Happy Fossil Friday!
Earlier this week a new paper from CCNHM paleontologist Robert Boessenecker was published in the open access journal PLoS One, reporting new anatomical details for the extinct dolphin Albertocetus meffordorum. The paper was a collaboration between College of Charleston and the New York Institute of Technology, with scientific contributions from NYIT student Erum Ahmed and paleontologist Jonathan Geisler. The paper is open access, meaning that it can be read for free and downloaded directly from the PLoS One web page, here.
Background: What is Albertocetus?
Albertocetus meffordorum was named quite recently (Uhen, 2008) and based on a single skull from the Belgrade Formation at Onslow Beach in North Carolina. The Belgrade Formation is late Oligocene in age, approximately 25-27 million years old (Ma). Albertocetus was the first xenorophid dolphin reported from outside the Charleston area in South Carolina. The xenorophids are a formerly poorly-known group of very early-diverging echolocating whales (Odontoceti). Originally reported in the 1930s by a partial skull, the xenorophids are now known to be a somewhat diverse group of dolphins including large fish-eating dolphins like Xenorophus sloanii, the earliest dolphins for which we have abundant evidence of echolocation – the somewhat smaller Cotylocara macei and Echovenator sandersi, and the tiniest cetacean to ever evolve, the ~1 meter long dwarf toothless short-snouted dolphin Inermorostrum xenops. Most xenorophids have a long, gharial-like snout with double-rooted teeth and pits in the snout and jaw for the teeth to interlock. Albertocetus was the first xenorophid to be described with a braincase and earbones – critically expanding the known anatomy of this group. Phylogenetic analyses have consistently placed xenorophids at the very base of the echolocating whales, though several groups were already contemporaries.
What’s new about Albertocetus?
The new fossils described by Boessenecker et al. (2017) include a number of specimens from South Carolina and North Carolina. Two skulls from the lower Oligocene Ashley Formation (~28-30 Ma) near Charleston, South Carolina, preserve much of the same anatomy as the original ‘type specimen’ (a skull at the Smithsonian) but one of these is associated with a ~50% complete vertebral column and several ribs. This skeleton, collected by College of Charleston students in the 1990s, also has part of a mandible and an earbone that was removed from the skull (in the type specimen, the earbone is still embedded in the skull). The second skull was CT scanned and a digital endocast was studied – in other words, the internal surface of the braincase was reconstructed into a 3D model to examine the anatomy of the brain.
The vertebral column includes a number of tail vertebrae, which are wider than would be expected for an odontocete. In modern cetaceans – both baleen whales and dolphins – these vertebrae are somewhat narrower than tall, corresponding to the narrow tail stock (caudal peduncle) seen in iconic photographs of whale tail flukes displayed whilst diving. This suggests that a narrow tail stock evolved independently within later odontocetes and baleen whales.
The brain endocast is interesting, as it is anatomically intermediate between the ancestors of modern whales, the Basilosauridae, and modern dolphins. The brain is still somewhat elongate and conical, and has enlarged temporal lobes and relatively small frontal and parietal lobes. In contrast, modern dolphins have greatly enlarged frontal and parietal lobes and proportionally smaller temporal lobes. This brain anatomy suggests that the large brains of the xenorophids evolved early, and perhaps in parallel with modern dolphins – becoming enlarged via a different pathway and using different parts of the brain for cognition.
Because animals – particularly extinct ones – cannot be given IQ tests, the encephalization quotient (EQ) is used as a rough approximation of animal intelligence. It’s essentially a ratio of brain mass to body mass. In this study, the brain volume of Albertocetus was carefully calculated and body mass estimated based off of skull size using equations developed for other cetaceans. Because a large mass of venous tissue is present along the sides and bottom of the brain of cetaceans, a correcting factor was also used to estimate how much volume was actually brain matter versus venous tissue – neither fossilizes and both soft tissues occupy the same space in the skull. Even still, Albertocetus was found to have the highest EQ for any early Oligocene cetacean – surpassed in the late Oligocene by a new, undescribed species of its’ bigger cousin Xenorophus, and by more modernized dolphins in the Miocene and Pliocene. This highlights a rapid increase in brain size across the Eocene-Oligocene boundary – archaeocetes ancestors were not exactly packing supercomputers upstairs.
What caused this rapid increase in brain size? Some flawed studies have suggested that global cooling at this time caused odontocete brains to increase in size as brains produce heat (thermogenesis) and it helped with odontocete body temperature regulation in cooling climates (Manger, 2006). However, there are numerous problems with this study, which has been largely discredited by a large team of experts on dolphin cognition and evolution (Marino et al., 2008). For example – why didn’t baleen whales show a similar increase in brain size across the boundary? Rather than using brains for thermogenesis, the earliest odontocetes were echolocators like Echovenator sandersi (Churchill et al., 2016). Large brains are necessary for complex cognition, and echolocation takes a lot of brain power. Albertocetus was another xenorophid with the same facial and basicranial features indicative of echolocation. Boessenecker et al. (2017) reiterate the conclusions of earlier work that brain size exploded as echolocation evolved – a ‘key innovation’ in the evolution of the odontocetes.
Final message – amateur collections and sample sizes in paleontology
The study of fossil whales and dolphins is often plagued by overemphasis on single specimens: rather than describing a suite of fossils when reporting a new species, only a single fossil is often reported, even if others are known in museum collections. This new study reported multiple new specimens including a skeleton, a skull, and isolated earbones – drastically expanding the known anatomy of Albertocetus as well as providing new information on individual and growth related variation in early odontocetes. Most of these specimens were all collected by amateur collectors, and we would know none of this if paleontologists had not worked with amateur fossil collectors. This new scientific study highlights the importance of embracing larger sample sizes and, most critically, working with amateur fossil collectors to study fossils.
Dr. Boessenecker did an interview with PLOS that you can read here.
- D. Uhen. 2008. A new Xenorophus-like odontocete cetacean form the Oligocene of North Carolina and a discussion of the basal odontocete radiation. Journal of Systematic Palaeontology 6(4):433-452 [Article Link]
- Churchill, M. Martinez-Cáceres, C. Muizon, J. Mnieckowski, and J. H. Geisler. 2016. The Origin of High-Frequency Hearing in Whales. Current Biology 26:1-6 [Article Link]
- Boessenecker RW, Ahmed E, Geisler JH (2017) New records of the dolphin Albertocetus meffordorum (Odontoceti: Xenorophidae) from the lower Oligocene of South Carolina: Encephalization, sensory anatomy, postcranial morphology, and ontogeny of early odontocetes. PLOS ONE 12(11): e0186476. https://doi.org/10.1371/journal.pone.0186476
Manger, P. R. (2006), An examination of cetacean brain structure with a novel hypothesis correlating thermogenesis to the evolution of a big brain. Biological Reviews, 81: 293–338. doi:10.1017/S1464793106007019 [Article Link]
Marino, L., Butti, C., Connor, R. C., Fordyce, R. E., Herman, L. M., Hof, P. R., Lefebvre, L., Lusseau, D., McCowan, B., Nimchinsky, E. A., Pack, A. A., Reidenberg, J. S., Reiss, D., Rendell, L., Uhen, M. D., Van der Gucht, E. and Whitehead, H. (2008), A claim in search of evidence: reply to Manger’s thermogenesis hypothesis of cetacean brain structure. Biological Reviews, 83: 417–440. doi:10.1111/j.1469-185X.2008.00049.x [Article Link]