\n <\/p>\n
This great white shark (Carcharodon carcharias) is still a shark. <\/span>Credit: Dave Fleetham\/Design Pics Editorial\/UIG via Getty<\/span><\/p>\n In a 1981 magazine essay, the evolutionary biologist Stephen Jay Gould let readers in one of his field\u2019s counter-intuitive truths. Aquatic animals, including lungfish and coelacanths, are more closely related to tetrapods \u2014 four-limbed vertebrates \u2014 than to salmon, sticklebacks and many other things people call \u2018fish\u2019 \u2014 or, as Gould quipped, \u201cthere is surely no such thing as a fish\u201d.<\/p>\n Sharks could be in a similar situation. A genomic study of dozens of shark species and their close relatives suggests that the ocean\u2019s top predators might also not be a natural biological group, contrary to what studies using more-limited genetic data have suggested.<\/p>\n The analysis, posted last month on the bioRxiv preprint server, finds that, when researchers look at some \u2018ultra-conserved\u2019 parts of the genome, a peculiar family of sharks called Hexanchiformes might be part of an evolutionary lineage that is distinct from the group that includes all other sharks, as well as skates and rays1.<\/p>\n The results, which haven\u2019t been peer reviewed, suggest that most animals that people call sharks are more closely related to rays and skates than to hexanchiform shark species \u2014 just as Gould pointed out was the case for some species called fishes. Biologists call such groups paraphyletic.<\/p>\n Whether a grouping of animals is paraphyletic or not doesn\u2019t keep most scientists awake at night. But accurate family trees, or phylogenies \u2014 including one for sharks \u2014 help researchers to chart the evolution of key traits. \u201cHaving an accurate phylogeny is a way forward for understanding the processes that have shaped life,\u201d says Gavin Naylor, an evolutionary biologist at the Florida Museum of Natural History in Gainesville.<\/p>\n Sharks, along with rays, skates and other sea creatures with a cartilaginous skeleton, are part of a group called Chondrichthyes, which shared a common ancestor with bony fish that lived more than 400 million years ago.<\/p>\n Animals resembling modern sharks have existed for much of this time, says Naylor. \u201cThese things have been around looking the way they look, or at least recognizable as sharks, for 330 million years.\u201d<\/p>\n Yet researchers are unsure how the different members of Chondrichthyes \u2014 which is one of the major jawed-vertebrate groups \u2014 are related to one another. Anatomical studies have concluded2 that rays and skates, together known as batoids, were either distinct from all sharks, or members of a shark subgroup.<\/p>\n What were the first animals? The fierce sponge\u2013jelly battle that just won\u2019t end<\/p>\n<\/p>\n Studies of limited genetic data have also tended to class sharks and batoids as distinct evolutionary groups. Increasingly, researchers are redrawing the animal tree of life using entire genomes \u2014 sometimes with bewildering and controversial conclusions. But sharks have yet to receive such attention.<\/p>\n To plug this gap, evolutionary biologists Thomas Near and Chase Brownstein at Yale University in New Haven, Connecticut, analysed previously published genomes of 48 species of shark, ray and other animals representing all the major lineages of Chondrichthyes. They looked at two kinds of data: 840 protein-coding sequences that are shared across species and around 350 \u2018ultra-conserved\u2019 regions, which are slowly evolving sequences that probably serve important, but often unknown, functions.<\/p>\n The protein-coding genes confirmed existing \u2018monophyletic\u2019 shark family trees, with rays and skates classed as distinct from all sharks. The ultra-conserved sequences, however, suggested that sharks were paraphyletic: the hexanchiform sharks formed a lineage distinct from the one that gave rise to all other sharks, rays and skates. Hexanchiformes, which include cow sharks and frilled sharks, have six or seven gill slits instead of the usual five, as well as primitive-looking jaws and eel-like bodies.<\/p>\n One implication of this tree is that flat-bodied animals, including manta rays, evolved from shark-like ancestors. \u201cThis result implies that rays and skates are just another type of shark, and that the shark body plan came first,\u201d says Brownstein.<\/p>\n He and Near favour the hypothesis that sharks are paraphyletic, in part because this tree was more strongly supported by their analysis than was a monophyletic one. Sequencing more shark species and looking at other types of genetic marker might be needed to determine which of the two trees is correct.<\/p>\n","protected":false},"excerpt":{"rendered":" This great white shark (Carcharodon carcharias) is still a shark. Credit: Dave Fleetham\/Design Pics Editorial\/UIG via Getty In a 1981 magazine essay, the evolutionary biologist Stephen Jay Gould let readers in one of his field\u2019s counter-intuitive truths. Aquatic animals, including lungfish and coelacanths, are more closely related to tetrapods \u2014 four-limbed vertebrates \u2014 than to<\/p>\n","protected":false},"author":1,"featured_media":46706,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[58],"tags":[2906,17919,11002,19203,7128,6331,3751],"class_list":{"0":"post-46705","1":"post","2":"type-post","3":"status-publish","4":"format-standard","5":"has-post-thumbnail","7":"category-science","8":"tag-family","9":"tag-genomes","10":"tag-ocean","11":"tag-predators","12":"tag-shake","13":"tag-shark","14":"tag-tree"},"_links":{"self":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/posts\/46705","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=46705"}],"version-history":[{"count":0,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/posts\/46705\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=\/wp\/v2\/media\/46706"}],"wp:attachment":[{"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=46705"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=46705"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/naijaglobalnews.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=46705"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Shifting definitions<\/h2>\n