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Hindfins of Ichthyosaurus: effects of large sample size on ‘distinct’ morphological characters

  • JUDY A. MASSARE (a1) and DEAN R. LOMAX (a2)

The abundance of specimens of Ichthyosaurus provides an opportunity to assess morphological variation without the limits of a small sample size. This research evaluates the variation and taxonomic utility of hindfin morphology. Two seemingly distinct morphotypes of the mesopodium occur in the genus. Morphotype 1 has three elements in the third row: metatarsal two, distal tarsal three and distal tarsal four. This is the common morphology in Ichthyosaurus breviceps, I. conybeari and I. somersetensis. Morphotype 2 has four elements in the third row, owing to a bifurcation. This morphotype occurs in at least some specimens of each species, but it has several variations distinguished by the extent of contact of elements in the third row with the astragalus. Two specimens display a different morphotype in each fin, suggesting that the difference reflects individual variation. In Ichthyosaurus, the hindfin is taxonomically useful at the genus level, but species cannot be identified unequivocally from a well-preserved hindfin, although certain morphologies are more common in certain species than others. The large sample size filled in morphological gaps between what initially appeared to be taxonomically distinct characters. The full picture of variation would have been obscured with a small sample size. Furthermore, we have found several unusual morphologies which, in isolation, could have been mistaken for new taxa. Thus, one must be cautious when describing new species or genera on the basis of limited material, such as isolated fins and fragmentary specimens.

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Arkhangelsky, M. S. & Zverkov, N. G. 2014. On a new ichthyosaur of the genus Undorosaurus . Proceedings of the Zoological Institute, RAS 368, 187–96.
Bakker, M. A. de, Fowler, D. A., Oude, K. den, Dondorp, E. M., Garrido Navas, M. C., Horbanczuk, J. O., Sire, J.-Y., Szczerbinska, D. & Richardson, M. K. 2013. Digit loss in archosaur evolution and the interplay between selection and constraints. Nature 500, 445–9.
Bardet, N. & Fernández, M. 2000. A new ichthyosaur from the Upper Jurassic lithographic limestone of Bavaria. Journal of Paleontology 74, 503–11.
Brusatte, S. L., Young, M. T., Challands, T. J., Clark, N. D. L., Fischer, V., Fraser, N. C., Liston, J. J., Macfadyen, C. J., Ross, D. A., Walsh, S. & Wilkinson, M. 2015. Ichthyosaurs from the Jurassic of Skye, Scotland. Scottish Journal of Geology 51, 4355.
Caldwell, M. W. 1997. Limb ossification patterns of the ichthyosaur Stenopterygius, and a discussion of the proximal tarsal row of ichthyosaurs and other neodiapsid reptiles. Zoological Journal of the Linnean Society 120, 125.
Cooper, L. N., Berta, A., Dawson, S. D. & Reidenberg, J. S. 2007. Evolution of hyperphalangy and digit reduction in the cetacean manus. The Anatomical Record 290, 654–72.
Cooper, L. N. & Dawson, S. D. 2009. The trouble with flippers: a report on the prevalence of digital anomalies in Cetacea. Zoological Journal of the Linnean Society 155, 722–35.
Delsett, L. L., Roberts, A. J., Druckenmiller, P. S. & Hurum, J. H. 2017. A new ophthalmosaurid (Ichthyosauria) from Svalbard, Norway, and evolution of the ichthyopterygian pelvic girdle. PLoS ONE 12, e0169971. doi: 10.1371/journal.pone.0169971.
Druckenmiller, P. S. & Maxwell, E. E. 2010. A new Lower Cretaceous (lower Albian) ichthyosaur genus from the Clearwater Formation, Alberta, Canada. Canadian Journal of Earth Sciences 47, 1037–53.
Fabrezi, M., Abdala, V. & Martínez Oliver, M. I. 2007. Developmental basis of limb homology in lizards. The Anatomical Record 290, 900–12.
