On the identity of historical specimen TM 8501 and the taxonomy of North American Basilosauridae (Mammalia, Cetartiodactyla, Archaeoceti)

Phylogeny and taxonomy of early whales have always been a complicated matter, partly due to confusing nineteenth-century descriptions and diagnoses and a profusion of given names. A largely complete cranium with associated postcrania that was discovered by Albert Koch in 1848, a historical specimen now preserved in Teylers Museum, Haarlem, the Netherlands (inv.nr. TM 8501), was given the name Zeuglodon hydrarchus Carus, 1849. It was never properly prepared, and it too became subject of taxonomic confusion and discussion. Here we present a revision of the specimen. Zeuglodon hydrarchus is currently a subjective junior synonym of Zygorhiza kochii (Reichenbach in Carus & Koch, 1847), a species that has remained difficult to diagnose, and probably consists of multiple taxa. The diagnostic characters separating Zygorhiza kochii from Dorudon serratus Gibbes, 1845 have been a longstanding issue in basilosaurid paleontology and a source of contention. Here, key specimen TM 8501 is CT-scanned to reveal obscured morphology, allowing for a more detailed taxonomic assessment. The cranial variation exhibited by Zygorhiza kochii and the similar Dorudon serratus is compared to that exhibited in other basilosaurid taxa. TM 8501 belongs to a group of specimens tentatively referred to as Zygorhiza kochii, separated from Dorudon serratus by the presence of crenulated cingula on P2-4, an anterior process on the frontals of adult specimens, an elongated narial process of the premaxillae, medial maxillae which contact the frontals, the presence of a posteriorly projecting point at mid-height of the nuchal crest, and a single-rooted p1. The type specimen of Zygorhiza kochii remains problematic. Several specimens previously considered Zy. kochii are here reidentified as D. serratus.