(F) Schematic representation of hypothesized water flow (blue arrows) through the oral cavity during filter feeding, drawn from CT data after the mandibles were digitally articulated with the cranium.

(B) Oblique anteromedial view of filter-feeding slots (indicated by arrows) between molars and p4. Orientation of view is shown in (A).

ap, anterior process of petrosal; cp, conical apophysis; fr, fenestra rotunda; Fr, frontal; lt, ventrolateral tuberosity; mf, fossa for malleus; Mx, maxilla; Na, nasal; ol, outer lip of bulla; os, occipital shield; Pa, parietal; pc, pars cochlearis; pbf, posterior facet for bulla; pgp, postglenoid process; pp, posterior process of bulla; Px, premaxilla; sp, sigmoid process; Sq, squamosal; sc, sagittal crest; Vo, vomer; zy, zygomatic process; VII canal for facial nerve. Scale bars in (A)–(C), 10 mm. Scale bars in (D)–(I), 5 mm. Blue denotes dental wear and red denotes dental erosion.

(H and I) Right bulla in (H) dorsal view and left petrosal in (I) ventrolateral view. Portions in gray are reconstructed.

(A–E) Cranium in (A) lateral and (B) dorsal views. For comparison, (C) shows a dorsal view of the archaeocete Zygorhiza kochii (USNM 11962). Also shown of Coronodon are the left P3 in (D) labial and (E) lingual views.

Coronodon havensteini. Genus is Greek for “crown tooth,” referring to the multi-cusped molars. The species name recognizes Mark Havenstein, who discovered the holotype.

Stratigraphic revision of the Cooper Group and the Chandler Bridge and Edisto Formations in the Coastal Plain of South Carolina.

Coronodon has the following mysticete synapomorphies: supraoccipital level with temporal fossa (character 25: state 1), broad basioccipital crests (39: 2), all cusps of posterior teeth subequal (99: 1), upturned antorbital process of maxilla (100: 1), and splayed basal cusps on posterior teeth (206: 1) ( Figures 1 S1 , and S2 Data S1 ). Like some archaeocetes [], its rostrum is twisted counterclockwise in anterior view ( Figure 3 ). Coronodon havensteini is unique in having anterior lower molars labially overlapping posterior lower molars ( Figure 2 ).

Feeding Behavior

9 Uhen M.D. Form, Function, and Anatomy of Dorudon atrox (Mammalia, Cetacea): An Archaeocete from the Middle to Late Eocene of Egypt. 10 Fahlke J.M. Bite marks revisited – evidence for middle-to-late Eocene Basilosaurus isis predation on Dorudon atrox (both Cetacea, Basilosauridae). 10 Fahlke J.M. Bite marks revisited – evidence for middle-to-late Eocene Basilosaurus isis predation on Dorudon atrox (both Cetacea, Basilosauridae). 11 Foote A.D.

Newton J.

Piertney S.B.

Willerslev E.

Gilbert M.T.P. Ecological, morphological and genetic divergence of sympatric North Atlantic killer whale populations. 12 Ford J.K.B.

Ellis G.M.

Matkin C.O.

Wetklo M.H.

Barrett-Lennard L.G.

Withler R.E. Shark predation and tooth wear in a population of northeastern Pacific killer whales. Toothed mysticetes evolved from archaeocetes, a paraphyletic group ancestral to all extant cetaceans. Archaeocetes are interpreted as raptorial feeders: they hunted and caught prey with their teeth, one at a time. This inference is supported by fossilized stomach contents [] and bite marks on small archaeocetes []. Raptorial feeding is also indicated in Coronodon by the caniniform incisors and the truncated and worn crown of the right P2 ( Figure S1 ). Similar wear has been interpreted as being created by abrasion during feeding by the skeletons or other hard parts of prey [].

13 Busbey A.B. The structural consequences of skull flattening in crocodilians. 14 Fitzgerald E.M.G. A bizarre new toothed mysticete (Cetacea) from Australia and the early evolution of baleen whales. 1 Goldbogen J.A.

Calambokidis J.

Croll D.A.

McKenna M.F.

Oleson E.

Potvin J.

