|M. D. Uhen 2013||Members of the species Zygorhiza kochii have strong vertical ornamentation on the enamel of the teeth, and well-developed cingula on the premolars with additional cuspules on these cingula (e.g. Kellogg, 1936; Köhler & Fordyce, 1997). These features are not shown in the type specimen, as it lacks teeth. See the discussion below regarding the replacement of the holotype with a neotype specimen, USNM 11962, which does show this di- agnostic feature clearly (Kellogg, 1936, plates 10-15).
Additional cranial features were investigated as pos- sible diagnostic characteristics of Zygorhiza kochii. All proved to be either too difficult to assess, or proved to be variable in nature. For instance, the periotic has proven to be a good source of diagnostic characters among taxa of Neoceti, but there are a very limited number of periotics associated with teeth and skulls for Zygorhiza kochii. Some small differences in the shape of the pars cochlearis and the anterior processes were observed, but it is impossible to know whether this is due to intraspecific variation or possible interspecific variation.
Also, the area of the cranial vertex was investigated, but proved to be highly variable. Among the five skulls at- tributed to Zygorhiza that preserve this area (RMM 2739, TM 8501, USNM 11962, 16638, and 16639), all have nota- ble differences in the conformation of the sutures in this region. In RMM 2739, the premaxillae meet posterior to the external nares, excluding the anterior nasals from the narial opening. In USNM 16638, the lateral margin of the nasal presents a “notch” into which the posteriormost end of the premaxilla is sutured. TM 8501, USNM 11962 and USNM 16638 all have a large anterior process of the fron- tal (narial process of the frontal). RMM 2739, has a small anterior processes of the frontal, while USNM 16639 has none. Finally, in USNM 11962, the posterior end of the me- dial margin of the maxilla makes contact with the anterior process of the frontal, while in all of the others (this can- not be observed in TM 8501) it contacts the lateral margin of the nasal. Since none of these features seem to correlat- ed to one another, for the time being, Zygorhiza kochii is considered to be a highly variable species in these respects.
|P. D. Gingerich 2015||Zygorhiza kochii, the only species of the genus recognized at present, is distinctive in being the smallest archaeocete known from the Yazoo Formation of late Priabonian, late Eocene, age. Skeletally mature specimens have occipital condyles measuring 110–120 mm across their outer margins (Kellogg, 1936, p. 246–247), in contrast to contemporaneous Pontogeneus brachyspondylus (including Cynthiacetus maxwelli) measuring 142 mm (Uhen, 2005, p. 163) and Basilosaurus cetoides measuring 145 mm (Kellogg, 1936, p. 246–247). Z. kochii has posterior thoracic vertebrae (e.g., T11) measuring 69 mm in centrum length and 79 mm in transverse diameter (Kellogg, 1936, p. 143), in contrast to contemporaneous P. brachyspondylus (C. maxwelli) measuring 85+ mm in centrum length and 112 mm in transverse diameter (Uhen, 2005, p. 163), and B. cetoides measuring 302 mm in centrum length and 177 mm in transverse diameter (Kellogg, 1936, p. 246–247). Zygorhiza resembles contemporaneous Pontogeneus (including Cynthiacetus) in having unusually large vertebrarterial canals perforating the transverse processes of cervical vertebrae C3 and C4 (Kellogg, 1936, p. 133; Uhen, 2005, p. 158), which is a conspicuous difference from the smaller canals of slightly older Dorudon (Uhen, 2004, p. 69) and contemporaneous Basilosaurus (Kellogg, 1936, p. 39).|