Fernández, M. 2007. Redescription and phylogenetic position of Caypullisaurus (Ichthyosauria: Ophthalmosauridae). Journal of Paleontology 81, 368–75.
Fischer, V. 2012. New data on the ichthyosaur Platypterygius hercynicus and its implications for the validity of the genus. Acta Palaeontologica Polonica 57, 123–34.
Fischer, V., Appleby, R. M., Naish, D., Liston, J., Riding, J. B., Brindley, S. & Godefroit, P. 2013. A basal thunnosaurian from Iraq reveals disparate phylogenetic origins for Cretaceous ichthyosaurs. Biology Letters 9, 20130021. doi: 10.1098/rsbl.2013.0021.
Ji, C., Jiang, D.-Y., Motani, R., Rieppel, O., Hao, W.-C. & Sun, Z.-Y. 2016. Phylogeny of the Ichthyopterygia incorporating recent discoveries from South China. Journal of Vertebrate Paleontology 36, e1025956. doi: 10.1080/02724634.2015.1025956.
Johnson, R. 1977. Size independent criteria for estimating relative age and the relationships among growth parameters in a group of fossil reptiles (Reptilia: Ichthyosauria). Canadian Journal of Earth Sciences 14, 1916–192.
Johnson, R. 1979. The osteology of the pectoral complex of Stenopterygius Jaekel (Reptilia, Ichthyosauria). Neues Jahrbuch für Geologie un Paläontologie, Abhandlungen 159, 4186.
Kolb, C. & Sander, P. M. 2009. Redescription of the ichthyosaur Platypterygius hercynicus (Kuhn 1946) from the Lower Cretaceous of Salzgitter (Lower Saxony, Germany). Palaeontographica Abteilung A−Palaozoologie−Stratigraphie 288, 151–92.
Lomax, D. R., Larkin, N. R., Boomer, I., Dey, S. & Copestake, P. 2017. The first known neonate Ichthyosaurus communis skeleton: a rediscovered specimen from the Lower Jurassic, UK. Historical Biology, published online 3 October 2017. doi: 10.1080/08912963.2017.1382488.
Lomax, D. R. & Massare, J. A. 2015. A new species of Ichthyosaurus from the Lower Jurassic of west Dorset, England. Journal of Vertebrate Paleontology 35, e903260. doi: 10.1080/02724634.2014.903260.
Lomax, D. R. & Massare, J. A. 2017. Two new species of Ichthyosaurus from the lowermost Jurassic (Hettangian) of Somerset, U.K. Papers in Palaeontology 3, 120.
Lomax, D. R., Massare, J. A. & Mistry, R. T. 2017. The taxonomic utility of forefin morphology in Lower Jurassic ichthyosaurs: Protoichthyosaurus and Ichthyosaurus . Journal of Vertebrate Paleontology 37, e1361433. doi: 10.1080/02724634.2017.1361433.
Maisch, M. W. 1998. A new ichthyosaur genus from the Posidonia Shale (Lower Toarcian, Jurassic) of Holzmaden, SW Germany with comments on the phylogeny of post-Triassic ichthyosaurs. Neues Jahrbuch fȕr Geologie und Paläontologie, Abhandlungen 209, 4778.
Maisch, M. W. 2010. Phylogeny, systematics, and the origin of the Ichthyosauria—the state of the art. Palaeodiversity 3, 151214.
Massare, J. A. & Lomax, D. R. 2014. An Ichthyosaurus breviceps collected by Mary Anning: new information on the species. Geological Magazine 151, 21–8.
Massare, J. A. & Lomax, D. R. 2016a. Composite specimens of Ichthyosaurus in historic collections. Paludicola 10, 207–40.
Massare, J. A. & Lomax, D. R. 2016b. A new specimen of Ichthyosaurus conybeari (Reptilia: Ichthyosauria) from Watchet, Somerset, U.K. and a re-examination of the species. Journal of Vertebrate Paleontology 36, e1163264. doi: 10.1080/02724634.2016.1163264.
Massare, J. A. & Lomax, D. R. 2017. A taxonomic reassessment of Ichthyosaurus communis and I. intermedius and a revised diagnosis for the genus. Journal of Systematic Palaeontology 16, 263–77.
Maxwell, E. E. 2012. Unravelling the influences of soft tissue flipper development on skeletal variation using and extinct taxon. Journal of Experimental Zoology 318, 545–54.
Maxwell, E. E. & Caldwell, M. W. 2006. A new genus of ichthyosaur from the Lower Cretaceous of western Canada. Palaeontology 49, 1043–52.
Maxwell, E. E., Dick, D., Padilla, S. & Parra, M. L. 2016. A new ophthalmosaurid ichthyosaur from the Early Cretaceous of Colombia. Papers in Palaeontology 2, 5970.
Maxwell, E. E., Scheyer, T. M. & Fowler, D. A. 2014. An evolutionary and developmental perspective on the loss of regionalization in the limbs of derived ichthyosaurs. Geological Magazine 151, 2940.
Maxwell, E. E., Zammit, M. & Druckenmiller, P. S. 2012. Morphology and orientation of the ichthyosaurian femur. Journal of Vertebrate Paleontology 32, 1207–11.
McGowan, C. 1972. The distinction between latipinnate and longipinnate ichthyosaurs. Life Science Occasional Papers, Royal Ontario Museum 20, 18.
McGowan, C. 1974. A revision of the latipinnate ichthyosaurs of the Lower Jurassic of England (Reptilia: Ichthyosauria). Life Sciences Contributions, Royal Ontario Museum 100, 130.
McGowan, C. & Motani, R. 2003. Ichthyopterygia, Part 8. Handbook of Paleoherpetology. Munich: Verlag Dr. Friedrich Pfeil, 175 pp.
Motani, R. 1999. On the evolution and homologies of ichthyopterygian forefins. Journal of Vertebrate Paleontology 19, 2841.
Motani, R., Minoura, N. & Ando, T. 1998. Ichthyosaurian relationships illuminated by new primitive skeletons from Japan. Nature 39, 255–7.
O'Keefe, F. R., Sidor, C. A., Larsson, H. C. E., Maga, A. & Ide, O. 2006. Evolution and homology of the astragalus in early amniotes: new fossils, new perspectives. Journal of Morphology 267, 415–25.
Oster, G. O., Shubin, N., Murray, J. D. & Alberch, P. 1988. Evolution and morphogenetic rules: the shape of the vertebrate limb in ontogeny and phylogeny. Evolution 42, 862–84.
Rieppel, O. 1993. Studies on skeleton formation in reptiles. IV. The homology of the reptilian (amniote) astragalus revisited. Journal of Vertebrate Paleontology 13, 3147.
Scheyer, T. M., Neenan, J. M., Bodogan, T., Furrer, H., Obrist, C. & Palmondon, M. 2017. A new, exceptionally preserved juvenile specimen of Eusaurosphargis dalsassoi (Diapsida) and implications for Mesozoic marine diapsid phylogeny. Scientific Reports 7, 4406. doi: 10.1038/s41598-017-04514-x.
Shubin, N. H. & Alberch, P. 1986. A morphogenetic approach to the origin and basic organization of the tetrapod limb. In Evolutionary Biology (eds Hecht, M. K., Wallace, B. & Prance, G. T.), pp. 319–87. New York: Plenum Press.
Zammit, M., Norris, R. M. & Kear, B. P. 2010. The Australian Cretaceous ichthyosaur Platypterygius australis: a description and review of postcranial remains. Journal of Vertebrate Paleontology 30, 1726–35.
Zverkov, N. 2017. Mesopodial elements in hindlimbs of ichthyosaurs and plesiosaurs: controversial interpretations and possible solution. Abstract for the International Meeting: Secondary Adaptation of Tetrapods to Life in Water, Berlin, 8, 39.
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