Beak deviations in the skull of Franciscana dolphins Pontoporia blainvillei from Argentina

Abstract

The Franciscana dolphin Pontoporia blainvillei is characterized by a long rostrum, a feature that is shared with the families formerly classified as river dolphins (Pontoporiidae, Platanistidae, Iniidae, Lipotidae). Although there are occasional reports on the existence of beak deformations, very little published information exists describing this process. The object of the present study was to describe and quantify the beak anomalies of Franciscana dolphins from the coastal waters of Argentina. Of 239 skulls analyzed 12% showed beak deviations (BD), affecting the premaxillary-maxillary and dentary bones to different extents. The occurrence of BD in the dentary bone represented 58%, whereas premaxillary-maxillary BDs were observed in 14% of the studied specimens, while the complete rostrum (dentary, premaxillary and maxillary) was affected in 28% of the skulls. Dorsoventral axis BD was more frequent than lateral BD (48 and 38%, respectively), and double BD was only observed in the dentary bone. Most of the BD observed in this study could be classified as mild/moderate, and we assume that it did not affect the feeding activities of individuals; however, 2 specimens (<1%) showed a severe and complex curvature that probably did affect them. The cause of these anomalies (natural or anthropogenic origins) is unknown but may be related to important parasite loads, heavy metal and organic contaminants and plastic ingestion that could affect the coastal dolphin in different ways. A more detailed and thorough study of these cranial anomalies is necessary.

Extensive Diversity and Disparity of the Early Miocene Platanistoids (Cetacea, Odontoceti) in the Southeastern Pacific (Chilcatay Formation, Peru)

“…Unfortunately, E. riveroi is only known from this single skull and consequently it is not possible to check if this peculiar morphology is an anomaly present in a single individual, or rather a distinctive character of the species. In support of the "anomaly" hypothesis, a similar torsion has been observed in some skulls of P. gangetica [113], Inia geoffrensis [144], and Pontoporia blainvillei [114,145]. Even if the causes of this and other cranial anomalies observed on the skull of extant "river dolphins" are still unclear, Gerholdt [145] speculated that a slight twisting due to a trauma in a young individual of P. blainvillei could increase disproportionally in the adult due to the positive allometric growth of the rostrum.…”

“…The most peculiar feature of this new species is its curved, sinusoidal rostrum in lateral view, combined with large and proportionally larger, probably procumbent, upper incisors. A similar sinusoidal rostrum, but not associated with large and procumbent anterior teeth, has been observed in some skulls of Pontoporia blainvillei [114]. Large, but not procumbent anterior teeth, sometimes combined with an upward but not sinusoidal curvature of the rostrum are also present in Platanista gangetica [23,149].…”

“…As already mentioned above, all squalodelphinids exhibit a left-side torsion of the rostrum. Being present in the five skulls of N. vanbenedeni currently known, this character could be considered a distinctive character of this family rather than an individual 'anomaly' as interpreted in some extant longirostrine odontocetes [113,114,144,145]. Although a similar twist of the rostrum is observed in protocetids and basilosaurids [48,157], none of the extant cetaceans consistently shows such a character.…”

“…The rostrum of Ensidelphis clearly differs from the rostrum of the extant Platanista by not displaying a marked transverse compression, a character also observed in a fragmentary early Miocene fossil rostrum referred to Platanistinae [100]. Moreover, the rostrum of the E. riveroi holotype is markedly bent towards the right, a possibly natural condition also observed in some adult females of extant Platanista gangetica [113] and in some specimens of Pontoporia blanivillei [114] (see below for a possible interpretation of this peculiar asymmetry).…”

“…Among other squalodelphinids, we also observed a curved (but not sinusoidal) rostrum in the cranium MUSM 1403 of Huaridelphis raimondii (but not in the holotype) and in the crania MUSM 1395 and MUSM 3897 of Notocetus vanbenedeni (but not in the holotype and the other referred specimens). Among extant odontocetes, although not associated with procumbent incisors a sinusoidal shape of the rostrum was observed in a few crania of Pontoporia blainvillei [114], whereas a rostrum raising anterodorsally, but lacking the anterior downward counter-curvature and, again, the procumbent incisors, was noted in one cranium of the brevirostrine pontoporiid Brachydelphis mazeasi [122] and in some crania of Platanista gangetica [113,123] and of few delphinid species (e.g., Delphinus delphis MZUF 12484, G.B. personal observation; Delphinus capensis, Figure 2 in [124] ).…”