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Biogeography and Taxonomy of Extinct and Endangered Monk Seals Illuminated by Ancient DNA and Skull Morphology

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Biogeography and Taxonomy of Extinct and Endangered Monk Seals Illuminated by Ancient DNA and Skull Morphology

Dirk-Martin Scheel et al. Zookeys.

Abstract

Extinctions and declines of large marine vertebrates have major ecological impacts and are of critical concern in marine environments. The Caribbean monk seal, Monachus tropicalis, last definitively reported in 1952, was one of the few marine mammal species to become extinct in historical times. Despite its importance for understanding the evolutionary biogeography of southern phocids, the relationships of M. tropicalis to the two living species of critically endangered monk seals have not been resolved. In this study we present the first molecular data for M. tropicalis, derived from museum skins. Phylogenetic analysis of cytochrome b sequences indicates that M. tropicalis was more closely related to the Hawaiian rather than the Mediterranean monk seal. Divergence time estimation implicates the formation of the Panamanian Isthmus in the speciation of Caribbean and Hawaiian monk seals. Molecular, morphological and temporal divergence between the Mediterranean and "New World monk seals" (Hawaiian and Caribbean) is profound, equivalent to or greater than between sister genera of phocids. As a result, we classify the Caribbean and Hawaiian monk seals together in a newly erected genus, Neomonachus. The two genera of extant monk seals (Monachus and Neomonachus) represent old evolutionary lineages each represented by a single critically endangered species, both warranting continuing and concerted conservation attention and investment if they are to avoid the fate of their Caribbean relative.

Keywords: Ancient DNA; Panamanian Seaway; Phocidae; extinction; mitochondrial DNA; systematics.

Figures

Figure 1.
Figure 1.
Distributions of the three monk seal species. The range for the Caribbean monk seal is taken from Adam (2004) and is based on documented populations and archeological evidence. The range of the Mediterranean monk seal illustrates both historical (lighter shading) and current (darker shading) distributions.
Figure 2.
Figure 2.
Maximum likelihood phylogram inferred from cytb sequence data using the GTR + Γ4 substitution model. Node support is expressed as the percent proportion of 1000 bootstrap pseudoreplicates that agree with the bipartitions on the best ML tree (above internode branches) as well as the aLRT SH-like score (below internode branches). Support values above 80% for both measures are shown. Black boxes indicate nodes recovered with >0.88 posterior probability in Bayesian analyses. The scale bar indicates the number of substitutions per site.
Figure 3.
Figure 3.
Time-calibrated phylogeny of the seals estimated from combined nuclear and mitochondrial data. Time scale is in millions of years before present. Note that the chronogram has been pruned to show only true seals and immediate pinniped outgroups. Node bars show the 95% HPD intervals for divergence time estimates and mean ages are labeled for the two divergence times within the monk seals. Labeling at the top indicates water circulation through the Central American Seaway, the circle and associated wavy blue lines indicate a period during which water circulation periodically ceased and resumed but a shallow seaway remained open.
Figure 4.
Figure 4.
Genetic distances between currently recognized taxonomic units within Phocidae derived from logdet distances for cytb. Distances within: a Phoca b Pusa c Phoca versus Halichoerus d Pusa versus Halichoerus e Phoca versus Pusa f Histriophoca versus Pagophilus g Phocini h Phocinae i Monachus j Mirounga k Lobodontini, and l Monachini.
Figure 5.
Figure 5.
Lateral views of crania of a Monachus monachus b Neomonachus schauinslandi, and c Neomonachus tropicalis. Arrows indicate the more developed occipital crest and zygomatic arches, and deeper snout of Monachus compared to Neomonachus species.
Figure 6.
Figure 6.
Ventral views of palates of a Monachus monachus b Neomonachus schauinslandi, and c Neomonachus tropicalis. The tooth row of Monachus is more crowded, likely as a result of the shorter rostrum, and this results in a more obliquely oriented set of post-canine teeth and the lack of a diastema between the upper canine and the first premolar. In Neomonachus, there is a distinct diastema between C1 and P1, and the post-canine teeth are arranged more linearly. The upper incisor arcade of Monachus is slightly parabolic due to the posterior placement of the lateral incisors, and the anterior premaxilla appears slightly curved. In Neomonachus, the incisor arcade is linear and the anterior premaxilla is straight.
Figure 7.
Figure 7.
Dorsal view of rostra of a Monachus monachus b Neomonachus schauinslandi, and c Neomonachus tropicalis. Monachus exhibits a well-developed antorbital process on the maxilla, immediately inferior to the fronto-maxillary suture. The process is reduced or absent in Neomonachus. The nasals of Monachus are short and triangular, tapering smoothly posteriorly to produce a point at their union. The nasals of Neomonachus are longer and do not taper smoothly.
Figure 8.
Figure 8.
Ventral views of crania of a Monachus monachus b Neomonachus schauinslandi, and c Neomonachus tropicalis, showing the pterygoid region. Neomonachus exhibits a well-developed, laterally flared pterygoid hamulus that is visible in dorsal view. The hamulus may be spatulate (Neomonachus schauinslandi) or hook-like (Neomonachus tropicalis). The hamular process is absent or medially flared in Monachus, and is not visible in dorsal view.
Figure 9.
Figure 9.
Posteroventral view of the basicranium and left bulla in a Monachus monachus b Neomonachus schauinslandi, and c Neomonachus tropicalis. The bulla of Monachus is bordered posteriorly by a ventrally expanded posterior portion of the petro-mastoid complex. The petrosal abuts the bulla’s posterior wall and in ventral view forms the entire lateral and anterolateral border of the posterior lacerate foramen. In Neomonachus, the posterior part of the petrosal is visible in the posterior lacerate foramen but remains superior to the bulla. In ventral view, this gives the impression that the anterior border of the posterior lacerate foramen is formed entirely by the bulla. The posterior carotid canal opens posteroventrally in Monachus. This apparently results from a relatively complete “ring-like” opening, formed by the bulla. This form of opening is apparent in subadult and juvenile Monachus, suggesting that it is not dependent on ontogenetic development or the robusticity of the Monachus cranium relative to Neomonachus. In contrast, the posterior carotid canal of Neomonachus opens directly posteriorly, the opening being an incomplete ring and the dorsal border formed by a flattening of the bulla, perhaps resulting from the bulla’s extension over the petrosal.
Figure 10.
Figure 10.
Plots of mean upper (a) and lower (b) relative post-canine tooth size. Relative tooth size is computed by dividing the mesio-distal length of each tooth by the length of the 3rd premolar (which is typically largest) in the same row.
Figure 11.
Figure 11.
Medial view of right dentaries of a Monachus monachus b Neomonachus schauinslandi, and c Neomonachus tropicalis. The mandibular foramen is situated inferior to the mandibular notch in Monachus, and opens immediately to the medial surface of the ramus. In Neomonachus, the foramen is anteriorly displaced and is set in a groove or sulcus that extends from inferior to the mandibular notch. Also note the expanded rugose area for insertion of the pterygoid muscles in Monachus. This region is poorly developed in Neomonachus.

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