Life restoration of Ankylorhiza, shown here attacking plunge-diving gannets. Illustration by the author.
Introduction
At long last, a new fossil cetacean from the Oligocene of South Carolina - it's been since 2023 that I last named one! This new study is based on a single specimen, CCNHM 105, from the Ashley Formation of South Carolina. I recently have gotten back to work on Charleston fossil cetaceans, and started by revising this manuscript - originally submitted a few years ago. Unlike the beautiful skeleton we reported in 2020 and referred to "Squalodon" tiedemani - which formed the basis for the new genus, Ankylorhiza - this specimen has only a handful of vertebrae, a bunch of mandibular teeth, a couple ribs, and the mandibles.
The skull and skeleton of Ankylorhiza tiedemani, CCNHM 103. Photo by the author.
You can read these older posts from 2020 (part 1 here, part 2 here) when we published Ankylorhiza for more, but some quick background: fossils of a large dolphin including isolated teeth and scraps of rostra and mandibles were reported as early as the mid 19th century from the "Ashley Phosphate Beds" (see this post for background on local stratigraphy in Charleston). In the 1970s, Al Sanders of the Charleston Museum dug up ChM PV 2764, which is a spectacularly preserved skeleton with a complete skull of a large stem odontocete from a then-unnamed rock unit overlying the Ashley Formation - later named the Chandler Bridge Formation. Al nicknamed this "Genus Y" in a 1976 paper with Frank Whitmore on the Oligocene fossil record of cetaceans. Additional fossils were found later, including CCNHM 103, which I began studying when I arrived in Charleston in 2015. It had a few differences with ChM PV 2764, and so my colleagues and eventual coauthors Brian Beatty and Jonathan Geisler nicknamed it "Genus Y not". Eventually I came to the conclusion that only one existing name for this extinct dolphin was based on fossils that were preserved well enough to build from, taxonomically speaking. We named a new genus, Ankylorhiza, for "Squalodon" tiedemani, which was based on a rostrum fragment dredged out of the Wando River. Ankylorhiza means "fused roots", and indeed, it seems to be the first odontocete to really evolve towards having single rooted teeth. Our study was mostly focused on the postcranial anatomy and the fact that Ankylorhiza's flipper and vertebral column indicated extensive parallel evolution of swimming adaptations within baleen whales and toothed whales. We found one skull from the Ashley Formation, CCNHM 220, that seemed to be referable to Ankylorhiza tiedemani.
New Specimens
Other specimens seemed to represent different species. A particularly large skull, CCNHM 1075, appears to represent a separate species - the same species as "Genus Y", ChM PV 2764 - also from the Chandler Bridge Formation. These have yet to be described. The more fragmentary specimen CCNHM 105 from the Ashley Formation - mentioned above - was unique amongst these in possessing serrated teeth! Not all of them, mind you, but many of the posterior cheek teeth are coarsely serrated, just like a shark tooth. For a long while I thought that this specimen may not be complete enough to name, but after further examination, I decided it was diagnoseable, and wrote it up. Just this XXX last week? today? the new species is named as Ankylorhiza serratidens.
The holotype (CCNHM 105) mandibles and lower teeth of Ankylorhiza serratidens. From Boessenecker (2026).
Ankylorhiza serratidens is distinguished first and foremost by the presence of serrations on the cheek teeth - absent in Ankylorhiza tiedemani and the "Genus Y" specimens CCNHM 1075 and ChM PV 2764. It's similar in size to Ankylorhiza tiedemani, but smaller than CCNHM 1075. The cheek teeth are also larger than those of Ankylorhiza tiedemani, by about 25% - and similar in size to "Genus Y". The teeth also have more strongly developed apicobasal ridges than Ankylorhiza tiedemani, where instead there are more faint striations on the enamel. This is particularly more obvious on the anterior teeth that have longer, narrower crowns. One last difference with Ankylorhiza tiedemani is the teeth of Ankylorhiza serratidens are slightly more 'cuspate' - there is one large posterior cusp on the fifth postcanine tooth, pc5 - and none at all on even the very last tooth position in A. tiedemani. This is shared with "Genus Y", which has one or two small cusps on the very last tooth.