Pyenson N.D.

Schorr G.

Shadwick R.E.

Tershy B.R. Scaling of lunge-feeding performance in rorqual whales: mass-specific energy expenditure increases with body size and progressively limits diving capacity. 15 Lambertsen R.H.

Ulrich N.

Straley J. Frontomandibular stay of Balaenopteridae: a mechanism from momentum recapture during feeding. 16 Fitzgerald E.M.G. The morphology and systematics of Mammalodon colliveri (Cetacea: Mysticeti), a toothed mysticete from the Oligocene of Australia. Zool. 17 Boessenecker R.W.

Fordyce R.E. Anatomy, feeding ecology, and ontogeny of a transitional baleen whale: a new genus and species of Eomysticetidae (Mammalia: Cetacea) from the Oligocene of New Zealand. 17 Boessenecker R.W.

Fordyce R.E. Anatomy, feeding ecology, and ontogeny of a transitional baleen whale: a new genus and species of Eomysticetidae (Mammalia: Cetacea) from the Oligocene of New Zealand. By contrast, other features suggest that Coronodon was less effective at raptorial feeding than archaeocetes. The latter resemble raptorial odontocetes in having a long, narrow rostrum, which likely allowed prey to be caught with only a turn of the head and minimal drag []. The rostrum of Coronodon is wider, as indicated by the straight sides and shorter mandibular symphysis ( Figures 1 and 2 ). In archaeocetes, the symphysis extends to p3, whereas the symphysis of Coronodon terminates anterior to the canine. Importantly, a wider rostrum in extant mysticetes is associated with a larger oral cavity, which is a critical adaptation for filter feeding []. Extant mysticetes also adjust the size of their oral cavity when feeding [], and some have suggested that loose rostral sutures may facilitate this []. Somewhat surprisingly, Coronodon has simple and open rostral sutures too, unlike the sutures of many other toothed mysticetes [].

9 Uhen M.D. Form, Function, and Anatomy of Dorudon atrox (Mammalia, Cetacea): An Archaeocete from the Middle to Late Eocene of Egypt. The premolars and molars of Coronodon differ from those of Basilosauridae ( Figures 2 A and 2G), the closest relatives of mysticetes among archaeocetes. The sides of the p4 in the latter are steeper: lines connecting the apices of three central cusps form an angle of 82° or 98° in Cynthiacetus (MNHN.F.PRU 102) and Dorudon (UM 10122), respectively, as compared to 155° in Coronodon. A smaller angle is more effective at puncturing prey because it concentrates and sustains the bite force on the central cusp. Even greater differences are seen in the molars. In Coronodon, the first two molars are subequal to the p4 and resemble it in having mesial and distal accessory cusps. By contrast, the molars of basilosaurids are much smaller (e.g., p4/m1 = 1.59 for Dorudon), and the lower molars lack mesial cusps []. Large molars are often indicative of greater mastication, but the pattern of wear makes this interpretation unlikely. Each lower molar has a labial wear facet that extends apically onto the base of the crown but remains far removed from the carinae ( Figure 2 C). As a result, the scissor-like shearing between upper and lower molars, as seen in protocetids like Georgiacetus, is absent. Although the posterior molars could have been used to impale prey, this behavior seems uncommon given the small size of most apical wear facets (1.6–4 mm; Figures 2 B and 2E) and the fact that the molars had reduced support from alveolar bone ( Figure 2 A). For each double-rooted tooth, only the distal half of each root is surrounded by alveolar bone, suggesting that the high occlusal pressures associated with macrophagy were rarely encountered.

6 Mitchell E.D. A new cetacean from the Late Eocene La Meseta Formation, Seymour Island, Antarctic Peninsula. 18 Fordyce R.E. Evolution and zoogeography of cetaceans in Australia. 16 Fitzgerald E.M.G. The morphology and systematics of Mammalodon colliveri (Cetacea: Mysticeti), a toothed mysticete from the Oligocene of Australia. Zool. 3 Marx F.G.

Hocking D.P.

Park T.

Ziegler T.

Evans A.R.

Fitzgerald E.M.G. Suction feeding preceded filtering in baleen whale evolution. 4 Marx F.G.

