Zygorhiza

Zygorhiza ('Yoke-Root') is an extinct genus of basilosaurid early whale known from the Late Eocene (Priabonian, 38–34 Ma) of Louisiana, Alabama, and Mississippi, United States, and the Bortonian (43–37 Ma on the New Zealand geologic time scale) to the late Eocene of New Zealand (43 to 33.9 million years ago).Specimens reported from Europe are considered Dorudontinae incertae sedis. Zygorhiza kochii, along with Basilosaurus under the designation 'prehistoric whales', is the state fossil of Mississippi. The mounted specimen in the Mississippi Museum of Natural Science in Jackson is commonly referred to as 'Ziggy'. Reichenbach (1847) erected Basilosaurus kochii for the posterior skull fragment MB Ma.43248, found in the Late Eocene (middle-late Priabonian) Ocala Limestone of Clarksville, Louisiana. Meanwhile, Muller (1851) erected a new subspecies of Zeuglodon brachyspondylus, Z. brachyspondylus minor, for not only MB Ma.43248 but also MB Ma.43247, TM 8501 (holotype of Zeuglodon hydrarchus Carus, 1849), and several vertebrae. In the late 19th century there was a debate whether large and small specimens attributed to Zeuglodon brachyspondylus (declared a nomen dubium by Uhen 2005) were separate species or not. Hoping to clarify things, Stromer 1903 restricted Z. brachyspondylus to the large fossils (including the Z. brachyspondylus lectotype) and created the subspecies Z. brachyspondylus minor for the small specimens which had previously been synonymized with Dorudon serratus. True 1908 proposed the genus Zygorhiza for the subspecies. Adopting True's generic name, Kellogg 1936 synonymized this subspecies with Basilosaurus kochii to form the new combination Zygorhiza kochii. Seeley 1876 named and described the species Zeuglodon wanklyni based on a skull collected by Dr. Arthur Wanklyn from the Barton Clays in southern England. This skull, however, was never deposited at the British Museum of Natural History and has not been since Seeley described it. Kellogg 1936, nevertheless, recombined it as Zygorhiza wanklyni and referred a posterior cervical vertebra from the same location to it. Uhen 1998 declared it nomen dubium. Köhler & Fordyce 1997 described an incomplete skull, four vertebrae, two teeth, and small fragments in early Bartonian sediments in New Zealand which they tentatively identified as Zygorhiza sp.. This is the oldest known Dorudontinae and the oldest known cetacean from the Southern Hemisphere In the U.S., Zygorhiza is known from the Gulf Coast, whilst Dorudon is known from southeastern Atlantic Coast. Outside North America, Zygorhiza has only been reliably identified in New Zealand, whereas Dorudon only in Egypt. It is possible that these non-overlapping distributions indicate differences in habitat preferences. Like other dorundontines, Zygorhiza had a body similar to modern cetaceans with flipper-like forelimbs, rudimentary hind limbs, a vertebral column adapted for oscillatory swimming, and a tail fluke. Marino et al. 2000 estimated the adult body mass for Zygorhiza to 3,351 kg (7,388 lb) based on an estimated body length of 520 cm (200 in). Using CT scans, they estimated the brain weight to 738.2 g (26.04 oz), resulting in a EQ value of 0.26 (compared to 0.54 for a modern cetacean such as Cuvier's beaked whale.)