The left mandible and lower teeth of "Genus Y", Ankylorhiza sp. (CCNHM 1075). From Boessenecker (2026).
Though there are some loose teeth, there is no mandible preserved for Ankylorhiza tiedemani - so I decided to figure and provide brief comments on the mandible of "Genus Y", CCNHM 1075. The mandible of Anklyorhiza serratidens has a pointier coronoid process - this is that big fan-shaped process of bone on the back of the mandible that serves as the attachment point for the jaw-closing temporalis muscle. I'm not sure what the exact functional implications are, but it does suggest something different about the muscles involved in feeding. The coronoid on the more incomplete left mandible does seem like it would have been longer than that in CCNHM 1075, suggesting a longer lever arm - and perhaps higher bite force than in the larger unnamed species. More fundamentally, the mandibular symphysis is much longer in Ankylorhiza serratidens - it is broad and flattened, and constitutes about 40% of the length of the mandible, as opposed to the more indistinct symphyseal surface in CCNHM 1075, which is proportionally half as large, only 20% of the length of the mandible. This really implies a stronger bite force in Ankylorhiza serratidens - despite its smaller size than "Genus Y".
On a fundamental level, this new paper also reports the first mandibles for Ankylorhiza, period. The mandible looks a lot like later Squalodon, but is generally bigger. The tooth-bearing part of the mandible is more robust than what you see in the xenorophids. The mandibular symphysis is more strongly constructed than in Xenorophus, which is surprising, given that Xenorophus, and other xenorophids, generally have more elongate, delicate snouts. In Echovenator sandersi, the mandibles seem to be fused together - but they're glued together. In other specimens, the symphyseal surface is surprisingly smooth, like Xenorophus - one of our surprising findings when we published our Xenorophus monograph in 2023.
Serrated Teeth in extinct Cetaceans
What are serrations? Why evolve serrations? Serrations - sometimes called denticles - are little bumps or blades on the cutting edge or 'carina' of a tooth and separated by little notches. Sometimes these are tiny and restricted to the enamel - in other words, if you pulled the enamel off of the underlying dentin, the dentin would have a smooth cutting edge and no evidence of serrations. This is the case in many sharks, reptiles, and the serrations in the canines of the sabertoothed cat Smilodon. To a degree, this is a function of how large each serration is, how deep the inter-serration notches are, and how thick or thin the enamel is. In Ankylorhiza serratidens, the serrations are very large - on average, around 1 mm - and the underlying enamel is exposed in wear facets on the tips of each serration. In many theropod dinosaurs, the notches are quite deep, and so there are deep 'interdental folds' that are developed in the dentine; this is a result of the much thinner enamel present in dinosaurs and reptiles. Mammals generally develop absurdly thick enamel. In comparison, the serrations in theropod dinosaurs are much smaller than Ankylorhiza serratidens: perhaps 1/3 the size, usually around 0.3-0.4 mm or so, versus 0.9-1.1 mm in our new dolphin. Whenever it comes to dental structures, mammals sometimes just don't know how to do anything 'small'. Tiny serrations have also been reported in marine reptile teeth - mostly marine crocodilians and ichthyosaurs - but those are often even tinier, 0.1-0.3 mm. Oddly enough, the serrations in Ankylorhiza serratidens compares best with the serrations in the teeth of great white sharks.
