CETACEA, Brisson, 1762

Benton, MJ, Donoghue, PCJ, Vinther, J, Asher, RJ, Friedman, M & Near, TJ, 2015, Constraints on the timescale of animal evolutionary history, Palaeontologia Electronica (Florence, Italy) 15 (1), pp. 1-107 : 67-68

CROWN CETACEA (72)

Node Calibrated. Divergence of baleen and toothed whales.

Fossil Taxon and Specimen. Llanocetus denticrenatus (USNM 183022; Mitchell, 1989) from the Eocene La Meseta Formation of Seymour Island, Antarctica (Mitchell, 1989).

Phylogenetic Justification. Phylogenetic analysis of living and fossil mysticetes (Steeman, 2007) shows that Llanocetus is the closest relative of the toothless mysticetes (Chaeomysticeti Mitchell, 1989), based on two apomorphies: the premaxilla is straight in lateral view (not abruptly depressed anterior to nasals), and the occipital condyles are not situated on a swelling, but are rather levelled with the skull.

Minimum Age. 33.9 Ma

Soft Maximum Age. 56 Ma

Age Justification. The upper La Meseta Formation of Seymour Island, Antarctica is dated as latest Priabonian (late Eocene) on the basis of a wide

variety of fossils, including palynomorphs (Mitchell, 1989). Elsewhere, the La Meseta Formation appears to be restricted in age to the eaely Eocene (Ypresian), but upper units run to the end of theEocene on Seymour Island (Reguero et al., 2013). Hence, the Priabonian record of Llanocetus serves as the minimum constraint for the crown cetacean divergence (33.9 Ma ± 0.0 Myr = 33.9 Ma).

The presence of a diverse, early Eocene artiodactyl record, including archaeocetes but no crown cetaceans during the Ypresian, suggests the base of the Eocene may serve as a soft maximum for the odontocete-mysticete divergence, dated at 55.8 Ma ± 0.2 Myr = 56.0 Ma ( Gradstein et al., 2012).

Discussion. The fossil record famously illuminates the transition from terrestrial artiodactyls to fully aquatic cetaceans in substantial detail (Thewissen et al., 2009). What is somewhat less appreciated by the lay public is the fact that the major divergence within cetaceans, i.e., that between baleen (mysticete) and toothed (odontocete) whales from an archaeocete common ancestor, is also well documented paleontologically. For example, Demere et al. (2008) discussed the stepwise-accumulation of mysticete characters evident using a phylogenetic and developmental understanding of the Tertiary cetacean record, particularly that from coastal regions of Australia and New Zealand. Thus, the Oligocene basal mysticete Janjucetus exhibts a wide, mysticete-like rostrum; more crownward taxa such as Mammalodon and aetiocetids show in addition lateral bowing of the mandibles and a laterally thin maxilla; aetiocetids (and crown mysticetes) show in addition an abundance of nutrient sulci and foramina on the palate indicative of the presence of baleen during life; and finally fossils such as Eomysticetus share with living mysticetes the absence of mineralized teeth as adults. Not all phylogenetic treatments are completely in agreement with this scenario; for example, Marx (2011: figure 3) places the phylogenetically basal-most mysticetes ( Janjucetus , mammalodontids, and aetiocetids) together in a clade, and Fitzgerald (2010, 2011) differs from Demere et al. (2008) in his interpretation of the evidence for lateral thinning of the maxilla and the extent of fusion of the mandibular symphysis in Janjucetus , and is less certain about the presence of baleen in aetiocetids.

Importantly, the geologically oldest mysticete - Llanocetus from the latest Eocene of Seymour Island, Antarctica - also shows palatal sulci and foramina consistent with the interpretation that it simultaneously possessed mineralized teeth and baleen, along with considerable diastemata between its maxillary teeth ( Fordyce 2003 and pers. comm.). Hence, there appears to be substantial mosaic evolution among basal mysticetes, with recent analyses ( Demere et al., 2008; Beatty and Dooley, 2010; Fitzgerald, 2010; Marx, 2011) consistent with the inference that loss of mineralized teeth took place independently of, and subsequent to, lateral thinning of the maxilla and lateral bowing of the mandibles.

By the late Eocene, archaeocetes had a nearglobal distribution, corresponding with the first appearance of crown-group cetaceans. The oldest odontocete is likely Simocetus from early Oligocene (Rupelian) deposits in the Alsea Formation of Oregon ( Fitzgerald, 2010: figure 48).