PBDB Taxon

Basic info	Taxonomic history	Classification	Included Taxa
Morphology	Ecology and taphonomy	External Literature Search	Age range and collections

Zygorhiza

Mammalia - Cetacea - Basilosauridae

Taxonomy

Zygorhiza was named by True (1908) [Sepkoski's age data: T Eo-m-u T Eo-u Sepkoski's reference number: 1022]. Its type is Basilosaurus kochii. It was considered monophyletic by Uhen (2008), Martinez-Cáceres and Muizon (2011).

It was assigned to Zeuglodontidae by Abel (1914); to Prozeuglodontidae by Moustafa (1954); to Dorudontidae by Kellogg (1928), Hay (1930), Kellogg (1936), Simpson (1945), Thurmond and Jones (1981), Hooker (1986); to Cetacea by Sepkoski (2002); to Basilosaurinae by Bouetel and Muizon (2006); to Dorudontinae by Fordyce (1982), Kohler and Fordyce (1997), McKenna and Bell (1997), Uhen (1998), Uhen (2002), van Vliet (2004), Uhen (2004), McLeod and Barnes (2008), Uhen (2008), Uhen (2009), Fordyce and Roberts (2009), Schouten (2011), Gao and Ni (2015); and to Basilosauridae by Carroll (1988), Benton (1993), Geisler and Sanders (2003), Fitzgerald (2010), Martinez-Cáceres and Muizon (2011), Uhen (2013), Boessenecker and Fordyce (2015), Gingerich (2015), Marx et al. (2016), Martínez-Cáceres et al. (2017), Berta (2017), Uhen (2018).

Species

Z. kochii (syn. Z. brachyspondylus, Z. minor, Zeuglodon brachyspondylum, Zeuglodon hydrarchus)

Year	Name and author
1908	Zygorhiza True p. 78
1914	Zygorhiza Abel p. 220
1928	Zygorhiza Kellogg p. 32 figs. Table 1
1930	Zygorhiza Hay p. 568
1936	Zygorhiza Kellogg p. 100
1945	Zygorhiza Simpson p. 100
1954	Zygorhiza Moustafa p. 87
1981	Zygorhiza Thurmond and Jones p. 192
1982	Zygorhiza Fordyce p. 422
1986	Zygorhiza Hooker p. 342
1988	Zygorhiza Carroll
1993	Zygorhiza Benton p. 761
1997	Zygorhiza Kohler and Fordyce p. 575
1997	Zygorhiza McKenna and Bell p. 369
1998	Zygorhiza Uhen p. 38
2002	Zygorhiza Sepkoski
2002	Zygorhiza Uhen p. 79
2003	Zygorhiza Geisler and Sanders p. 27
2004	Zygorhiza Uhen p. 12
2004	Zygorhiza van Vliet p. 143
2006	Zygorhiza Bouetel and Muizon p. 383
2008	Zygorhiza McLeod and Barnes p. 93
2008	Zygorhiza Uhen p. 562
2009	Zygorhiza Fordyce and Roberts p. 553
2009	Zygorhiza Uhen p. 93
2010	Zygorhiza Fitzgerald p. 370 figs. Table 1
2011	Zygorhiza Martinez-Cáceres and Muizon
2011	Zygorhiza Schouten p. 18
2013	Zygorhiza Uhen p. 11 figs. Figure 8
2015	Zygorhiza Boessenecker and Fordyce figs. Table 1
2015	Zygorhiza Gao and Ni p. 156 figs. Table 1
2015	Zygorhiza Gingerich p. 164
2016	Zygorhiza Marx et al. p. 101
2017	Zygorhiza Berta p. 159
2017	Zygorhiza Martínez-Cáceres et al. p. 11
2018	Zygorhiza Uhen

Diagnosis

Reference	Diagnosis
M. D. Uhen 2008	Zygorhiza is much smaller than both Basilosaurus and Cynthiacetus. Zygorhiza also lacks the elongate trunk vertebrae of Basilosaurus. Zygorhiza can be distinguished from Dorudon and Chrysocetus by the presence of accessory denticles on the cingula of the upper premolars (Köhler and Fordyce, 1997).
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.

Measurements

No measurements are available

Time interval	Ma	Country or state	Original ID and collection number
Bortonian	42.6 - 39.1	New Zealand (South Canterbury)	Z. sp. (32900)
Priabonian	38.0 - 33.9	USA (Louisiana)	Z. kochii (32925 55251 75638)
Priabonian	38.0 - 33.9	USA (Alabama)	Basilosaurus kochii (84754) Z. kochii (124707 131909 131910) Z. kochii, Dorudon sp. (28983)
Priabonian	38.0 - 33.9	United Kingdom (England)	Z. sp. (184754)
Priabonian	38.0 - 33.9	USA (Mississippi)	Z. kochii (45639 55869) Z. sp. (132843)
Late/Upper Eocene	37.2 - 33.9	New Zealand (Canterbury)	Z. sp. (45041)