Serrations function by concentrating the soft tissues of prey during biting into the notches - sort of like a pair of scissors held open. The more the tissue gets pushed into the notch, the more the notch is likely to cut or tear through it. Generally speaking, serrated teeth are considered to be an adaptation towards feeding on other tetrapods, which have thicker skin and tougher muscle fibers than fish. Fish meat is pretty soft in general, and so many species of carnivorous fish (and sharks) that feed exclusively on other fish tend to have teeth that are conical and sharp, sometimes fang-like - and they can easily puncture the soft muscle of fish. Scales are another issue altogether, but generally, it doesn't take much force to puncture the scales or dermal denticles of your average ray-finned fish or shark (we're ignoring gar, armored catfish, boxfish, and the like for the purpose of this discussion). There is actually surprisingly little in the published literature on just how soft fish meat is, say, in comparison to chicken or beef. I think that's an entire aspect of feeding adaptations that has just been completely ignored, and I think someone could do an entire master's or doctoral thesis on the puncture and cutting performance of different tooth types on both 'tetrapod' meat (tastes like chicken or beef) versus fish meat. From my own experience: rockfish, trout, salmon, mahi mahi, tilapia, snapper, halibut, cod - all of these are very soft. "Chilean sea bass" (Patagonian toothfish) is even flabbier (disgusting ass fish, in my opinion). The only fish I've ever had with the texture approaching beef or chicken was swordfish - and, mind you, that could simply be a result of cooking (I can't do uncooked fish; cold smoked salmon is the closest I can get to sushi, I'm afraid).
One of the teeth of Ankylorhiza tiedemani specimen CCNHM 220 which has a few serrations, but not quite as well-developed as in Ankylorhiza serratidens. Additionally, there were a mix of doubtfully attributed teeth with this specimen when we arrived in 2015, and it's possible that this could belong to a different specimen altogether. One particularly serrated tooth stored with CCNHM 220 had different coloration and certainly represented a different specimen. Photo by the author.
This is a long winded way of saying that when serrations evolve in a particular marine predator lineage, it's a good sign that they have "graduated" from a piscivorous (fish-eating) diet to feeding on other marine tetrapods with tough, fibrous skin and muscle. This has happened many times within sharks - the two most obvious examples are the evolution of serrations within the megatoothed (Otodontidae) shark lineage about 40 million years ago, with the evolution of Carcharocles auriculatus (great, great, great grandparent of Carcharocles megalodon), and the comparably more recent evolution of the species of great white sharks with serrated teeth, Carcharodon hubbelli and Carcharodon carcharias, around 6-7 million years ago.
Surprisingly, serrated teeth are nothing new in whaleontology - though not much attention has been directed to them. Sporadic serrations - a few here and there - have been reported on the teeth of the more famous Miocene shark-toothed dolphin Squalodon, and from several species. Coronodon has a couple of teeth with a handful of serrations on them, and there are a few basilosaurids, like Basilosaurus isis (thank to Eli Amson for alerting me to this), with a few serrations on some of the cheek teeth. What is curious about these examples are that all of them have accessory cusps on the teeth, and in general, it's possible that having basilosaurid-like accessory cusps may simply act as gigantic serrations. There aren't many cetaceans with simple, triangular teeth and serrations - making Ankylorhiza a bit unique as it is one of the only stem odontocetes with a markedly simplified dentition
The teeth in the front of the mouth of Ankylorhiza serratidens are procumbent, just like in Ankylorhiza tiedemani - indicating that they probably weren't used for biting, since biting applies force in a direction perpendicular to the orientation of the incisor teeth. We proposed that Ankylorhiza tiedemani rammed prey with these procumbent, nearly tusk-like teeth, and indeed, these anterior teeth were much more highly worn than other teeth in the mouth. It's certainly possible that Ankylorhiza tiedemani rammed prey to death as well. That the anterior teeth are similar is not surprising. But, what's really interesting about Ankylorhiza serratidens is that the cheek teeth with serrations are actually mostly towards the back of the toothrow - closer to the jaw joint, where the most bite force is located. This doesn't suggest that Ankylorhiza serratidens was chewing its food like a land mammal - but that it could bite a bit through tough food using the posterior most teeth, perhaps aided by some shake-feeding.
