A close relative of the Amazon river dolphin in marine deposits: a new Iniidae from the late Miocene of Angola

Abstract

Background: A few odontocetes (echolocating toothed cetaceans) have been able to independently colonize freshwater ecosystems. Although some extant species of delphinids (true dolphins) and phocoenids (porpoises) at least occasionally migrate upstream of large river systems, they have close relatives in fully marine regions. This contrasts with the three odontocete families only containing extant species with a strictly freshwater habitat (Iniidae in South America, the recently extinct Lipotidae in China, and Platanistidae in southeast Asia). Among those, the fossil record of Iniidae includes taxa from freshwater deposits of South America, partly overlapping geographically with the extant Amazon river dolphin Inia geoffrensis, whereas a few marine species from the Americas were only tentatively referred to the family, leaving the transition from a marine to freshwater environment poorly understood.

Methods: Based on a partial odontocete skeleton including the cranium, discovered in late Miocene (Tortonian-Messinian) marine deposits near the estuary of the Cuanza River, Angola, we describe a new large iniid genus and species. The new taxon is compared to other extinct and extant iniids, and its phylogenetic relationships with the latter are investigated through cladistic analysis.

Results and discussion: The new genus and species Kwanzacetus khoisani shares a series of morphological features with Inia geoffrensis, including the combination of a frontal boss with nasals being lower on the anterior wall of the vertex, the laterally directed postorbital process of the frontal, the anteroposterior thickening of the nuchal crest, and robust teeth with wrinkled enamel. As confirmed (although with a low support) with the phylogenetic analysis, this makes the new taxon the closest relative of I. geoffrensis found in marine deposits. The geographic provenance of K. khoisani, on the eastern coast of South Atlantic, suggests that the transition from the marine environment to a freshwater, Amazonian habitat may have occurred on the Atlantic side of South America. This new record further increases the inioid diversity during the late Miocene, a time interval confirmed here as the heyday for this superfamily. Finally, this first description of a Neogene cetacean from inland deposits of western sub-Saharan Africa reveals the potential of this large coastal area for deciphering key steps of the evolutionary history of modern cetaceans in the South Atlantic.

Keywords: Amazon river dolphin; Angola; Cetacea; Iniidae; Inioidea; Kwanza basin; late Miocene.

Figure 1. Locality of the holotype of Kwanzacetuskhoisani.

Location map and geological overview of the discovery zone of K. khoisani in the Kwanza basin, Angola, modified from Cauxeiro, Durand & Lopez (2014). Image credit: Sylvain Adnet and Michel Lopez.

Figure 2. Stratigraphical context for the holotype of Kwanzacetus khoisani.

Stratigraphical architecture of the cliff south of Barra do Cuanza where the holotype of K. khoisani was discovered. (A) panoramic view; (B) interpreted line drawing; and (C) sedimentary column showing the main facies assemblage. In the lower part of the cliff, alternating hummocky cross stratified sandstones, mudstones, and black shales from the lower to middle Miocene (sequence 1) are slightly tilted southwards and obliquely truncated by the major erosional unconformity (sequence 2). This erosional surface is onlapped by an overall fining upward sequence dated from the late Miocene. This sequence shows upper shoreface sand burrowed by Ophiomorpha (sequence 3) passing upwards to burrowed fine sand to clayey silt from lower shoreface environment (sequence 4). The upper part of the cliff is composed of an upward coarsening sequence, from fine to coarse sand and gravel (sequence 5), which marks the overall progradation of the paleo-Cuanza delta during the Pliocene. This sequence is floored by a marine ravinement lag deposit. Abbreviations: F fossil locality (see also Fig. 3); M mudstone; W wackestone; P packstone; G grainstone; B boundstone; C clay; Si silt; f fine sand; m medium sand; c coarse sand; G gravel; HCS hummocky cross stratification; TaCS tabular cross stratification; TrCS trough cross stratification. Image credit: Sylvain Adnet and Michel Lopez.

Figure 3. Location of the holotype of Kwanzacetuskhoisani on the cliff (indicated in Fig. 2).

Photographs before (A) and after (B) the extraction of the skull. Orange Fe-hydroxide halos in the sand indicate Ophiomorpha burrowing. Photo credit: Michel Lopez.

Figure 4. Dorsal view of the cranium of Kwanzacetus khoisani.

