Zygorhiza kochii

Abstract

Zygorhiza kochii (Reichenbach in Carus & Koch, 1847) Basilosaurus kochii Reichenbach in Carus & Koch, 1847: 13. Zeuglodon hydrarchus Carus, 1849: 17-18 Zeuglodon brachyspondylus minor Müller, 1851: 240 (in part). Zygorhiza minor Kellogg, 1928: 40. Zygorhiza kochii Kellogg, 1936: 101. TYPE MATERIAL. — Holotype. United States • 1 specimen (braincase with partial periotic attached); Tennessee, Montgomery County, Clarksville; [36°31’47”N, 87°21’34”W]; Jackson group undifferentiated (formerly “Ocala limestone”); MB [MB.Ma.43248]. T YPE LOCALITY. — Collected near Clarksville, exact locale unclear (Uhen 2013a). T YPE HORIZON AND AGE. — Formerly “Ocala limestone”, currently Jackson group undifferentiated (Uhen 2013a). R EFERRED SPECIMENS. — TM 8501, USNM 11962, RMM 3927 / AUMP 2368. DISTRIBUTION. — All specimens assigned to Zygorhiza kochii here were recovered from the Pachuta Marl Member of the Yazoo Formation of late Priabonian age from Alabama (Uhen 2013a; Gingerich 2015b). AMENDED AND DIFFERENTIAL DIAGNOSIS. — Adult Zygorhiza kochii are mid-sized basilosaurids (condylobasal length c. 83 cm), smaller than Basilosaurus spp. (Kellogg 1936), Basiloterus (Gingerich et al. 1997), Cynthiacetus spp. (Martínez-Cáceres et al. 2017), Eocetus (Gingerich & Zouhri 2015), Perucetus (Bianucci et al. 2023) and Masracetus (Gingerich 2007), larger than Chrysocetus (Uhen & Gingerich 2001; Uhen 2013a), Saghacetus (Martínez-Cáceres et al. 2017), Ocucajea (Uhen et al. 2011), Stromerius (Gingerich 2007) and Tutcetus (Antar et al. 2023). The posterior narial region of Zygorhiza kochii is recognizable by the extended narial process of the premaxillae, which about the nasals along c. 80% of their length (autapomorphy; this study); the medial margin of the maxillae contacting the frontals (shared with Saghacetus osiris; this study); and a short (c. 25% nasal length), triangular anterior process on the frontals (shared with Cynthiacetus spp.; this study). The occipital shield of Zygorhiza kochii is anterodorsally oriented such that the occipital condyles are visible in dorsal view (differing from Saghacetus, Cynthiacetus spp.; this study) and bears a nuchal crest with a posteriorly projecting point at mid-height (shared only with Basilosaurus spp.; this study). The tympanic bulla of Zygorhiza kochii lacks the square shape characteristic of Supayacetus (Uhen et al. 2011).The dentition of Zygorhiza kochii is recognized by the crenulated cingulum (Kellogg 1936; Uhen 2013a; shared with Pachycetus (Gol’din et al. 2014) and an unnamed New Zealand basilosaurid (Köhler & Fordyce 1997) and enamel bearing on P2-4 prominent vertical striae (autapomorphy; Kellogg 1936; Uhen 2013a) on the P2-4. The p1 is single-rooted (differing from Dorudon spp.; this study). The middle cervical vertebrae show large vertebrarterial foramina (diameter greater than the thickness of the upper bounding bone), shared with Dorudon serratus, Cynthiacetus spp. (Gingerich 2015b). The posterior thoracic, lumbar, and caudal vertebrae show no elongation, with the height, width and length of centra being approximately equal, differing from Pachycetinae, Basilosaurus spp. (Uhen 2013a), Basiloterus (Gingerich et al. 1997), Eocetus (Gingerich & Zouhri 2015), Saghacetus (Martínez-Cáceres et al. 2017) and Stromerius (Gingerich 2007). The neural arches are likewise not elongated, and no sign of pachyostosis is notable (differing from Pachycetinae; Gingerich et al. 2022). The metapophyses on the lumbar vertebrae are not anteriorly elongated (differing from Stomerius; Gingerich 2007). The manubrium of Zygorhiza kochii is not T-shaped, unlike Supayacetus (Uhen et al. 2011) and Pachycetus (Gol’din et al. 2014). DISCUSSION Basilosaurus kochii is the oldest name given to the small basilosaurid from Koch’s Hydrarchos, followed by ‘ Zeuglodon hydrarchus ’. Müller (1851) then lumped both taxa together with D. serratus into Zeuglodon brachyspondylus minor. Müller noted that he was unsure whether these small specimens represented adults of a separate species, or juveniles of the larger ‘ Zeuglodon brachyspondylus ’ (now Cynthiacetus maxwelli Uhen, 2005). True (1908) then erected the genus Zygorhiza for Zeuglodon brachyspondylus minor, but did not specify a full species name. True (1908) additionally only used ontogenetically variable characters such as the robustness of the snout and spacing of teeth to separate Zy. kochii from D. serratus. Kellogg (1928) elevated the subspecies Ze. brachyspondylus minor to a species level resulting in the combination Zygorhiza minor. In his 1936 monograph, Kellogg recognized that Z. kochii was the proper combination for the taxon but did not