Taphonomy of the Ankylorhiza serratidens Holotype
The partial skeleton of Ankylorhiza serratidens has some interesting taphonomic features, and while the real 'meat' of the manuscript is about the feeding ecology of this new giant dolphin, there is evidence of several critters munching on the skeleton after the dolphin's death. The ribs of the specimen have quite a few bite marks on them, probably from sharks - most of these are short, linear gouges in the bone surface. One is certifiably a trace from a shark - a series of subparallel, wavy grooves, like distorted ridges on the surface of an old record. These are marks from the individual serrations on a large shark tooth - in this case, probably formed from a bite from the extinct megatoothed shark, Carcharocles angustidens. This trace has actually been named as an ichnospecies - Knethichnus parallelum. I've reported similar traces already from other Oligocene cetaceans, and so this isn't terribly surprising.
What is perhaps somewhat more interesting are a series of irregular crater-like holes in the bone surface. The mandibles are particularly 'bioeroded' by these craters, resulting in areas of bone that have a surface resembling the surface of the moon. These can be a challenge to prepare - brave (or inexperienced) preparators might try to prep the matrix out of these craters, which usually results in further damage and the accidental 'expansion' of them. More experienced preparators recognize that these are not natural to the bone anatomy, and leave matrix inside, prepping it 'flush' with the bone surface. Neither option is ideal for a taphonomist - however, given how important Oligocene fossil cetaceans are, I've deferred to preserving the anatomy at virtually every opportunity when I've been preparing these myself (or advising preparators).
These traces have been reported increasingly more frequently in Oligocene and Eocene rocks, and attributed to the bone-eating worm Osedax. Osedax is a sessile annelid worm that grows roots into bone and dissolves away the calcium phosphate mineral structure of bone, and is thought to metabolize either the bone itself or collagen. These worms have a stalk and gills that extend outside of the bone - and can be retracted inside the cavity that they erode. Osedax erode a small circular hole through the bone surface, which in the case of borings in Oligocene cetaceans, typically rapidly expands just underneath the bone forming a roughly inverted hemispherical excavation with many branch-like tunnels that extend radially away from the center. The hemispherical central cavity is protected by a wafer-thin layer of bone. This typically flakes away, exposing a crater-like boring underneath. As part of my PhD research, I reported on relatively large craters in the mandible of the holotype specimen of the eomysticetid whale Waharoa ruwhenua, where the limestone outside the mandible was pushed into the cavity as a result of post-burial compaction of the sediment, and the thin 'skin' of bone sagged into the craters. Further, these had many small boreholes, just as had been reported for modern Osedax borings.
The holotype specimen of Ankylorhiza serratidens has a number of these borings on the mandibles and vertebrae, and even some on the teeth. These are surprisingly common in the Ashley and Chandler Bridge formations of South Carolina, and as I noted in a 2014 paper, seem to be surprisingly common in Oligocene marine vertebrate bones worldwide - these crater-like scars had previously been figured in cetacean, sirenian, sea turtle, and fish remains from North America, New Zealand, and Europe. Though Eocene examples have been reported and are evident even in some figures from Remington Kellogg's massive tome on the archaeocetes, they are in general far less common than in Oligocene cetaceans. The most unusual, and largest, Osedax traces in Ankylorhiza serratidens occur on the atlas vertebra. Here there are rather large funnel-like borings with a deep central fissure and a shallow zone surrounding it with small, branching tunnels that are more parallel with the bone surface. These appear to have not been prepared mechanically - some of these specimens can be cleaned with running water, and where you can just wash away the sediment with a hose or under a sink, Osedax traces fare the best as the little crumbs of bone between bioeroded tunnels flake away frustratingly easy. These do not seem to match the described traces of any modern Osedax, nor have I really seen this morphology elsewhere.
The rather nice skull of Ankylorhiza tiedemani that I described back in 2020 (specimen CCNHM 103). Photo by the author.
