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Porbeagle
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==Phylogeny and evolution== Several [[phylogenetic]] studies, based on [[morphology (biology)|morphological characters]] and [[mitochondrial DNA]] sequences, have established the [[sister species]] relationship between the porbeagle and the salmon shark (''L. ditropis''),<ref name="compagno1990"/><ref name="naylor et al"/> which occurs in place of it in the North Pacific.<ref name="francis et al"/> The genus ''Lamna'' evolved 65β45 [[mya (unit)|Mya]]. When its two extant species [[speciation|diverged]] from each other is uncertain, though the precipitating event was likely the formation of the [[Arctic ice pack|ice cap]] over the [[Arctic Ocean]], which would have isolated sharks in the North Pacific from those in the North Atlantic.<ref name="martin1996"/><ref name="musick et al"/> [[Fossil]]ized porbeagle remains are known from [[Late Miocene]] [[geologic timescale|epoch]] (about 7.2 Mya) deposits in [[Belgium]] and the Netherlands, [[Pliocene]] epoch (5.3β2.6 Mya) deposits in Belgium, [[Spain]], and [[Chile]], and [[Pleistocene]] epoch (2.6 Mya to 12,000 BP) deposits in the Netherlands.<ref name="mollen"/><ref name="long"/><ref name="bourdon"/> However, ''Lamna'' teeth that closely resemble those of the porbeagle have been found in the La Meseta Formation on [[Seymour Island]] off the [[Antarctic Peninsula]], which date to the middle to late [[Eocene]] [[epoch (geology)|epoch]] (50β34 Mya). Much taxonomic confusion remains regarding ''Lamna'' in the fossil record due to the high degree of variability in adult tooth morphology within species.<ref name="long"/><ref name="purdy and francis"/>
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