Updated November 20, 2016 based on input from Dr. RW Boessenecker who corrected a mistake I made reconstructing Tokarahia by noting the mandible was axially rotated in situ 180º. Correcting that error produces the bowed out Gothic arch set of mandibles, typical of mysticetes.
Several of the mistakes discussed below are based on the long-standing tradition that whales are monophyletic. That is, odontocetes (toothed whales) and mysticetes (baleen whales) were long thought to have a common ancestor with flukes and fins. That paradigm was overturned by the large reptile tree (LRT) earlier here. Workers thought that common ancestor would be an archaeocete, but, so far, all tested archaeocetes nest basal to extant odontocetes and were derived from tenrecs in the LRT.
Almost fifty years ago whale monophyly was questioned
by Van Valen (1968), who listed a number of traits that distinguish Odontoceti from Mysticeti. Unfortunately this was before computer-assisted phylogenetic analysis and neither desmostylians nor tenrecs were offered as basal taxa with legs. This was also long before any whales with legs had been discovered. Gotta give Van Valen credit for his insight way back then.
Yesterday we looked at the desmostylian ancestors of today’s mysticete (baleen) whales. Less than 24 hours ago I encountered for the first time Aetiocetus (Emlong 1966; Figs. 1, 2). I learned it has long been considered a toothed basal mysticete. Evidently some of the back teeth are leaf-shaped and all of the teeth are small and widely spaced. Most whale workers are happy with this hypothesis or relationships, but the LRT finds otherwise based on an expanded taxon list.
I added Aetiocetus
to the large reptile tree and, while given the opportunity to nest with mysticetes, Aetiocetus nested instead between Zygorhiza and Orcinus + Physeter, all members of the Odontoceti. As recently as 2015, Ekdale et al. (Fig. 2) were trying to use Aetiocetus to explain the origin of baleen in modern whales.
18 hours ago I encountered another mysticete,
Tokarahia kauaeroa (Boessenecker and Fordyce 2015; Late Oligocene; OU 2235), which has no teeth. Their reconstruction (Fig. 2) is
I thought Boessenecker and Fordyce changed the curve of the mandible in their reconstruction and did not follow the very narrow mandibles in restoring the largely missing or buried rostrum. They also moved the orbit anteriorly. I attempted a mistaken correction (Fig. 3) that was based on a narrow mandible interpretation. Dr Boessenecker reported the mandible was axially rotated in situ (Fig. 4). Those corrections was applied shortly thereafter. A good lesson in keeping an open mind.
12 hours ago
I also learned about Isanacetus laticephalus (Kimura and Ozawa 2002; early Miocene, 18 mya; MFM 28501; Fig. 5, 6). This fossil whale is indeed a mysticete. In the LRT it nests between the desmostylian Behemotops (presumably with at least front legs) and extant baleen whales.
Like all mysticetes and derived desmostylians
Isanacetus has a ventrally concave rostrum, a wide flat skull and other traits that distinguish mysticetes from odontocetes + tenrecs. Isanacetus is also one of the smallest known mysticetes, about twice the size of its current desmostylian sister, Behemotops (Fig. 6) and half again longer in the skull than the more completely known Desmostylus.
More backstory for those keenly interested
Below are some earlier and traditional reports on Aetiocetus and aetiocetids, which was considered by these authors to be related to mysticetes and cetotheres (extinct mysticetes).
Whitmore and Sanders 1976 report,
“The cheek teeth [of Aetiocetus] are leaf shaped, similar to those of Patriocetus, but smaller and with the roots coalesced. The triangular rostrum, reduced dentition, and the conformation of the posterior ends of the maxillae, premaxillae, and nasals (Ernlong, 1966:s) are characters that would be expected in the ancestor of the mysticetes. Thenius (1969:489) stated: “Even if Aetiocetus, because of its geologic age (upper Oligocene) cannot be a direct stem form of the cetotheres, yet this genus documents that a specific family (Aetiocetidae) must be classified as ancestor, the link between ancient and baleen whales.
“Among the few Cetacea known from deposits of middle Oligocene age are two occurrences of unmistakable Mysticeti. One of these, Mauicetus Benham, 1939, from New Zealafid, has long nasals embraced by premaxillae and maxillae which extend posteriorly to the level of the supraorbital process of the frontal, together with an anteriorly thrusting triangular supraoccipital. The Oligocene Mysticeti, had already evolved the elongated, edentulous rostrum, constituting 3/4 to 4/5 of total skull length, that typifies the modern baleen whales. The mandible of Oligocene Mysticeti was also edentulous and, like those of modem baleen whales, was long and slim.”
Berta and Demere 2005 reported,
“Aetiocetids are the most taxonomically and morphologically diverse clade of toothed mysticetes known from the late Oligocene of the eastern and western North Pacific. Aetiocetids can be distinguished from other toothed mysticetes by the following unequivocal synapomorphies: lobate or triangular parietal-frontal suture; zygomatic process of squamosal expanded near anterior end; “window” in the palate exposing vomer; short, broad extension of the palatine that overlaps the pterygoid; and exoccipital developed ventrally as an anteriorly directed posterior sinus.
The presence of palatal nutrient foramina associated with the upper teeth in all aetiocetids suggests that these toothed mysticetes had already evolved some type of baleen. The form and function of this rudimentary baleen is currently unknown, but the fact that these archaic mysticetes also possessed procumbent anterior teeth, broad diastemata, and posterior teeth with sharply pointed cusps, accessory denticles, and longitudinal enamel ridges suggests development of a specialized type of filter feeding differing from that of other toothed and edentulous mysticetes.”
these authors had not expanded their taxon set to include desmostylians, which pull mysticetes away from odontocetes + tenrecs.
Eckland et al. 2015 report
“The origin of baleen in mysticetes heralded a major transition during cetacean evolution. Extant mysticetes are edentulous in adulthood, but rudimentary teeth develop in utero within open maxillary and mandibular alveolar grooves. The teeth are resorbed prenatally and the alveolar grooves close as baleen germ develops.”
That’s all well and good, but you really need a wide gamut taxon inclusion set that includes whales, tenrecs and desmostylians to find the diphyletic origins of extant whales and in desmostylians one should look for the origin of baleen, as discussed earlier here.
Berta A and Demere TA 2005. Phylogenetic relationships among the diverse toothed mysticete clade the aetiocetidae and reconsideration of the filter feeding niche. Evolution of aquatic tetrapods. Fourth triennial convention abstracts May 16-20 2005, Akron, OH, USA.
Boessenecker RW and Fordyce RE 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. in press. doi:10.1111/zoj.12297.
Demere TA 2005. Palate vascularization in an Oligocene toothed mysticete (Cetacea: Mysticeti): Aetiocetidae); implications for the evolution of baleen. Evolution of aquatic tetrapods. Fourth triennial convention abstracts May 16-20 2005, Akron, OH, USA.
Ekdale EG, Demere TA and Berta A 2015. Vacularization of the gray whale palate (Cetacea, Mysticeti, Eschrichtius robustus): soft tissue evidence for an alveolar source of blood to baleen. · February 2015.
Whitmore FC Jr and Sanders AE 1976. Review of the Oligocene Cetacea. US Geological Survey Staff — Published Research. Paper 237.
Van Valen L 1968. Monophyly or diphyly in the origin of whales”. Evolution. 22 (1):37–41.