recognize the ontogenetic nature of most of True’s characters, and expanded the list of referred specimens to over twenty, based on these characteristics. This issue was later recognized by Uhen (1998). When describing MMNS VP 130, Daly (1999) attempted to identify new diagnostic features. However, given the specimen described in Daly (1999) is currently seen as D. serratus (Uhen 2013a), all features identified by Daly are in fact shared by both taxa. Uhen (2004) attempted to separate Zy. kochii from members of Dorudon, however was only working from a single specimen, thus not all identified features are applicable to the specimens assigned to Zy. kochii. Uhen (2013a) examined the known cranial sutures in specimens identified as Zygorhiza and found them unusually variable for a single species. Uhen (2013a) also analyzed periotics identified as Zygorhiza, but found no significant variation. Upon recognizing this situation, Uhen (2013b) petitioned the ICZN to designate USNM 11962 as the neotype specimen of Zygorhiza kochii. Gingerich (2015a; 2015b) argued that stratigraphic context, and size disparities, allowed for proper assignment of the original holotype to Zygorhiza kochii. Gingerich (2015a) further argued that the size of the vertebrarterial foramina on the cervical vertebrae distinguished Zy. kochii from members of genus Dorudon, but this observation appears entirely based on D. atrox. Gingerich (2015b) also noted that the type of Zy. kochii is neither lost nor destroyed. The ICZN (2017) rejected Uhen’s proposal on the grounds that it was premature. When skulls assigned to Zygorhiza kochii and Dorudon serratus are compared, two morphotypes can be recognized; an overview of this is given in Table 3. The first contains USNM 11962 (Figs 5B; 6B), TM 8501 (Figs 5A; 6B), MSC 3927 (Figs 5C; 6C), and tentatively MB Ma.43248 (Figs 5D; 6D). These specimens are characterized by; the presence of a small triangular anterior process on the frontals, elongated narial processes of the premaxillae, medial maxillae which contact the frontals, the presence of a posteriorly projecting point on the nuchal crest at mid-height, and a single-rooted p1. The skull of MSC 3927 is heavily crushed (see Fig. 6C), affecting perceived layout of facial bones. The nasals have been pressed downwards, and the premaxillae folded over the top, resulting in the nasals being excluded from the narial fossa. The taphonomic nature of this morphology is clear from the asymmetry in dorsal view, and the difference in height between the premaxillae and nasals. The second morphotype contains USNM 16639 (Figs 5G; 6G), MMNS VP 130 (Figs 5H; 6H), FMNH PM-459 (Figs 5I; 6I). This morphotype is characterized by; the lack of an anterior process on the frontals, medial maxillae that do not contact the frontals, premaxillae that terminate roughly halfway along the lateral margin of the nasals, a nuchal crest which is rounded in lateral view and whose posterior margin is roughly equal to the exoccipital process, and a single-rooted p1. MCZ 8763 (Figs 5E; 6E), UNSM 13773 (Kellogg 1936: fig. 33), and USNM 16638 (Figs 5F; 6F) are also assigned to this morphotype as juveniles, but possess an anterior process on the frontals. MCZ 8763 possessed a large anterior process on the frontals as seen by the morphology of the nasals, USNM 16638 and USNM 13773 both show a smaller anterior process on the frontals, and are ontogenetically older than MCZ 8763 (Kellogg 1936). The nasal-frontal contact in basilosaurids is not a lateral-lateral suture, rather the nasals extend over the dorsal surface of the frontals’ anterior margin (Fahlke 2012: fig. 1). Thus, the anterior process on the frontals could be ontogenetically variable within Dorudon serratus, extending over the dorsal surface of the frontals with ontogeny to cover the anterior process on the frontals. The crenulated cingulum on P2-4 considered diagnostic of Zygorhiza by Uhen (1998, 2013a) is not preserved in all assigned specimens; it is present in USNM 11962 (Kellogg 1936), and MSC 3927 (Daly 1999), but USNM 16639 and FMMH PM-459 lack the upper premolars, while the P2-4 of USNM 16638 are deciduous. Gingerich (2015b) describes the p1 of Zygorhiza as being double-rooted, and cites Kellogg (1936). The cited text however refers to the P1, not p1. Peredo et al. (2018) describe the morphology as double-rooted within a single alveolus, however this feature appears to differ between specimens. MMNS VP 130 (Uhen 2000), and FMNH PM-459 (William Simpson pers. comm.) appear to possess separate alveoli, while TM 8501 and USNM 11962 do preserve only a single alveolus but no tooth.

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