Photograph (A) and corresponding line drawing (B) of the cranium of the holotype of K. khoisani CZA 1-2 in dorsal view. Light grey for sediment and plaster; dark grey for attached bone fragments; hatched areas for major break surfaces; dotted lines for interpretation of unclear sutures. Scale bar equals 100 mm. Photo credit: Olivier Lambert.

Figure 5. Lateral view of the cranium of Kwanzacetus khoisani.

Photograph (A) and corresponding line drawing (B) of the cranium of the holotype of K. khoisani CZA 1-2 in right lateral view. Light grey for sediment and plaster; dark grey for attached bone fragments; hatched areas for major break surfaces; dotted lines for interpretation of unclear sutures. Scale bar equals 100 mm. Photo credit: Olivier Lambert.

Figure 6. Ventral view of the cranium of Kwanzacetus khoisani.

Photograph (A) and corresponding line drawing (B) of the cranium of the holotype of K. khoisani CZA 1 in ventral view. Note that the detached, tooth-bearing anterior part of the rostrum figured in dorsal and right lateral view is not present here. Light grey for sediment and plaster; hatched areas for major break surfaces. Scale bar equals 100 mm. Photo credit: Olivier Lambert.

Figure 7. Posterior view of the cranium of Kwanzacetus khoisani.

Photograph (A) and corresponding line drawing (B) of the cranium of the holotype of K. khoisani CZA 1 in ventral view. Light grey for sediment and plaster; dark grey for attached bone fragments; hatched areas for major break surfaces. Scale bar equals 100 mm. Photo credit: Olivier Lambert.

Figure 8. Detail of the basicranium, orbit, and palate of the cranium of Kwanzacetus khoisani.

Photograph (A) and corresponding line drawing (B) of the right side of the basicranium, orbit, and palate of the holotype of K. khoisani CZA 1-2 in ventrolateral and slightly anterior view. Light grey for sediment and plaster; dark grey for attached bone fragments; hatched areas for major break surfaces. Scale bar equals 100 mm. Photo credit: Olivier Lambert.

Figure 9. Additional views of the cranium of Kwanzacetus khoisani.

(A) right posterolateral and slightly dorsal view of the holotype of K. khoisani CZA 1-2; (B) anterior and slightly dorsal view; (C) right anterolateral and slightly dorsal view. Dotted lines for main sutures and other bone outlines. Scale bar equals 100 mm. Photo credit: Olivier Lambert.

Figure 10. Dorsal view of the cranium of the extant iniid Inia geoffrensis (Amazon river dolphin).

Photograph of the facial region of the cranium of I. geoffrensis (MUSM DPV CE 9) in dorsal view, showing several morphological features shared with Kwanzacetus khoisani and, for part of them, with other inioids and some early delphinidans (see text for details). Photo credit: Giovanni Bianucci.

Figure 11. Maxillary teeth of Kwanzacetus khoisani.

(A) two more anterior right maxillary teeth of the holotype of K. khoisani CZA 2 in lingual view; (B) detail of one of these teeth in labiodistal view; (C) three more posterior right maxillary teeth CZA 3 in lingual view; (D) detail of one tooth in labiomesial view; (E) detail of the same tooth in labiodistal and slightly occlusal view; (F) detail of another tooth in labiomesial view. Dotted lines for deep occlusion facets. Scale bars equal 10 mm. Photo credit: Olivier Lambert.

Figure 12. Cervical vertebrae of Kwanzacetuskhoisani.

(A–D), axis of the holotype of K. khoisani CZA 4 in anterior (A), left lateral (B), posterior (C), and ventral (D) views; E–F, cervical ?C3-C4 CZA 5 in anterior (E) and right lateral (F) views. Scale bar equals 50 mm. Photo credit: Olivier Lambert.

Figure 13. Phylogenetic relationships of Kwanzacetus khoisani.

Phylogenetic tree showing the relationships of K. khoisani with other early diverging delphinidans, as obtained from our parsimony analysis of morphological data, constrained with a molecular tree as backbone. Other odontocete clades are collapsed to facilitate reading. K. khoisani falls as an iniid, displaying close relationships with the extant Inia geoffrensis. Stars identify species with a strictly freshwater distribution. Numbers indicate bootstrap values.