Vascularization of the Gray Whale Palate (Cetacea, Mysticeti, Eschrichtius robustus): Soft Tissue Evidence for an Alveolar Source of Blood to Baleen

Abstract

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. Arteries supplying blood to highly vascularized epithelial tissue from which baleen develops pass through lateral nutrient foramina in the area of the embryonic alveolar grooves and rudimentary teeth. Those vessels are hypothesized to be branches of the superior alveolar artery, but branches of the greater palatine arteries may play a role in the baleen vascularization. Through a combination of latex injection, CT, and traditional dissection of the palate of a neonatal gray whale (Eschrichtius robustus), we confirm that the baleen receives blood from vessels within the superior alveolar canal via the lateral foramina. The greater palatine artery is restricted to its own passage with no connections to the baleen. This study has implications for the presence of baleen in extinct taxa by identifying the vessels and bony canals that supply blood to the epithelium from which baleen develops. The results indicate that the lateral foramina in edentulous mysticete fossils are bony correlates for the presence of baleen, and the results can be used to help identify bony canals and foramina that have been used to reconstruct baleen in extinct mysticetes that retained teeth in adulthood. Further comparisons are made with mammals that also possess oral keratin structures, including ruminants, ornithorhynchid monotremes, and sirenians. Anat Rec, 298:691-702, 2015. V C 2015 Wiley Periodicals, Inc.Key words: baleen; gray whale; Eschrichtius robustus; vascularization; palateIn this article, we investigate the blood supply to the baleen and surrounding epithelium covering the hard palate in a neonatal gray whale (Eschrichtius robustus). Baleen is a neomorphic keratinous structure that is unique to mysticete cetaceans (baleen whales) and functions as a natural sieve to trap prey during filter feeding (Pivorunas, 1979;Werth, 2001Werth, , 2004Werth, , 2013. Baleen develops as a series of transverse laminae (main and minor plates) that form a pair of bilateral racks along the ventrolateral borders of the maxillae. The racks of baleen laminae (or baleen racks) are oriented parasagittally, and they are deeply embedded within the highly Additional Supporting Information may be found in the online version of this article.

Hydration affects the physical and mechanical properties of baleen tissue

“…This, in turn, suggests that baleen, owing to its eruption and use in an exclusively aqueous environment, is unlike other mammalian epidermal alpha-keratins that are formed and used in air. As Szewciw et al [13] explained, baleen is different from other mammalian keratins in that it is never air-dried ( in vivo ) during any stage of development, most notably as it forms and emerges from the gums [43–45], but also during any later portion of life history. Szewciw et al .…”

Decoupling Tooth Loss from the Evolution of Baleen in Whales

“…Specifically, argued that foramina on the lateral margins of the palate, medial of tooth alveoli, were homologous with baleen innervation and vascularization in extant mysticetes (Figure 4). Consequently, artistic reconstructions of select aetiocetids depict them as toothbearing cetaceans with a functional rack of baleen medial to the dental row (see Hypothesis 2, Figure 1; Ekdale et al, 2015). There are several reasons to question this reconstruction as an overinterpretation.…”

“…While recent work suggests the baleen in extant gray whales (Eschrichtius robustus) may be vascularized via branches of the superior alveolar artery (Ekdale et al, 2015), it remains unclear how these branches could supply both teeth and baleen, as would be necessary for extant mysticetes in utero, or for aetiocetids if these possessed both types of feeding structures. Second, compared aetiocetids only to species of Balaenoptera, although balaenids lack lateral foramina entirely and Eschrichtius and Caperea display very different patterns (Figure 4).…”

“…The second (medial baleen hypothesis) suggests that incipient baleen evolved medial to a functional dental row Ekdale et al, 2015). The third (posterior baleen hypothesis) suggests that functional baleen evolved posterior to vestigial adult teeth retained at the distal tip of the rostrum and dentaries, with the dentition and baleen aligned in the rostrum (Boessenecker and Fordyce, 2015a,b).…”

A new Miocene baleen whale from the Peruvian desert

“…The extent of this divergence, as well as its absence further posteriorly, may partially be due to the post-mortem distortion of the rostrum and/or rostral kinesis. The palatal surface of each maxilla bears two sets of sulci and foramina: a medial one consisting of anteroposteriorly arranged sulci transmitting the greater palatine artery; and a lateral one consisting of a series of radially arranged sulci near the lateral margin of the rostrum, which originally housed branches of the superior alveolar artery supplying the baleen racks [46] (figure 8). Posteriorly, the maxilla forms a V-shaped suture with the palatine and gives rise to a well-developed infraorbital plate with a clearly defined embayment for the jugal.…”