Tuesday, February 25, 2014

When will we find more Eocene odontocetes and mysticetes?

Baleen whales (Mysticeti) and toothed whales (Odontoceti) are nearly universally considered to share a sister-group relationship, and constitute a monophyletic clade termed Neoceti (also known as Autoceta). Odontocetes and mysticetes are generally considered to have diverged and diversified during the Oligocene, and neither group really has an extensive fossil record prior to the Oligocene. On the other hand, archaeocetes – a paraphyletic assemblage of stem cetaceans leading up to neocetes – generally are considered to be restricted to the Eocene. For paleocetologists, the Eo-Oligocene boundary is colloquially thought of as the archaeocete-neocete split. However, many early Oligocene neocetes are relatively derived, and few are really archaeocete-like – in other words, few appear to exhibit archaeocete-like morphology with only a couple of acquired neocete synapomorphies. This begs the question of when exactly Neoceti evolved.

The early Oligocene dolphin Simocetus rayi.

Early Oligocene Neoceti

Although the majority of the Oligocene record of fossil cetaceans is limited to the late Oligocene (owing to generally low sea levels and widespread erosion of early Oligocene strata), a few notable records of early Oligocene cetaceans are worth discussing. In particular are the fossil cetaceans of the Alsea Formation in west central Oregon. Although considered by Fordyce (2002) to be late Oligocene, paleomagnetic data indicate it is probably early Oligocene in age (Prothero, 2001). First and foremost is Simocetus rayi – the only formally described cetacean from this unit, a bizarre agorophiid-grade dolphin described by Fordyce (2002). Simocetus is pretty derived, and not really archaeocete-like in many regards. Another agorophiid-grade dolphin is an unnamed tusked odontocete, preliminarily reported by Fordyce et al. (2012) at the Society of Vertebrate Paleontology meeting in Raleigh, North Carolina. This undescribed dolphin is also fairly derived and fairly removed from the odontocete stem. A third cetacean from the Alsea Formation is the world’s earliest toothless mysticete (an eomysticetid in my personal opinion), which remains undescribed and was preliminarily reported by Mark Uhen (2007). A fourth cetacean which I spotted at the USNM in 2012 is an undescribed aetiocetid with a complete braincase and somewhat basilosaurid-like mandible. The Alsea Formation demonstrates that the early Oligocene was populated by the same sorts of cetaceans known from the late Oligocene. Most notably, the presence of an early chaeomysticete – in my opinion an eomysticetid – indicates that mysticete evolution is telescoped into a 5 million year interval (or less), given our current understanding of the timing of mysticete origins.

Holotype mandible fragment of the late Eocene mysticete Llanocetus denticrenatus from the La Meseta Formation of Seymour Island. There is much more material awaiting description.

Known records of neocetes in the Eocene

As could be surmised from the presence of relatively derived Neoceti in the early Oligocene, a few – but only a few – records of latest Eocene mysticetes and odontocetes exist. First and foremost, the only named Eocene neocete is the earliest known baleen whale, Llanocetus denticrenatus from the La Meseta Formation of Seymour Island in Antarctica. It was named by Ed Mitchell in 1989, and the miserable scraps he designated as the holotype were originally collected in the mid 1970’s. My Ph.D. adviser, Ewan Fordyce, returned to the locality in 1986 and collected the rest of the specimen, which is a rather large skull and mandible in addition to some postcrania. The type specimen dates from just below the Eo-Oligocene boundary and is approximately ~34 Ma in age. Accordingly, Llanocetus is perhaps one of the most basilosaurid-like mysticetes. There is also an undescribed odontocete preliminarily reported from about the Eo-Oligocene boundary within the Lincoln Creek Formation in Washington state, U.S.A. (Barnes and Goedert, 2000). These specimens – although only barely scraping into the Eocene – do demonstrate that odontocetes and mysticetes did evolve before the end of the Eocene.

Phylogenetic relationships and stratigraphic ranges of Basilosauridae, from Gol'Din and Zvonok 2013.

