Anacoracid sharks and calcareous nannofossil stratigraphy of the mid-Cretaceous ‘upper’ Gearle Siltstone and Haycock Marl in the lower Murchison River area, Western Australia

Abstract

Siversson, M., Cook, T.D., Ryan, H.E., Watkins, D.K., Tatarnic, N.J., Downes, P.J. & Newbrey, M.G. 5 June 2018. Anacoracid sharks and calcareous nannofossil stratigraphy of the mid-Cretaceous Gearle Siltstone and Haycock Marl in the lower Murchison River area, Western Australia. Alcheringa 43, 85–113.

Extensive bulk sampling over the past 20 years and greatly improved stratigraphic control permitted a meaningful revision of previously described anacoracid sharks from the ‘upper’ Gearle Siltstone and lower Haycock Marl in the lower Murchison River area, Western Australia. Isolated teeth of anacoracids are rare in the lower three (Beds 1–3) of four stratigraphic units of the ‘upper’ Gearle Siltstone but relatively common in the uppermost layer (Bed 4) and in the lower part of the overlying Haycock Marl. On the basis of calcareous nannofossils, Beds 1 and 2 of the ‘upper’ Gearle Siltstone can be placed in the uppermost upper Albian calcareous nannofossil Subzone CC9b whereas Bed 3 can be referred to the lowermost Cenomanian CC9c Subzone. Bed 1 yielded fragments of strongly serrated anacoracid teeth as well as a single, smooth-edged tooth. The samples from Beds 2 and 3 contained a few small fragments of serrated anacoracid teeth. Bed 4 is barren of calcareous nannofossils but the presence of a dentally advanced tooth of the cosmopolitan lamniform genus Cretoxyrhina in combination with the age of the overlying Haycock Marl indicate deposition within the younger half of the Cenomanian. The unit produced teeth of two anacoracids; Squalicorax acutus sp. nov. and S. bazzii sp. nov. The basal, laminated part of the Haycock Marl is placed in the uppermost upper Cenomanian part of CC10b. It yielded Squalicorax mutabilis sp. nov. and S. aff. S. bernardezi. Exceptionally well-preserved teeth of the former species span a 5:1 size ratio range for teeth of comparable jaw position. The teeth reveal strong ontogenetic heterodonty with a large increase in the relative size of the main cusp with age and the transition from a vertical distal heel of the crown in very young juveniles to a sub-horizontal, well demarcated heel in ‘adult’ teeth. An isolated phosphatic lens in the lower part of the Haycock Marl produced calcareous nannofossils indicative of the CC10b SubZone, most likely the lowermost lower Turonian part. It contains teeth of Squalicorax mutabilis sp. nov., S. aff. S. bernardezi, and S. sp. C.

Mikael Siversson* [[email protected]], Helen E. Ryan [[email protected]] and Peter Downes [[email protected]] Department of Earth and Planetary Sciences, Western Australian Museum, 49 Kew Street, Welshpool, Western Australia 6106, Australia; David K. Watkins [[email protected]] Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, NE 68588, USA; Todd D. Cook [[email protected]] School of Science, Penn State Behrend, 4205 College Drive, Erie, PA 16563, USA; Nikolai J. Tatarnic† [[email protected]] Department of Terrestrial Zoology, Western Australian Museum, 49 Kew Street, Welshpool, Western Australia 6106, Australia; Michael G. Newbrey‡ [[email protected]] Department of Biology, Columbus State University, Columbus, GA 31907-5645, USA. *Also affiliated with: School of Molecular and Life Sciences, Curtin University, Kent Street, Bentley, WA 6102, Australia. †Also affiliated with: Centre for Evolutionary Biology, University of Western Australia, Crawley, Western Australia 6009. ‡Also affiliated with: Canadian Fossil Discovery Centre, 111-B Gilmour Street, Morden, Manitoba R6 M 1N9, Canada.

http://zoobank.org/urn:lsid:zoobank.org:pub:97D5131F-C0D5-4A7E-9C9A-0FDF13BFCBBB

http://zoobank.org/urn:lsid:zoobank.org:act:5977DCC2-355C-4732-8B0A-4BD0EABBA8DE

http://zoobank.org/urn:lsid:zoobank.org:act:2D7C4147-B756-4434-847A-B0C1C6D167DF

http://zoobank.org/urn:lsid:zoobank.org:act:33F3B55E-41E0-45B3-8296-A3B95C17B41D

Key words:

• Cretaceous
• elasmobranchs
• Gearle
• Haycock
• nannofossils
• Squalicorax
• teeth

Acknowledgements

We thank two anonymous reviewers and editor Benjamin Kear for thoughtful remarks that improved the manuscript. Eugeny Popov assisted us with information and photographs of specimens kept in museums in Russia. Funding for this research was provided by the 2012 Endeavour Research Fellowship Award to TDC (#3392-2012) and Royal Tyrrell Museum Cooperating Society grant to MGN. We are also grateful for the support provided by the Department of Earth and Planetary Sciences, Western Australian Museum and the School of Molecular and Life Sciences, Curtin University.