Ankylorhiza serratidens in the Context of Early Cetacean Evolution
So, what does all this mean? Ankylorhiza is one small clade of gigantic early dolphins, and these species shared the Charleston Embayment with a wide diversity of small dolphins in several families including the Agorophiidae, Squalodontidae, Waipatiidae, and Xenorophidae - along with Coronodon and eomysticetids. Ankylorhiza serratidens itself only post-dates the presumed extinction of basilosaurid whales by about four or five million years - a LOT of evolution, and global environmental change - happens between the late Eocene and 'mid' Oligocene.
In Europe, there is a massive faunal change that occurs at the end of the Eocene, called Grande Coupure - the big break - and endemic European land mammals are generally replaced by a tranche of immigrant taxa from Asia. Though the faunal change occurring at the same time in the world's oceans is not quite so extensively studied, the changes are nonetheless profound. Archaeocete whales - at leas the worldwide basilosaurids - go extinct. There are some possible Oligocene archaeocetes, including the kekenodontids from New Zealand and Australia, and Coronodon may actually be an archaeocete - but both are generally quite rare. Within three million years or so, cetaceans are almost uniformly smaller. Simocetid and xenorophid dolphins have evolved, and with clear adaptations for echolocation. Small toothed baleen whales have also appeared, including some of the smallest cetaceans ever, like Fucaia buelli. Some aetiocetids have evidence of possessing baleen. The oldest baleen-bearing whales, the eomysticetids, have also appeared - indicating a much wider breadth of feeding ecologies than just a few million years prior during the Eocene, where basilosaurid skulls mostly look like cookie-cutter copies made at different sizes but with the same proportions. Other changes in the marine fauna are noteworthy. The paleophiid sea snakes - gigantic sea snakes measuring from 'only' five or six meters to longer than 10 meters - go extinct at the end of the Eocene. Pycnodont fish, an archaic group of shell-crushing fish that evolved during the Triassic period some 200 million years earlier, die off, and are largely replaced afterwards by fish in the Tetraodontiformes (pufferfish, porcupinefish, triggerfish, filefish) the Scaridae (parrotfish, hogfish, and wrasses), the unfortunately similarly named Sparidae (drumfish), and a continuing diversification of myliobatoid rays. Shark faunas change quite a bit, and there is a drastic change from lamniform (mackerel shark) dominated shark faunas to those dominated by carcharhiniform sharks (requiem sharks); essentially, sand tiger like species are common before, and reef sharks, tiger sharks, and snaggletooth sharks are common after. Small species of bony-toothed birds seem to go extinct, leaving only truly gigantic forms during the Oligocene and onwards. This disruption of the marine ecosystem seems to be the most extreme change since the Cretaceous-Paleogene extinction - and is probably just as severe, if not moreso, than the period of faunal change at the end of the Pliocene epoch. While these trends are evident in the shallow marine fossil record,
What caused this? The biggest culprit seems to be Antarctica. And, to address this, we have to step back to the early Jurassic, 200 million years ago. The supercontinent Pangea began to break up, presaged by the largest volcanic event in earth's history - the eruption of CAMP, the Central Atlantic Magmatic Province. Not a single eruption, but a series of many that flooded a rift zone in Pangea, and the volcanic rocks left behind are found in the southeastern USA, Brazil, northwestern Africa, and southern Europe. Within a few million years rifting began in earnest. The first rift separated North America, Europe, and Asia from the southern Pangean landmass, known as Gondwana: South America, Africa, Madagascar, India, Australia, Antarctica, and Zealandia. Most of these, including India, South America, Africa, and Zealandia rifted away during the Mesozoic - but during the Paleocene and most of the Eocene, you could walk from what is now Brazil to Australia through Antarctica. And in fact, we do find marsupials and unique South American mammals in Eocene rocks of Antarctica. A shallow sea finally formed during the mid-late Eocene - but there was still continental crust forming a continental shelf between each of these three landmasses. During the latest Eocene, deep, oceanic crust formed as the continents were truly separated, resulting in the beginning of the circum-Antarctic current. Antarctica was temperate during the Eocene, without an ice cap, though it almost certainly snowed and was covered with boreal forest. In the southern hemisphere, a gulf stream-like current introduced warm, wet air to the south pole and kept it relatively warm. By the middle Oligocene, a small ice cap was present - as Antarctica was now thermally isolated by a cold water current that permitted (at least a limited degree) of runaway glaciation. Antarctica would end up having ice-free valleys and regions with vegetation until the late Miocene, and didn't really become completely glaciated until the last 7-8 million years or so.