The earliest Basilosauridae – middle Eocene

One problem inherent with a late or even latest Eocene origin of Neoceti is that it would telescope the majority of basilosaurid evolution into a 5 Ma period during the Priabonian and late Bartonian stages of the Eocene. The oldest records of traditionally identified basilosaurids are only about 40 Ma, only 5-6 Ma older than Llanocetus. However, a recent discovery of a basilosaurid from the Bartonian (late Middle Eocene) of Ukraine suggests a reinterpretation of “Eocetuswardii from similarly aged strata in the eastern USA. Gol’din and Zvonok (2013) named a new genus and species, Basilotritus uheni – which has vertebrae like “Eocetuswardii with the tympanic bulla of a basilosaurid. Gol’Din and Zvonok (2013) transferred “Eocetuswardii to Basilotritus, recombining it as Basilotritus wardii. In additition to the recognition of both species of Basilotritus as early basilosaurids, a couple other middle Eocene basilosaurids have been reported, including a braincase from the Bartonian of New Zealand identified as Zygorhiza sp. (Kohler and Fordyce, 1997), and Ocucajea and Supaycetus from the Bartonian of Peru (Uhen et al., 2011). Unfortunately, the protocetid-basilosaurid transition is poorly known, although several protocetids exhibit derived basilosaurid-like features, including Georgiacetus, Babiacetus, and Eocetus, and the earlier basilosaurids like Basilotritus and Supaycetus are a bit more plesiomorphic than other basilosaurids.

In summary, there are numerous derived Neoceti from the early Oligocene, a couple of genuine records of Neoceti from the latest Eocene – and lastly, the expanded fossil record of basilosaurids now ameliorates the problem of a formerly telescoped record of the family. More records of early odontocetes and mysticetes from the Eocene does not sound like such an strange idea anymore, but is in fact now predicted by the fossil record. We have little evidence of it, but improved sampling of late Eocene marine rocks – especially from poorly sampled areas (in terms of Eocene rocks) like the Pacific Northwest and the west coast of South America – may yield more records of early Neoceti.

References Cited

L. G. Barnes and J. L. Goedert. 2000. The world's oldest known odontocete (Mammalia, Cetacea). Journal of Vertebrate Paleontology 20(3):28A.

R. E. Fordyce. 2002. Simocetus rayi (Odontoceti, Simocetidae, new family); a bizarre new archaic Oligocene dolphin from the eastern North Pacific. Smithsonian Contributions to Paleobiology 93:185-222.

R.E. Fordyce, E.M.G. Fitzgerald, G. Gonzalez Barba. 2012. Long-tusked archaic odontocetes from Oregon and Baja California Sur, eastern Pacific Margin. Journal of Vertebrate Paleontology 32:3:95.

P. Goldin and E. Zvonok. 2013. Basilotritus uheni, a New Cetacean (Cetacea, Basilosauridae) from the Late Middle Eocene of Eastern Europe. Journal of Paleontology 87(2):254-268.

R. Kohler and R. E. Fordyce. 1997. An archaeocete whale (Cetacea: Archaeoceti) from the Eocene Waiho Greensand, New Zealand. Journal of Vertebrate Paleontology 17(3):574-583.

E. D. Mitchell. 1989. A new cetacean from the late Eocene La Meseta Formation, Seymour Island, Antarctic Peninsula. Canadian Journal of Fisheries and Aquatic Sciences 46(12):2219-2235.

Prothero DR, Bitboul CZ, Moore GW, Niem AR. 2001. Magnetic stratigraphy and tectonic rotation of the Oligocene Alsea, Yaquina, and Nye formations, Lincoln County,
Oregon. In: Prothero DR, ed. Magnetic stratigraphy of the Pacific coast Cenozoic. Pacific Section SEPM (Society for Sedimentary Geology) 91:184–194.

M. D. Uhen (2007): The earliest toothless mysticete: A chaeomysticetan from the early Oligocene Alsea Formation, Toledo, Oregon. – Journal of Vertebrate Paleontology 27/3 Suppl.: 161A.

M. D. Uhen, N. D. Pyenson, T. J. DeVries, M. Urbina, and P. R. Renne. 2011. New middle Eocene whales from the Pisco Basin of Peru. Journal of Paleontology 85(5):955-969.

No comments: