Timing of deep-sea adaptation in dogfish sharks: insights from a supertree of extinct and extant taxa

Dogfish sharks (Squaliformes) constitute a monophyletic group of predominantly deep-water neoselachians, but the reasons and timing of their adaptation to this hostile environment remain ambiguous. Late Cretaceous dogfish sharks, which generally would be associated with deep-water occur predominantly in shallow water environments. Did the end-Cretaceous mass extinction event that eliminated large numbers of both terrestrial and aquatic taxa and clades including sharks trigger the evolutionary adaptation of present deep-water dogfish sharks? Here, we construct, date, and analyse a genus-level phylogeny of extinct and living dogfish sharks to bring a new perspective to this question. For this, eleven partial source trees of dogfish shark interrelationships were merged to create a comprehensive phylogenetic hypothesis. The resulting supertree is the most inclusive estimate of squaliform interrelationships that has been proposed to date containing 23 fossil and extant members of all major groups. †Eoetmopterus represents the oldest dalatoid. †Microetmopterus, †Paraphorosoides, †Proetmopterus and †Squaliogaleus are stem-group dalatoids in which bioluminescence most likely was not developed. According to our analyses, bioluminescence in dogfish sharks was already developed in the early Late Cretaceous indicating that these sharks adapted to deep-water conditions most likely at about 100 Mya. The advantage of this reconstruction is that the fossil record is used directly for age node estimates rather than employing molecular clock approaches.