Zygorhiza True 1908

Abstract

Genus Zygorhiza True, 1908 Zygorhiza True, 1908: 78. TYPE SPECIES. — Basilosaurus kochii Reichenbach in Carus & Koch, 1847 (priority over original Zygorhiza minor due to seniority). DIAGNOSIS. — Same as for species (see below). DISTRIBUTION. — Same as for species (see below). DISCUSSION True(1908) erected Zygorhiza as a new genus forMüller’s(1851) Zeuglodon brachyspondylus minor, and argued for the separation Dorudon serratus from this new genus. Zeuglodon brachyspondylus minor was originally described by Müller as a morphotype of Zeuglodon brachyspondylus, either a new species or juvenile form. The lectotype of Zeuglodon brachyspondylus minor as per Uhen (2013b) is MB.Ma 43247, a partial braincase (Müller et al. 1849: pl. XXVI, fig. 1). TM 8501 and MB.Ma 43248 (type of Zy. kochii). are paralectotypes of Ze. brachyspondylus minor. Kellogg referred two other species to the genus, ‘ Zy. juddi ’ Seeley, 1881 and ‘ Zy. wanklyni ’ Seeley, 1876. Gingerich et al. (2022) discussed the affinities of ‘ Zy. wanklyni ’ and referred it to Pachycetinae. Uhen (1998, 2013a) noted the lack of diagnostic features on ‘ Zy. juddi ’ and thus considered it a nomen dubium. These interpretations are followed here. Köhler & Fordyce (1997) assigned a series of specimens probably belonging to a single individual to Zygorhiza sp. based on being a ‘dorudontine’ with crenulated cingula on the preserved premolars, and an expanded posterior root on P3 (Köhler & Fordyce 1997). The specimens described differ from the North American form in a few diagnostically relevant features. The P3’s enamel is largely smooth, unlike in USNM 11962 (Kellogg 1936). The cervical vertebrae possess small vertebrarterial foramina unlike TM 8501. The thoracic vertebrae show mild elongation, being notably longer than tall or wide. In Zygorhiza (Kellogg 1936; this study) and D. serratus (Daly 1999) the greatest centrum measurement is width. This elongation is notable as no known ‘dorudontine’ possesses elongated thoracic vertebrae. Even in Saghacetus and Stromerius Gingerich, 2007 the thoracic vertebrae are shortened, despite mild elongation being present in the lumbar vertebrae (Uhen 2004; Gingerich 2007; Uhen et al. 2011; Martínez-Cáceres et al. 2017). Thus, it seems that Köhler and Fordyce’s ‘ Zygorhiza ’ sp. represents a member of another, probably new genus.

Authors 3

1. Jonas W. P. Hakkens

   Utrecht University

   Affiliation as printed

   Utrecht University, Faculty of Geosciences, Department of Earth Sciences, Princetonlaan 8 a, 3584 CB Utrecht (the Netherlands) j. w. p. hakkens @ students. uu. nl and jonashakkens @ gmail. com (corresponding author)

2. Jelle W. F. Reumer Aachen Faculty of Geosciences Department of Earth Sciences

   Utrecht University · Naturalis Biodiversity Center · Natural History Museum Rotterdam

   Affiliation as printed

   Utrecht University, Faculty of Geosciences, Department of Earth Sciences, Princetonlaan 8 a, 3584 CB Utrecht (the Netherlands) and Naturalis Biodiversity Center, Darwinweg 2, 2333 CR, Leiden (the Netherlands) and Natural History Museum Rotterdam, Westzeedijk 345, 3015 AA, Rotterdam (the Netherlands) j. w. f. reumer @ uu. nl

3. Anne S. Schulp Aachen Faculty of Geosciences Department of Earth Sciences

   Utrecht University · Naturalis Biodiversity Center · Teylers Museum

   Affiliation as printed

   Utrecht University, Faculty of Geosciences, Department of Earth Sciences, Princetonlaan 8 a, 3584 CB Utrecht (the Netherlands) and Naturalis Biodiversity Center, Darwinweg 2, 2333 CR, Leiden (the Netherlands) and Teylers Museum, Spaarne 16, 2011 CH, Haarlem (the Netherlands) a. s. schulp @ uu. nl