A new clade of putative plankton-feeding sharks from the Upper Cretaceous of Russia and the United States

<div><p>ABSTRACT</p><p><i>Eorhincodon casei</i> from Russia and <i>Megachasma comanchensis</i> from the United States are two Cretaceous taxa initially described as putative planktivorous elasmobranchs, but the type specimens of these two taxa were subsequently reinterpreted to represent taphonomically abraded teeth of an odontaspidid, <i>Johnlongia</i> Siverson (Lamniformes: Odontaspididae). Here, we redescribe the type materials of ‘<i>E. casei</i>’ and ‘<i>M. comanchensis</i>’ and describe additional specimens of these species from other Late Cretaceous localities in Russia and the United States. These specimens demonstrate that (1) the two fossil taxa are valid species; (2) they warrant the establishment of a new genus of presumed planktivorous sharks, <i>Pseudomegachasma</i>, gen. nov., to accommodate the two species; and (3) the new genus is sister to <i>Johnlongia</i> and together constitute a new subfamily Johnlonginae, subfam. nov., tentatively placed in the family Odontaspididae sensu stricto. This taxonomic placement indicates that the putative planktivorous clade was derived from a presumed piscivorous form (<i>Johnlongia</i>), with an implication that <i>Pseudomegachasma</i>, gen. nov., evolved a plankton-eating habit independent of the four known planktivorous elasmobranch clades (Rhincodontidae, Megachasmidae, Cetorhinidae, and Mobulidae). It also indicates that planktivorous diets evolved independently at least three times in the order Lamniformes (i.e., Megachasmidae, Cetorhinidae, and Odontaspididae), and more significantly, <i>Pseudomegachasma</i>, gen. nov., would represent the oldest known plankton-feeding elasmobranch in the fossil record. The present fossil record suggests that <i>Pseudomegachasma</i>, gen. nov., evolved in a relatively shallow-water environment in Russia in the early Cenomanian or earlier and subsequently migrated to the North American Western Interior Seaway by the mid-Cenomanian.</p><p>http://zoobank.org/urn:lsid:zoobank.org:pub:D5D0400FD438-4A95-8301-DD47991572F6</p><p>SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP</p></div

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

<p> 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. <i>Alcheringa</i> 43, 85–113.</p> <p>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 <i>Cretoxyrhina</i> 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; <i>Squalicorax acutus</i> sp. nov. and <i>S</i>. <i>bazzii</i> sp. nov. The basal, laminated part of the Haycock Marl is placed in the uppermost upper Cenomanian part of CC10b. It yielded <i>Squalicorax mutabilis</i> sp. nov. and <i>S</i>. aff. <i>S</i>. <i>bernardezi</i>. 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 <i>Squalicorax mutabilis</i> sp. nov., <i>S</i>. aff. <i>S</i>. <i>bernardezi</i>, and <i>S</i>. sp. C.</p> <p><i>Mikael Siversson* [</i><i>mikael</i>.<i>siversson@museum</i>.<i>wa</i>.<i>gov</i>.<i>au</i><i>], Helen E</i>. <i>Ryan [</i><i>helen</i>.<i>ryan@museum</i>.<i>wa</i>.<i>gov</i>.<i>au</i><i>] and Peter Downes [</i><i>peter</i>.<i>downes@museum</i>.<i>wa</i>.<i>gov</i>.<i>au</i><i>] Department of Earth and Planetary Sciences</i>, <i>Western Australian Museum</i>, <i>49 Kew Street</i>, <i>Welshpool</i>, <i>Western Australia 6106</i>, <i>Australia</i>; <i>David K</i>. <i>Watkins [</i><i>dwatkins@unl</i>.<i>edu</i><i>] Department of Earth and Atmospheric Sciences</i>, <i>University of Nebraska</i>, <i>Lincoln</i>, <i>NE 68588</i>, <i>USA</i>; <i>Todd D</i>. <i>Cook [</i><i>tdc15@psu</i>.<i>edu</i><i>] School of Science</i>, <i>Penn State Behrend</i>, <i>4205 College Drive</i>, <i>Erie</i>, <i>PA 16563</i>, <i>USA</i>; <i>Nikolai J</i>. <i>Tatarnic† [</i><i>nikolai</i>.<i>tatarnic@museum</i>.<i>wa</i>.<i>gov</i>.<i>au</i><i>] Department of Terrestrial Zoology</i>, <i>Western Australian Museum</i>, <i>49 Kew Street</i>, <i>Welshpool</i>, <i>Western Australia 6106</i>, <i>Australia; Michael G</i>. <i>Newbrey‡ [</i><i>newbrey_michael@columbusstate</i>.<i>edu</i><i>] Department of Biology</i>, <i>Columbus State University</i>, <i>Columbus</i>, <i>GA 31907</i>-<i>5645</i>, <i>USA. *Also affiliated with: School of Molecular and Life Sciences</i>, <i>Curtin University</i>, <i>Kent Street</i>, <i>Bentley</i>, <i>WA 6102</i>, <i>Australia</i>. <i>†Also affiliated with: Centre for Evolutionary Biology</i>, <i>University of Western Australia</i>, <i>Crawley</i>, <i>Western Australia 6009</i>. <i>‡Also affiliated with: Canadian Fossil Discovery Centre</i>, <i>111-B Gilmour Street</i>, <i>Morden</i>, <i>Manitoba R6 M 1N9</i>, <i>Canada</i>.</p> <p><a href="http://zoobank.org/urn:lsid:zoobank.org:pub:97D5131F-C0D5-4A7E-9C9A-0FDF13BFCBBB" target="_blank">http://zoobank.org/urn:lsid:zoobank.org:pub:97D5131F-C0D5-4A7E-9C9A-0FDF13BFCBBB</a></p> <p><a href="http://zoobank.org/urn:lsid:zoobank.org:act:5977DCC2-355C-4732-8B0A-4BD0EABBA8DE" target="_blank">http://zoobank.org/urn:lsid:zoobank.org:act:5977DCC2-355C-4732-8B0A-4BD0EABBA8DE</a></p> <p><a href="http://zoobank.org/urn:lsid:zoobank.org:act:2D7C4147-B756-4434-847A-B0C1C6D167DF" target="_blank">http://zoobank.org/urn:lsid:zoobank.org:act:2D7C4147-B756-4434-847A-B0C1C6D167DF</a></p> <p><a href="http://zoobank.org/urn:lsid:zoobank.org:act:33F3B55E-41E0-45B3-8296-A3B95C17B41D" target="_blank">http://zoobank.org/urn:lsid:zoobank.org:act:33F3B55E-41E0-45B3-8296-A3B95C17B41D</a></p