton (DONMG:1983.98 (Fig1)), collected from Upper Jurassic Pliensbachian deposits near Charmouth in Dorset1, was recently identified as a new species (Lomax and Massare, in press2). However, it was clear that at some point in the past some of the forefin bones had been deliberately set within a filler. The skeleton was purchased by the museum in 1983 and no details about the discovery, excavation, conservation or mounting of the specimen were available. As the morphology of the visible humerus appeared to be unusual (a potentially diagnostic feature), the authors had to be clear exactly which bones were in situ and which – if any – had been introduced (i.e. from other specimens). Therefore the current positions of the forefin bones (including the humerus ) had to be recorded, the paint had to be removed, the exact outline of the filler recorded and the filler removed along with any ‘introduced’ bones – which included one of the humeri . This meant also that the in situ and much less visible humerus had to be completely prepared and removed, to check if it was a perfect symmetrical match or not (it was). Because the morphology of the humerus would be the prime defining feature of the new species, the opportunity was taken to photograph, video, illustrate, CT scan and mould the humeri before re-setting them in the matrix. A set of good quality casts are now stored with the specimen for future study. Abstract Investigation of the forefin References 1 Lomax, D. R. (2010). An Ichthyosaurus (Reptilia, Ichthyosauria) with gastric contents from Charmouth, England: First report of the genus from the Pliensbachian. Paludicola, 8: 22–36. 2 Lomax, D. R. and Massare, J. A. (In press). A new species of Ichthyosaurus from the Lower Jurassic of West Dorset, England. Journal of Vertebrate Paleontology. Acknowledgements Thanks are due to: Peter Tandy (Earth Sciences Department of the Natural History Museum, London) for the XRD analysis of the filler; Dr John Hutchinson (The Royal Veterinary College) for the CT scans; Elizabeth Hall for illustrating the humerus; Peter Robinson (Doncaster Museums and Art Gallery) for managing the CIRCA project; and the Esmée Fairbairn Foundation for funding the Doncaster CIRCA project which included this preparation work. Finally, we thank Prof. Judy Massare for collaborating with DRL and helping to determine the true importance of the specimen. Firstly, the positions of all the bones in the forefin were photographed (Fig 2) and also traced on to paper as a record of their exact arrangement (their orientation as well as position), and the bones were numbered. An airabrasive unit using compressed air lightly laced with sodium bicarbonate was used to remove the varnish on the bones and to remove the paint on the matrix and filler so that the bone, the matrix and the filler could all be distinguished from one another more easily and the exact extent of the filler assessed and recorded (Fig 3). The filler was then gently removed with scalpels and vibrating preparation pens and the numbered bones embedded within the filler were removed and stored, leaving only those in situ (Figs 4 & 5). The bones removed included most of the proximal phalanges and one humerus . Some filler was kept for analysis as the collector and the preparator of the specimen are still unknown but it would be useful to track them down t o find out more about the specimen’s discovery. The type of filler used could help with this as people often stick to materials they know. XRD analysis (Fig 6) revealed that the filler is a mixture of quartz, calcite and cristobalite – it has yet to be determined what product has these constituent parts (it did not have the texture of plaster or of resins such as epoxy or polyester etc). Left: Fig 5, the filler being removed from the forefin. Right: Fig 6, the results of the XRD analysis of the filler material. The amount of time spent working on this single specimen (eleven days including some conservation work not described here) was easily justified as this ichthyosaur is the single most important specimen in the palaeontology collection of Doncaster Museum and Art Gallery and it was in the process of being described as a new species. The specimen was enhanced by this work because a much greater knowledge was gained of the exact arrangement of the in situ bones of the forefin and in particular how the ex-situ humerus related to the in-situ humerus , leading to a greater confidence in the identification of the specimen within its wider taxonomy. Preparing and removing these bones was the only way of achieving this because the bones concerned were too small and too close together to resolve the issue by X-raying and/or CT scanning the bones in situ , especially considering the density of the matrix, the plaster and the wooden frame they were all surrounded by . If the preparation work described here had not been undertaken, determining the specimen as a new species would have been exceedingly difficult, if not impossible. Left: Fig 9, the illustrations made of the right humerus. A. Dorsal, B. Anterior, C. Ventral, D. Proximal, E. Distal views – Courtesy of Elizabeth Hall. Above right: Fig 10, the casts of the humeri. ¹ Cambridge University Museum of Zoology, Downing St, Cambridge, CB2 3EJ & freelance conservator and curator at Natural-History-Conservation.com ² School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester, UK. 3 Doncaster Museum & Art Gallery, Chequer Road, Doncaster, South Yorkshire, DN1 2AE. Nigel R. Larkin¹ & Dean R. Lomax 2, 3 A humerus tale: physically investigating and recording a new species of Ichthyosaurus