Friday 23 May 2014

The origin and diversification of Monk Seals.

The Caribbean Monk Seal, Monachus tropicalis, has not been seen in the wild since 1952, and is now generally excepted as being extinct. Its two surviving relatives, the Mediterranean Monk Seal, Monachus monachus, and the Hawaiian Monk Seal, Monachus schauinslandi, are both considered to be critically endangered; with about 350-450 surviving Mediterranean Monk Seals split into a number of small populations in the Mediterranean Sea and Atlantic coast of Mauritania, Western Sahara and the Canary Islands, and about 1000 Hawaiian Monk Seals scattered throughout the Hawaiian Islands. These small population sizes make studying the Seals hard, and relationships within the group are not well understood.

In a paper published in the journal ZooKeys on 14 May 2014, a team of scientists led by Dirk-Martin Scheel of the Leibniz Institute for Zoo and Wildlife Research describe the results of a phylogenic study of Monk Seals, which used DNA recovered from museum specimens to elucidate relationships within the group.

Lateral views of crania of (a) the Mediterranean Monk Seal (b) the Hawaiian Monk Seal, and (c) the Caribbean Monk Seal. Arrows indicate the more developed occipital crest and zygomatic arches, and deeper snout of the Mediterannean Monk Seal compared to the Hawaiian and Caribbean Monk Seals. Scheel et al. (2014).

This analysis revealed that the extinct Caribbean Monk Seal was significantly more closely related to the Hawaiian Monk Seal than it was to the Mediterranean Monk Seal, and indeed that the genetic gulf between the Mediterranean and New World Monk Seals was great enough to justify them being placed in separate genera. Since the Mediterranean species was the first described in the genus Monachus, the two remaining species are assigned to a new genus, Neomonachus, meaning ‘new-Monachus’, a reference to both the new nature of the genus and its New World distribution.

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 archaeological evidence. The range of the Mediterranean monk seal illustrates both historical (lighter shading) and current (darker shading) distributions. Scheel et al. (2014).

Using dating methods based upon genetic divergence, Scheel et al. suggest that Mediterranean Monk Seals diverged from New World Monk Seals about 6.30 million years ago, in the Late Miocene, and that the two New World species diverged about 3.67 million years ago, in the Pliocene. This later divergence fits with the timing of the closure of the Panama Seaway around the beginning of the Pliocene, which cut off the Atlantic and Pacific Oceans (they were later reconnected in the south by the separation of Antarctica and South America at the end of the Pliocene).

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. Scheel et al. (2014).

Most of the Seals most closely related to Monk Seals are restricted to the Southern Ocean, making it hard to determine through genetic analysis where the group originated. However there are numerous fossils of Seals deemed to be closely related to the group around the shores of the Atlantic, suggesting this is a more likely point of origin then the Pacific. Scheel et al. suggest that either the last common ancestor lived in the Atlantic and that the ancestral groups became divided when one group crossed to the other side of this ocean, though they do not have the means to determine whether this was an east-west or west-east migration.

See also…

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