Friday, September 11, 2015

[PaleoMammalogy • 2015] Waharoa ruwhenua • 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

Waharoa ruwhenua  Boessenecker & Fordyce, 2015
Alternative life restorations of with erupted permanent dentition
Holotype cranium (OU 22044) in dorsal view and ventral view: (A) photograph; (B) interpretive line drawing.


The Eocene history of cetacean evolution is now represented by the expansive fossil record of archaeocetes elucidating major morphofunctional shifts relating to the land to sea transition, but the change from archaeocetes to modern cetaceans is poorly established. New fossil material of the recently recognized family Eomysticetidae from the upper Oligocene Otekaike Limestone includes a new genus and species, Waharoa ruwhenua, represented by skulls and partial skeletons of an adult, juvenile, and a smaller juvenile. Ontogenetic status is confirmed by osteohistology of ribs. Waharoa ruwhenua is characterized by an elongate and narrow rostrum which retains vestigial alveoli and alveolar grooves. Palatal foramina and sulci are present only on the posterior half of the palate. The nasals are elongate, and the bony nares are positioned far anteriorly. Enormous temporal fossae are present adjacent to an elongate and narrow intertemporal region with a sharp sagittal crest. The earbones are characterized by retaining inner and outer posterior pedicles, lacking fused posterior processes, and retaining a separate accessory ossicle. Phylogenetic analysis supports inclusion of Waharoa ruwhenua within a monophyletic Eomysticetidae as the earliest diverging clade of toothless mysticetes. This eomysticetid clade also included Eomysticetus whitmorei, Micromysticetus rothauseni, Tohoraata raekohao, Tokarahia kauaeroa, Tokarahia lophocephalus, and Yamatocetus canaliculatus. Detailed study of ontogenetic change demonstrates postnatal elaboration of the sagittal and nuchal crests, elongation of the intertemporal region, inflation of the zygomatic processes, and an extreme proportional increase in rostral length. Tympanic bullae are nearly full sized during early postnatal ontogeny indicating precocial development of auditory structures, but do increase slightly in size. Positive allometry of the rostrum suggests an ontogenetic change in feeding ecology, from neonatal suckling to a more specialized adult feeding behaviour. Possible absence of baleen anteriorly, a delicate temporomandibular joint with probable synovial capsule, non-laterally deflected coronoid process, and anteroposteriorly expanded palate suggests skim feeding as likely mode of adult feeding for zooplankton. Isotopic data in concert with preservation of young juveniles suggests the continental shelf of Zealandia was an important calving ground for latitudinally migrating Oligocene baleen whales.

Figure 1: Geologic and geographic context of Waharoa ruwhenua localities.
(A) map of South Island, New Zealand; (B) map of Waitaki Valley Regions showing location of Hakataramea Quarry, “The Earthquakes,” and Springside; (C), stratigraphic column of “The Earthquakes,” modified from Fordyce (1994); (D), stratigraphic column of Hakataramea Quarry, modified from (Gottfried, Fordyce & Rust, 2012).

Systematic Paleontology

MAMMALIA Linneaus, 1758
CETACEA Brisson, 1762
MYSTICETI Gray, 1846

EOMYSTICETIDAE Sanders & Barnes, 2002

Waharoa new genus

Type and only species: Waharoa ruwhenua.
Diagnosis of Genus: same as for the type and only known species.

Etymology: Waharoa, meaning long mouth; from the Māori waha (mouth) plus roa (long).

Waharoa ruwhenua new species

Etymology: Ruwhenua, from the Maori for ru (shaking) and whenua (land), a translation of the “The Earthquakes” locality.
Type locality: “The Earthquakes,” 5 km east southeast of Duntroon, North Otago, South Island, New Zealand (Figs. 1 and 2A).

Figure 3: Holotype cranium (OU 22044) of Waharoa ruwhenua in dorsal view. 
(A) photograph; (B) interpretive line drawing. Stippling denotes matrix, cross-hatching denotes broken surfaces.

Figure 4: Holotype cranium (OU 22044) of Waharoa ruwhenua in ventral view.
(A) photograph; (B) interpretive line drawing. Stippling denotes matrix, cross-hatching denotes broken surfaces.

Figure 5: Holotype cranium (OU 22044) of Waharoa ruwhenua.
(A) right lateral view; (B) interpretive line drawing; (C) left lateral view; (D) interpretive line drawing; (E) detail of basicranium in ventral view.

The new eomysticetid Waharoa ruwhenua is the first stem mysticete, and the first early neocete, for which an ontogenetic series of fossils is available for study. Amongst eomysticetids, Waharoa has a gracile skull, anteriorly oriented zygomatic processes, small periotics with a short anteroposteriorly directed and smooth posterior bullar facet, narrow and dorsoventrally shallow tympanic bullae, transversely wide atlases and axes and posterior cervicals with dorsoventrally deep vertebral foramina. External morphology, suture development, and osteohistology clearly identifies the smallest specimen (OU 22075) as a young juvenile, a slightly larger specimen as mature juvenile (OU 22163), and the holotype as an old adult. Several craniomandibular changes are noted through ontogeny, including anteroposterior lengthening of the rostrum, nasals, and mandibles, decrease in the size of the symphyseal groove, elaboration of the sagittal and nuchal crests, lengthening of the tympanic bulla, and increase in diameter of the facial canal of the periotic. Postnatal growth of the feeding apparatus is more extreme even than within modern rorquals, indicating that the long rostrum of eomysticetids is not simply a primitively inherited condition but that an elongate feeding apparatus was positively selected for. Distinct glenoid fossae indicate the presence of synovial craniomandibular joints, and in concert with the delicate posterior mandible indicate that Waharoa was likely not capable of rapid lunge feeding like rorquals. The lack of lateral palatal sulci from the anterior third of the palate may suggest the absence of baleen from the rostral terminus, perhaps forming a subrostral gap functionally analogous to that of balaenids and permitting skim feeding behavior. Osteohistology indicates that Waharoa primitively retained dense osteosclerotic ribs but lost localized pachyosteosclerosis characteristic of archaeocetes. Discovery of several juvenile Waharoa from New Zealand suggests that the continental shelf of Zealandia served as a calving ground for some of the earliest toothless mysticetes, perhaps serving as a warm-water winter habitat prior to a latitudinal migration to productive Antarctic waters. The radiation of early skim feeders such as putative late Oligocene balaenids may have contributed to the demise of Waharoa and other eomysticetids at the end of the Oligocene.

Figure 29: Alternative life restorations of Waharoa ruwhenua.
(A) with erupted permanent dentition; (B) without dentition.
Boessenecker, R,W, and Fordyce, R.E. 2015. 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. PeerJ 3:e1129 10.7717/peerj.1129