New material of Natchitochia jonesi and a comparison of the innominata and locomotor capabilities of Protocetidae

Abstract

New material of Natchitochia from the Bartonian Archusa Marl Member is described here, including thoracic, lumbar, sacral, and caudal vertebrae, an innominate, proximal femur, and pedal? phalanx. The vertebrae and innominate are similar to those of Qaisracetus and Georgiacetus. The structure of the caudal vertebrae support previous observations that as sacral vertebrae disconnect from the sacrum, they become caudalized, developing hemal processes on the posteroventral margins of the bodies, reminiscent of chevron bones associated with true caudal vertebrae. The innominate of Natchitochia shares an elongate ilium and pubis with Qaisracetus and Georgiacetus, which differ from the innominata of the more apomorphic archaeocetes. Comparison of archaeocete innominata and sacra in a phylogenetic context indicates that the apomorphic sacrum composed of 4 vertebrae (Pakicetus, Ambulocetus, Rodhocetus, Maiacetus) was reduced to 3 (Qaisracetus) to 2 (Protocetus?, Natchitochia) to 0 (Georgiacetus, Basilosauridae), while the innominata remained robust, supporting a large hind limb until the origin of the Basilosauridae. In Georgiacetus, the innominate is large but detached from the vertebral column, preventing the use of the hind limb in terrestrial locomotion. More crownward cetaceans for which the innominate is known display greatly reduced innominata and hind limbs are disconnected from the vertebral column.

Finding sacral: Developmental evolution of the axial skeleton of odontocetes (Cetacea)

“…Ancestral (archaeocete) cetaceans mimicked terrestrial mammals in most aspects of axial morphology, and the fossil record documents preliminary steps in column reorganization (Bebej, ul-Haq, Zalmout, & Gingerich, 2012;Uhen, 2014). Like many terrestrial mammals, protocetid archaeocetes had six lumbar vertebrae and four sacral vertebrae.…”

“…Like many terrestrial mammals, protocetid archaeocetes had six lumbar vertebrae and four sacral vertebrae. These losses occurred in a posterior to anterior direction, with the result that precaudal vertebrae with apparently lumbar morphology existed both anterior and posterior to remaining sacrals in transitional taxa (Uhen, 2014). These losses occurred in a posterior to anterior direction, with the result that precaudal vertebrae with apparently lumbar morphology existed both anterior and posterior to remaining sacrals in transitional taxa (Uhen, 2014).…”

“…Derived protocetids showed progressive modification of the sacrals: both their articulations with the pelvis and with each other were lost (Bebej et al, 2012;Gingerich, Raza, Arif, & Zhou, 1994;Hulbert et al, 1998;Uhen, 2014). Derived protocetids lacked the sacral series completely, but their lumbar counts were enhanced by four, strongly suggesting that four dedifferentiated sacral homologs were present at the posterior end of the lumbar series (Buchholtz, 2007;Uhen, 2014). Derived protocetids lacked the sacral series completely, but their lumbar counts were enhanced by four, strongly suggesting that four dedifferentiated sacral homologs were present at the posterior end of the lumbar series (Buchholtz, 2007;Uhen, 2014).…”

“…Recently, Buchholtz (2007) and Uhen (2014) hypothesized that the columns of living cetaceans exhibit a novel modular organization. Hypothesis 1 is discordant with the extreme reduction in lumbar count in some odontocetes: the loss of all lumbar vertebrae at the expense of a fixed count of sacral homologs is implied.…”

“…Maiacetus data fromGingerich et al (2009), Qaisracetus data fromGingerich et al (2001), Rodhocetus data fromGingerich et al (1994), Basilosaurid data fromUhen (2014) and fromBebej et al (2012). Maiacetus data fromGingerich et al (2009), Qaisracetus data fromGingerich et al (2001), Rodhocetus data fromGingerich et al (1994), Basilosaurid data fromUhen (2014) and fromBebej et al (2012).…”