By Robert Boessenecker (@CoastalPaleo)
Happy Fossil Friday!
Last week saw the publication of a new paper naming our spectacularly preserved toothed baleen whale Coronodon havensteini. The article was published in Current Biology by Jonathan Geisler and Brian Beatty from NYIT, CCNHM donor and namesake Mace Brown, and of course one of our own paleontologists, Robert Boessenecker. The paper has been in the works for years and we are elated, if not relieved, that it is finally published. This blog post will consider some of the highlights of the paper, but first – a little back story is needed.
Modern baleen whales (Mysticeti) are filter feeders, and do not have adult teeth. They do, however, form teeth as embryos which are then quickly resorbed – this in itself is not surprising as having teeth is the primitive condition for mammals. Instead, as adults, baleen whales possess racks of baleen that hangs from the upper jaw – a highly specialized type of gingival (gum) soft tissue that is made of keratin and is essentially the same type of tissue that makes up horn, fingernail, hooves, etc. Because it is a soft tissue, it typically does not fossilize (though some rare examples definitely exist), and just like fingernails, grows continuously during life (and length is maintained by the tongue rasping away at the inside). Modern baleen whales feed in three ways: gulping large volumes of water after schools of krill and fish while swimming at high speed (humpback/blue whales – rorquals), sucking up sediment filled with tiny burrowing crustaceans (gray whales), and cruising slowly though “clouds” of tiny copepods with the mouth open (right/bowhead whales). Given that modern baleen whales lack adult teeth, and all species in the other group of cetaceans – the echolocating whales, or Odontoceti – possess teeth (given their more common, yet evolutionarily misleading name, the “toothed whales”) have teeth – we should expect a baleen whale ancestor with teeth.
A few confusing fossils were reported in the late 1960s from the Pacific Northwest, originally interpreted as archaeocetes given the relatively primitive shape of the skull and, in one case, retention of teeth. One of these was collected, prepared, and published by legendary fossil collector Douglas Emlong – and named Aetiocetus cotylalveus. It was later recognized as an baleen whale, and in the 1990s, a number of additional fossils revealed that there was an entire diverse family of aetiocetids in the North Pacific, now known from Japan, British Columbia, Washington, Oregon, California, and Baja California. Other advances in the last fifteen years included the discovery of additional aetiocetids, the discovery of the bizarre toothed mysticete Janjucetus from Australia, and the redescription of another former archaeocete, Mammalodon (closely related to Janjucetus). A critical scientific advance was the detection of palatal foramina in the toothed mysticete Aetiocetus weltoni (Deméré et al., 2008) – these are holes that transmit blood vessels to “feed” baleen, and are enlarged in all modern baleen whales and all extinct baleen whales that lack teeth, and are thus a bony correlate indicating the presence of baleen. This landmark paper proposed a “stepwise” transition from teeth to baleen in the Mysticeti: a tooth-only stage (e.g. Janjucetus), a stage including teeth and baleen at the same time (Aetiocetus weltoni), followed by loss of the teeth (modern baleen whales). Some eomysticetid whales – higher up the tree than the aetiocetids, and formerly considered to be toothless – had baleen along most of the palate, with a few vestigial “buck-teeth” in the front of the jaws, revealing a fourth stage between aetiocetids and modern mysticetes (Boessenecker and Fordyce (2015).
This study has drawn substantial criticism, with certain papers pointing out that the foramina are of uncertain origin, or perhaps too small to infer the presence of baleen. A recent study (Marx et al., 2016) showed a unique pattern of tooth wear on an aetiocetid lovingly nicknamed “Alfred” where the base of the crowns were in pristine condition but the tips of the teeth were worn and scratched – they proposed that the pristine crown was covered in enlarged gums (gums do not typically cover tooth enamel in most healthy mammals). These researchers hypothesized that aetiocetids simply had enlarged gums rather than baleen – and palatal foramina would supply blood to the thickened gums instead, as an intermediate condition between “teeth only” and “baleen only”. Another recent hypothesis is that because modern suction feeding odontocetes have reduced teeth, a hypothetical toothless and baleen-less intermediate may have been possible (Peredo et al., 2017). A very recent paper reported a very early baleen whale (Mystacodon selenensis) with teeth and possible suction feeding habits (Lambert et al., 2017).
Coronodon havensteini sheds new light on this debate. The holotype specimen (a holotype is the specimen which a new species is based on) is CCNHM-108, and is a large (~1 meter long) skull with mandibles, many of the teeth, earbones, cervical and thoracic vertebrae, and a bunch of ribs. Other specimens that may belong to this species are curated at Charleston Museum and have been known about since the 1980s but never formally published. CCNHM-108 is the most complete of the infamous “Charleston toothed mysticetes”, and was collected by licensed diver Mark Havenstein from the Ashley Formation in the Wando River in the early 2000s – the species name is in honor of Mark’s discovery. The genus name refers to the beautiful crown-shaped cheek teeth. The specimen was donated to CCNHM shortly after its founding, and was placed on display in the whale room in 2014.