Tsai C.-H.

Fordyce R.E. A new Early Oligocene toothed ‘baleen’ whale (Mysticeti: Aetiocetidae) from western North America: one of the oldest and the smallest. 19 Peredo C.M.

Pyenson N.D.

Boersma A.T. Decoupling tooth loss from the evolution of baleen in whales. 20 Racicot R.A.

Deméré T.A.

Beatty B.L.

Boessenecker R.W. Unique feeding morphology in a new prognathous extinct porpoise from the Pliocene of California. 21 Heyning J.E.

Mead J.G. Suction feeding in beaked whales: morphological and observational evidence. 22 Werth A.J.

Straley J.M.

Shadwick R.E. Baleen wear reveals intraoral water flow patterns of mysticete filter feeding. 3 Marx F.G.

Hocking D.P.

Park T.

Ziegler T.

Evans A.R.

Fitzgerald E.M.G. Suction feeding preceded filtering in baleen whale evolution. 4 Marx F.G.

Tsai C.-H.

Fordyce R.E. A new Early Oligocene toothed ‘baleen’ whale (Mysticeti: Aetiocetidae) from western North America: one of the oldest and the smallest. 3 Marx F.G.

Hocking D.P.

Park T.

Ziegler T.

Evans A.R.

Fitzgerald E.M.G. Suction feeding preceded filtering in baleen whale evolution. 3 Marx F.G.

Hocking D.P.

Park T.

Ziegler T.

Evans A.R.

Fitzgerald E.M.G. Suction feeding preceded filtering in baleen whale evolution. 23 Gordon K.R. Models of tongue movement in the walrus (Odobenus rosmarus). Early studies speculated that toothed mysticetes used their teeth to filter feed [], an idea later described as the “dental filtration hypothesis.” However, the teeth of previously described toothed mysticetes are too few, too small, too simplified, or too worn to be effective in filtering []. This led to a spate of recent studies that have developed a new hypothesis: that filter-feeding mysticetes evolved from edentulous, suction-feeding whales that lacked baleen []. The molars of Coronodon are far larger than those of other toothed mysticetes and hearken back to the dental filtration hypothesis. Unlike archaeocetes and most neocetes, its upper teeth widely overlap its lower teeth instead of interdigitating with them. As a result, when the mouth is opened, the posterior teeth enclose diamond-shaped gaps (∼15 × 35 mm at m1 and m2) that could filter out prey of varying size. When the mouth is closed, the gaps in Coronodon would have been closed off by the crown of the opposing dentition. Even so, narrow slots (0.5–3 mm wide) between the imbricated lower molars remained open ( Figure 2 B), allowing even smaller prey to be filtered. The serrate borders of these slots are formed by small accessory cusps that point distally from the tooth preceding the slot and mesially from the tooth following the slot. In archaeocetes, the basal cusps are directed more apically, instead of mesially or distally ( Figure 2 G). Many of the basal cusps in Coronodon have minor, but distinct, apical wear, indicating that they were exposed and not covered in gingiva ( Table S2 ). The wear on the fairly sheltered, mesially directed cusps is unexpected and may have been formed by prey that accumulated along the slots during filtering. Such passive wear is quite common in marine mammals, with substrate being the best-documented culprit, particularly in porpoises []. Modern beaked whales use suction feeding to capture prey [], which impact their tusks upon entering the oral fissure. This can result in strong wear on the mesial side of the tooth, specifically that portion exposed during typical gape employed during feeding ( Figure 2 H). Interestingly, the wear of baleen in extant mysticetes seems driven by the intraoral flow of water and the prey and sediment carried with it []. Apical wear also occurs in some aetiocetids [], but we have come to a different interpretation because, in Coronodon, that wear also occurs on cusps sheltered by the preceding tooth. One unnamed aetiocetid (NMV P252567) has mesodistal grooves and large patches of wear on the lingual sides of its crowns []. We agree with the interpretation that this wear reflects suction feeding from the benthos [], as it mirrors the only extant mammal, Odobenus, that does this as a primary feeding mode []. Importantly, no comparable wear or grooves exist in the only known specimen of Coronodon.