Rise of the Cetacea: Part I – The Pakicetids

18 November 2013 by Travis Park, posted in Cetacea, Personal research

After a couple of frustrating months of issues and setbacks plaguing my attempts to process the data I collected from several fossil cetacean periotics at the Australian Synchrotron for my PhD research, the past fortnight has seen a breakthrough and I can now start actually playing around with the images in Avizo (cool image processing software). Hooray! Bearing this in mind, aside from the post where I detailed my research proposal, I haven’t been writing very much (or at all) about my PhD, or at the very least, material related to the overall topic of fossil cetaceans (whales and dolphins). I’ve decided to remedy this situation by writing a new series of posts covering the evolutionary history of cetaceans, starting with the semi-aquatic ‘walking whales’ and finishing with the two modern cetacean groups the mysticetes (baleen whales) and odontocetes (toothed whales). As the evolution of whales is one of the best if not the case study in macroevolution, there is already a plethora of information about this stuff on the web (e.g. here), but my hope that this will provide an intro to this fascinating subject, as well as potentially being a gateway drug of sorts to those who perhaps were unaware of this remarkable evolutionary narrative. This initial post will look at the very earliest whales, the semi aquatic Pakicetids.

A very simplified cladogram showing the major groups of whales during the course of their evolution. The group of of interest in this post, the pakicetids, are highlighted in blue. Reconstructions are by Carl Buell.

The Pakicetidae are known from the end of the Early Eocene to the beginning of the middle Eocene (approx. 50 – 45 Ma) and includes six named species in three genera: Pakicetus, Ichthyolestes and Nalacetus. The first of these taxa to be named was Ichthyolestes, which was first described in 1958 by Dehm and Oettingen-Spielberg 1958. They, perhaps understandably given there was no cetaceans of this kind known at the time, did not recognise its cetacean affinities, instead naming it as a mesonchynid. It wasn’t until the early 1980’s that their true cetacean nature was uncovered and understood (West, 1980; Gingerich and Russell, 1981).

A reconstruction of Pakicetus by the fantastic artist Carl Buell. Taken from Gatesy et al 2013.

 

Pakicetids, being the earliest whales, have a combination of morphological features from both terrestrial artiodactyls and also Cetacea. Their retained artiodactyl characteristics include elongate cervical vertebrae and limbs, an astragalus with the characteristic artiodactyl double pulleyed morphology, long metapodials, and the four sacral vertebrae were fused. The mandibular foramen was also small, a feature that rapidly changed as the descendants of these early whales became more and more adapted to hearing underwater. Morphological features for underwater hearing had already appeared, albeit in incipient forms. These included features such as a pachyosteosclerotic tympanic bulla (one of the middle ear bones) that also possessed an involucrum and a sigmoid process, features characteristic of cetaceans today (Uhen, 2010). The tympanic bulla also lacked a rostromedial connection to the periotic, allowing the tympanic to vibrate independently of the periotic (Nummela et al. 2004). This enhanced their ability to use bone conduction, which would have been the method by which sound was transmitted when underwater, although directional hearing underwater would still have been poor (Nummela et al. 2004, 2007).

A comparison between a typical land mammal ear (L) and the pakicetid ear (R). The key difference is the formation of the involucrum, enabling pakicetids to begin to hear underwater. Abbreviations: Coc, cochlea; EAM, external acoustic meatus; Inc, incus; Inv, involucrum; Mal, malleus; Man, mandible; MeTy, medial synostosis between periotic and tympanic bone, in cetaceans
this synostosis is absent and is homologous to a gap between these bones; OvW, oval window; Per, periotic bone; PeTy, joint between periotic and tympanic;
Sk, skull; Sta, stapes; TyBo, tympanic bone; TyMe, tympanic membrane; Modified from Nummela et al. 2004.

Unlike modern cetaceans, pakicetids could also still hear airborne sounds as they also possessed an external auditory meatus that permitted the transmission of them to the tympanic membrane, where the differential pressures cause it to vibrate and pass the sound to the three middle ear bones, the malleus, incus and stapes, which in turn transmit the sound to the inner ear. Other cetacean features found in pakicetids include dental characteristics such as lower molars that lack trigonid and talonid basins, upper molars that have a very small trigonid basin and anterior dentition that is in line with the cheek teeth.

Even though researchers are agreed that pakicetids were cetaceans, there has been debate in the past as to their lifestyle and the amount of time that the animals spent in the water. Thewissen et al (2001) argued that pakicetids were terrestrial animals based on the structure of the vertebrae and the post-cranial elements. They envisioned the pakicetids to be spending more time running and jumping around on land rather than swimming in rivers or the sea. Gingerich (2003) however disagreed with this interpretation, citing the fact that the relative lengths of the ilium and ischium (bones in the pelvic region) are more similar to those of an aquatic animal. Backing this up was studies of the microstructure of pakicetid bones by Madar (2007), who found them to be much denser than terrestrial mammals, a finding that also seems to support a more aquatic lifestyle for the group. Further support for an aquatic mode of living came from a couple of isotopic studies that showed that pakicetids most likely consumed freshwater prey, drank freshwater and most likely spent a large proportion of their lives in freshwater.

So the cetacean invasion of the water had begun. But most people would not think “thar she blows” if they saw a pakicetid alive today. The group still had to undergo a whole suite of changes to almost every aspect of their morphology. In the forthcoming posts in this series, we will see just how and when these changes are thought to have occurred and in turn, reveal one of the grandest ‘tails’ in the whole of evolution. I hope you’ll enjoy them.

 

References

Dehm R, Oettingen-Spielberg T. 1958. Palaontologische und geologische Untersuchungen im Terti¨ar von Pakistan, 2. Die mitteleocanen Saugetiere von Ganda Kas bei Basal in Nordwest-Pakistan. Bayer. Akad. Wiss. Math.-Naturwiss. Klasse 91:1–54

Gingerich PD, Russell DE. 1981. Pakicetus inachus, a new archaeocete (Mammalia, Cetacea) from the early-middle Eocene Kuldana Formation of Kohat (Pakistan). Contrib. Mus. Paleontol. Univ.Mich. 25(11):235–46

Gingerich PD. 2003a. Land-to-sea transition in early whales: evolution of Eocene Archaeoceti (Cetacea) in
relation to skeletal proportions and locomotion of living semiaquatic mammals. Paleobiology 29(3):429–54

Madar SI. 2007. The postcranial skeleton of early Eocene pakicetid cetaceans. J. Paleontol. 81(1):176–200

Nummela, S., Thewissen, J.G.M., Bajpai, S., Hussain, S.T. & Kumar, K., 2004. Eocene evolution of whale hearing. Nature 430, 776–778.

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.

Thewissen JGM, Williams EM, Roe LJ, Hussain ST. 2001. Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls. Nature 413:277–81.

Uhen, M.D., 2010. The origin(s) of whales. Annual Review of Earth and Planetary Sciences 38, 189–219.

West, Robert M (1980). "Middle Eocene large mammal assemblage with Tethyan affinities, Ganda Kas region, Pakistan". Journal of Paleontology 54 (3): 508–533.

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