Rise of the Cetacea: Part IV – The Protocetids
Since the 7th Triennial Meeting on the Secondary Adaptations of Tetrapods to Life in Water (or SecAd, as it’s somewhat more compactly known) was held in Washington D.C. last week, I thought that this would be an opportune time to continue my Rise of the Cetacea series. This time the spotlight is on the Protocetidae.
The protocetids consist of 16 genera split into 3 subfamilies. The increased aquatic capabilities of the protocetids becomes apparent when their distribution is analysed, this group can be considered the first widespread (almost global) cetacean group, with fossils known from India and Pakistan (where the group first evolved in the middle Eocene), as well as North Africa, West Africa, Europe, North America and South America (Uhen, 2010).
Despite this wide distribution, it appears that most protocetid genera were still capable of moving around on land. However Georgiacetus and Eocetus may be exceptions to this due to the pelvis lacking a bony connection to the vertebral column and a reduced pelvis respectively. Further evidence of protocetids retaining links to land were found in a specimen of Maiacetus that had a partial skeleton of a foetus preserved with it. The authors describing the specimen concluded that the position of the foetus within the thoracic region of the mother indicated that they gave birth on land. Conflicting evidence has been put forward by other researchers however based on necropsies of living cetaceans, where the foetus may have instead been displaced or even may instead represent a prey item that the individual had consumed prior to death.
Protocetids, like the ambulocetids and remingtonocetids, possessed a wide mandibular foramen, most likely housing a fat pad that provided a favourable pathway for sound to travel to the ears. The large, dense tympanic bulla and periotic retain close contact with the skull via the basiocciptial, although the periotic is surrounded by air sinuses. Despite these morphological adaptations for underwater hearing, protocetids still retained an external auditory meatus. The presence of this feature suggests that when these animals were on land they could still hear sounds via the land mammal hearing mechanism.
An exception to these hearing characteristics comes in the form of the most recent described protocetid taxon, Togocetus traversei (Gingerich & Cappetta, 2014). Found in phosphate deposits in Togo, this species possesses a relatively small mandibular canal, but yet still has large, dense tympanic bullae, perhaps demonstrating that these two features are not as tightly linked in terms of function of hearing as previously believed.
By this stage in cetacean evolution, the group had expanded across much of the globe, but it is not until the next part in this series that we see the first obligately aquatic whales, but that will have to wait until next time.
Philip D. Gingerich and Henri Cappetta, 2014. A New Archaeocete and Other Marine Mammals (Cetacea and Sirenia) from Lower Middle Eocene Phosphate Deposits of Togo. Journal of Paleontology, 109-129
Gatesy, J., Geisler, J.H., Chang, J., Buell, C., Berta, A., Meredith, R.W., Springer, M.S. & McGowen, M.R., 2013. A phylogenetic blueprint for a modern whale. Molecular Phylogenetics and Evolution 66, 479–506.
Nummela, S., Thewissen, J.G.M., Bajpai, S., Hussain, T., Kumar, K., 2007. Sound transmission in archaic and modern whales: anatomical adaptations for underwater hearing. Anatomical Record 290, 716–733.
Uhen MD. 1999. New species of protocetid archaeocete whale, Eocetus wardii (Mammalia, Cetacea) from the middle Eocene of North Carolina. J. Paleontol. 73(3):512–28
Uhen MD. 2008d. New protocetid whales from Alabama and Mississippi, and a new cetacean clade, Pelagiceti. J. Vertebr. Paleontol. 28(3):589–93.
Uhen, M.D., 2010. The origin(s) of whales. Annual Review of Earth and Planetary Sciences 38, 189–219.