The skull and dentition bear a number of striking resemblances to basilosaurid whales – the common ancestors of the odontocetes and mysticetes, and Coronodon would have thus greatly resembled a basilosaurid in many respects. Basilosaurids have an external nose far out on the rostrum and a series of denticulate shearing teeth, the largest being in the third and fourth premolars – the molars are smaller, with wear patterns indicative of sharing and in some perhaps a degree of crushing as well. Overall having bite force emphasized on the posterior premolars is reminiscent of mammalian carnivores like dogs. The rostrum, like most mammals, is tightly sutured together. Coronodon differs in several ways from the basilosaurid condition: 1) the rostrum is wider and somewhat flattened, a bit like modern baleen whales; 2) the blowhole is positioned further back, just in front of the eye sockets; 3) the bones of the snout are only lightly connected, permitting some flexibility of the rostrum, correlated with filter feeding (but the mechanism is still unknown); 4) the cheek teeth in the lower jaw (p2, p3, p4, m1, m2, m3) are all nearly the same size and shape (upper teeth are more scarcely known) – indicating that the molars have increased in size from basilosaurids; 5) the teeth overlap and have slots between them which water can pass through. Coronodon also exhibits some features of the basicranium that are unique to baleen whales, confirming it as an early transitional toothed baleen whale. Critically, Coronodon lacks palatal foramina – indicating the absence of baleen. It does, however, exhibit widespread enamel and dentine erosion on its teeth, indicating that much of the exposed roots and part of the enamel crown were covered by enlarged gums – as proposed for the aetiocetid “Alfred”, and confirmed here using evidence of periodontal disease, which appeared to be the norm for Coronodon as all specimens share some evidence of dental erosion.
These features all indicate that, despite certain features resembling archaeocetes, Coronodon was a baleen whale. The teeth, unlike basilosaurids, interlock in a way that formed a toothy “cage”; interdental slots permitted water to pass through, but not small prey. If the jaws were partially opened, diamond-shaped holes would be present between the upper and lower teeth, meaning that by changing how far open the mouth was, the filter feeding ‘size’ could be adjusted – making for a dynamic filter feeding device. Modern leopard and crabeater seals filter feed for krill and small fish – leopard seals seemingly rip penguin heads off only a few months of the year, and filter feed for most of it. They do not chew with their cheek teeth, and just nip with their canines and incisors – so tooth wear accumulates only on the teeth in the front of the jaw (Hocking et al., 2014). In Coronodon, we see the same pattern: tooth wear on the front teeth, and minimal tooth wear on the denticulate cheek teeth – which is very different from basilosaurids, despite dental similarities. This indicates that, like leopard seals, Coronodon avoided chewing hard prey with its cheek teeth, likely to preserve the shape of the dental filter.
So what does this all mean? Recent hypotheses have suggested that toothed mysticetes were suction feeders and did not filter feed, with true filter feeding occurring within the eomysticetid whales and further up the tree, with enlarged gums acting as a way to make a better oral “seal”, much like the enormous lips of a beluga or pilot whale. Aetiocetids occur farther up the tree than Coronodon – indicating that tooth-based filter feeding began very early in baleen whale evolution, suggesting that aetiocetids may have been too specialized on their own branch to weigh in on how baleen evolved. After all, functionally toothless baleen-bearing eomysticetid whales appear at the same time as the toothed aetiocetids. Filter feeding – whether with baleen or teeth – thus appears to be intimately linked with the earliest baleen whales, and perhaps the mammalodontids and aetiocetids reverted to earlier forms of feeding (e.g. fish eating) and experimented with their own feeding styles (suction feeding).
This is only the tip of the iceberg on Coronodon and other Coronodon-like whales; additional specimens of Coronodon exist in our collection here at CCNHM and at Charleston Museum, and there is a second possible genus known from two skulls, one at each museum – currently under study by Geisler, Beatty, and Boessenecker. Problematic teeth of a Coronodon-like whale have been found in the depths of the Lee Creek Mine in North Carolina. Stay posted for more whale news from CCNHM, and come visit and see the Coronodon holotype, on display!
- W. Boessenecker and R. E. Fordyce. 2015. A new genus and species of eomysticetid (Cetacea: Mysticeti) and a reinterpretation of ‘Mauicetus’ lophocephalus Marples, 1956: Transitional baleen whales from the upper Oligocene of New Zealand. Zoological Journal of the Linnean Society 175:607-660.
- A. Deméré and A. Berta. 2008. Skull anatomy of the Oligocene toothed mysticete Aetiocetus weltoni (Mammalia; Cetacea): implications for mysticete evolution and functional anatomy. Zoological Journal of the Linnean Society 154:308-352
- A. Deméré, A. Berta, and J. Gatesy. 2008. Morphological and molecular evidence for a stepwise evolutionary transition from teeth to baleen in mysticete whales. Systematic Biology 57:15-37.
- M. G. Fitzgerald. 2006. A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales. Proceedings of the Royal Society B 273:2955-2963
E.G. Ekdale, T. A. Deméré and A. Berta. 2015. Vascularization of the gray whale palate (Cetacea, Mysticeti, Eschrichtius robustus): soft tissue evidence for an alveolar source of blood to baleen.
Hocking, D.P., A.R. Evans, and E.M.G. Fitzgerald. 2013. Leopard seals (Hydrurga leptonyx) use suction and filter feeding when hunting small prey underwater. Polar Biology 36:211-222.
- H. Geisler, R. W. Boessenecker, M. Brown and B. L. Beatty. 2017. The Origin of Filter Feeding in Whales. Current Biology 27:1-7
- Lambert, M. Martínez-Cáeres, G. Bianucci, E. Steurbaut, M. Urbina and C. Muizon. 2017. Earliest Mysticete from the Late Eocene of Peru Sheds New Light on the Origin of Baleen Whales. Current Biology 27:17
- G. Marx, D. P. Hocking, T. Park, T. Ziegler, A. R. Evans and E. M. G. Fitzgerald. 2016. Suction feeding preceded filtering in baleen whale evolution. Memoirs of Museum Victoria 75:71-82