Silky Shark - Phylogeny and Evolution

Phylogeny and Evolution

Fossilized teeth belonging to the silky shark have been found in North Carolina: from the vicinity of two baleen whales, one in mud dating to the Pleistocene-Holocene (c. 12,000 years ago) and the other in Goose Creek Limestone dating to the Late Pliocene (c. 3.5 Ma), as well as from the Pungo River, dating to the Miocene (23–5.3 Ma). Fossil teeth have also been found in Pliocene strata at the Cava Serredi quarry in Tuscany, Italy. Carcharhinus elongatus, an earlier representative of its lineage with smooth-edged teeth, is known from Oligocene (34–23 Ma) deposits in the Old Church formation of Virginia, and the Ashley formation of South Carolina. A set of poorly described, Eocene (56–34 Ma) teeth resembling those of this species are known from Egypt.

Initial efforts to resolve the evolutionary relationships of the silky shark were inconclusive: based on morphology, Jack Garrick in 1982 suggested the blackspot shark (C. sealei) as its closest relative. In 1988, Leonard Compagno assigned it phenetically to an informal "transitional group" also containing the blacknose shark (C. acronotus), the blacktip reef shark (C. melanopterus), the nervous shark (C. cautus), the copper shark (C. brachyurus), and the night shark (C. signatus).

More recently, Gavin Naylor's 1992 phylogenetic analysis, based on allozyme sequence data, found that the silky shark is part of a group containing large sharks with a ridge between the dorsal fins. One branch within this group contains the sandbar shark (C. plumbeus) and the bignose shark (C. altimus), while the silky shark is the basal member of the other branch and the sister taxon to a clade containing the Caribbean reef shark (C. perezi), Galapagos shark (C. galapagensis), oceanic whitetip shark (C. longimanus), dusky shark (C. obscurus), and blue shark (Prionace glauca). Mine Dosay-Abkulut's 2008 ribosomal DNA analysis, which included the silky, blue, and bignose sharks, confirmed the closeness of those three species.