CHAEOMYSTICETI Mitchell, 1989 Chaeomysticeti

Ávila, SP, Cordeiro, R, Rodrigues, AR, Rebelo, AC, Melo, C, Madeira, P & Pyenson, ND, 2015, Fossil Mysticeti from the Pleistocene of Santa Maria Island, Azores (Northeast Atlantic Ocean), and the prevalence of fossil cetaceans on oceanic islands, Palaeontologia Electronica (New York, N. Y.) 23 (1), pp. 1-12 : 4-7

Infraorder CHAEOMYSTICETI Mitchell, 1989 Chaeomysticeti incertae sedis

Material examined. DBUA-F 1079 consists of a fragmentary mandibular corpus ( Figure 2 ).

Provenance. The specimen was collected September 17, 2012, at about 0.2 m above the contact between a basal conglomerate (facies 2, Figure 3 ) and the overlaying 2.6 m thick, yellowish, partly cross-laminated, volcaniclastic to bioclastic, uncemented sands (facies 4, Figure 3 ). These sediments correspond to a beach foreshore (intertidal) facies ( Ávila et al., 2015), and they are covered by aeolian dunes and colluvial–alluvial deposits ( Figure 3 ). The specimen was deposited in a northsouth direction, perpendicular to the present shoreline, in a subvertical position, and making a gentle slope of ~23° with the horizontal.

Description. As preserved, the fragment measures approximately 1.0 m in greatest length and approximately 0.2 m in greatest width. It is broadly cylindrical in presentation, dominated by one major surface that is convex and curved along the length of the major axis of the fragment; another surface, opposite of the latter one, is more planar, with a more gentle curve, largely paralleling the direction of the former one ( Figure 2 ). The best-preserved end of the fragment shows how both of these surfaces communicate enclosing a canal that extends similarly parallel to the major axis of the fragment. This inner surface of the bone forming this canal is notably smooth and maintains a consistent diameter (~ 3 cm) throughout its length. A small fragment at this best-preserved end, which roofs over this canal, suggests that in life the canal was likely completely enclosed conduit. The more poorly preserved end mostly consists of highly eroded fragments that belong to the planar surface. There are two, small notable pit-like depressions on the convex surface.

Remarks. Although DBUA-F 1079 merely consists of a mandibular corpus, some taxonomic precision can be inferred from its preserved morphology and provenance. First, the general complexity and texture of the preserved cortical bone surface and the exposed cancellous bone indicate that it belongs to Mammalia. Its morphology is inconsistent with rib or any other appendicular or axial postcranial material; moreover, both its consistently sub-cylindrical shape and the inclusion of a large canal preclude it from belonging to the rostrum or another part of the cranium. Its preservation in marine deposits on an oceanic island points strongly to it being a marine mammal; its absolute size (~1.0 m in its largest dimension) excludes it belonging to any lineage except for baleen whales ( Mysticeti ). Thus, on the whole, we argue that DBUA-F 1079 represents a fragment from the mandible of a mysticete.

The mandibular corpus represents a section comprising the approximate mid-length of the element. The maximum width of the fragment is approximately 17 cm; the maximum (and likely sub-complete) dorsoventral height is approximately 18 cm ( Figure 2.1-2 ). The large canal is thus the mandibular canal, which in life would house the nerves and vasculature that pass anteriorly from the mandibular fossa and exit laterally along mental foramina and, in extant mysticetes, also exit dorsally and anteriorly at relictual alveolar foramina ( Pyenson et al., 2012, 2013). Given the overall curvature and orientation of the inferred anatomical planes of the specimen, this fragment appears to represent a right mandible: the more planar surface is medial; and the more convex and curved surface is lateral; the best preserved end thus represents the posterior part of the fragment.

The small depressions on the lateral size of the mandible are too large, diffuse, irregular in shape, and too different in size (with respect to one another) to represent mental foramina, which have a clean, pinched aperture ( Figure 2.3 ). The bone modification in DBUA-F 1079 is unknown among other fossil cetaceans. The overall surface geometry does not match any of the categories of bone modification in marine vertebrate fossils (see Boessenecker and Perry, 2011, table 1), including known patterns generated from bites (e.g., punctures, tooth scrapes) or bioerosion (e.g., boring, bioencrustation). We suspect that they represent some unknown type of bone trauma, parasite or post mortem alteration of the bone surface .

Although DBUA-F 1079 lacks other major morphological features (e.g., coronoid process) that would permit easy and direct identification, based on the general curvature of the mandible, we propose that it does not belong to Eschrichtiidae , which generally exhibits mandibular rami that are straight and not bowed ( Noakes et al., 2013); also, the lateral breadth of the fragment is greater than all adult specimens Eschrichtius robustus (Pyenson and Goldbogen, unpublished data). Similarly, we exclude Neobalaenidae because the lateral breadth of the fragment far exceeds that for extant Caperea (which ranges between 3–4 cm, Pyenson and Goldbogen, unpublished data), and there are no extant or fossil Neobalaenidae from the North Atlantic Ocean ( Tsai and Fordyce, 2015). Given the size range and remaining taxonomic groups, we cannot discriminate among the latter possibilities: a medium-large species of Balaenopteridae ; Balaenidae ; or an extinct lineage of Mysticeti , possibly belonging to a very large member of Cetotheriidae sensu lato ( Marx, 2011). We thus conservatively argue for its taxonomic identification as a baleen-bearing mysticete ( Chaeomysticeti sensu Marx, 2011), likely within the crown of Mysticeti , but cannot exclude it belonging to an unknown stem lineage. Although both Balaenopteridae and Balaenidae have stratigraphic ranges through MIS 5e, this latter timeframe is younger that the youngest known fossil Cetotheriidae sensu stricto, a Middle Pleistocene occurrence of Herpetocetus from California ( Boessenecker, 2013). It is not clear if large “cetotheriids” in the same body size as midsize Balaenoptera sp. , such as Pelocetus ( Pyenson and Sponberg, 2011) were alive during the time that this specimen was buried.

Figures

FIGURE 2. Right mandibular corpus (DBUA-F 1079) belonging to a medium-large size Mysticeti. 1, dorsal view; 2, lateral view; and 3, inset magnification of unknown bone modification to the lateral surface (denoted by arrows). Abbreviation “mc” indicates the mandibular canal. Thick broken arrows show passages of the mandibular canal through the fragment.

FIGURE 3. Detailed composite stratigraphic column at Praia do Calhau. The numbers depicted in filled circles correspond to facies 1–7, which are described in Ávila et al. (2015).