This sudden cooling isn't necessarily the driver of marine vertebrate faunal change - but another effect of the newly formed current is thought to: a sudden boom in primary productivity. This started a little bit during the latest Eocene, but really spiked in the early Oligocene. Specifically, dissolved phosphorus became abundant, which helps support a large population of planktonic organisms, like krill. We're not certain if krill specifically were the dominant marine 'macro-plankton' in the southern Ocean during the Oligocene - it could just as well have been some other tiny arthropod - since krill, unfortunately, do not have a fossil record. We think that this boom in plankton drove experimentation with a new food source, leading to the rapid evolution of baleen in baleen whales, but also, a boom in food - perhaps combined with the loss of precise tooth occlusion in early odontocetes and mysticetes - led to a radiation in feeding morphology in both toothed mysticetes and odontocetes. This may be the first time that echolocating dolphins started diving for food, perhaps first an adaptation for feeding on prey at night - and then leading to deeper diving and searching for food beyond the photic zone.
It is precisely during this radiation in early Neoceti that Ankylorhiza evolves - alongside such short-lived experiments like the dwarf toothless suction feeding xenorophid Inermorostrum xenops, the possible tooth-filtering mysticete Coronodon, and small raptorial bug-eyed mysticetes like Janjucetus. Even Ankylorhiza is pushing the envelope: its species are the largest odontocetes to evolve until some point in the Miocene, and the large body size itself is an adaptation for being able to take a wider variety of prey, including lager sized prey. And, within that group of dolphins experimenting with gigantism comes one with serrated cheek teeth - an experiment within an experiment. Sadly, that experiment is short lived, as Ankylorhiza's fossil record terminates at the end of the Oligocene, and a few similar specimens persist into the early Miocene - like "Prosqualodon" hamiltoni from the Caversham Sandstone of New Zealand. The niche occupied by Ankylorhiza, and shortly thereafter vacated - did not remain vacated for long, as squalodontids took up the large macrophagous dolphin 'ecotype' during the early and middle Miocene, and then by macrophagous sperm whales. And, after those died off - killer whales during the Pleistocene.
If you're so inclined, you can find the new paper on Ankylorhiza serratidens here.
Further Reading
Bennion, R.F., Maxwell, E.E., Lambert, O., and Fischer, V. 2023. Craniodental ecomorphology of the large Jurassic ichthyosaurian Temnodontosaurus. Journal of Anatomy 244: 22-41.
Berta, A. and Lanzetti, A. 2020. Feeding in marine mammals: an integration of evolution and ecology through time. Palaeontologia Electronica 23 (a40): 1-41.Brink, K.S., Reisz, R.R., LeBlanc, A.R., Chang, R.S., Lee, Y.C., Chiang, C.C., Huang, T., and Evans, D.C. 2015. Developmental and evolutionary novelty in the serrated teeth of theropod dinosaurs. Scientific Reports 5: 12338. Marx, F.G. and Fordyce, R.E. 2015. Baleen boom and bust: a synthesis of mysticete phylogeny, diversity and disparity. Royal Society Open Science 2: 140434.
Nelson, M.D., Lambert, O., and Uhen, M.D. 2025c. Taxonomic revision of the family Squalodontidae (Cetacea, Odontoceti): emptying the wastebasket of fragmentary holotypes. Papers in Palaeontology 11: e70002.
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