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

Abstract

deep-sea adaptation in dogfish sharks: insights from a supertree of extinct and extant taxa. -Zoologica Scripta, 39, 331-342. 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. †Micro-etmopterus, †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.

Molecular phylogeny and node time estimation of bioluminescent Lantern Sharks (Elasmobranchii: Etmopteridae)

“…On the one hand fossil remains of etmopterids comprise fossilized teeth only and few studies exist dealing with the identification of general morphological tooth characteristics for identifying genera Kriwet and Klug, 2010;Straube et al, 2008). On the other side, dating of geological strata including fossil remains of Etmopteridae are partially debatable .…”

“…Our first point provides a minimum age for the root of the tree using the first unambiguous chimaeroid fossil dated to 374.5 Ma in the late Devonian (Venkatesh et al, 2007;Benton and Donoghue, 2007). Further, we restricted the minimum age of Squaliformes to a time window of 130-125 Ma ago in the early Cretaceous, as indicated by fossil findings of teeth of Protosqualus (Cappetta, 1987), apparently the oldest known representative of Squaliformes suggested by its tooth root morphology (Kriwet and Klug, 2010) and assuming that Protospinax is not a squaliform shark (Kriwet and Klug, 2004). Further calibration points within Squaliformes comprise the minimum age of Centroscymnus ranging from 83.5 to 70.6 Ma (Thies and Müller, 1993) and Centrophoridae with 70.6 to 65.5 Ma referring to articulated fossils from Sahel Alma, Lebanon (Cappetta, 1987) displaying the desired clear linkage to extant species.…”

Does the physiology of chondrichthyan fishes constrain their distribution in the deep sea?

“…3). For example, substantial hepatic accumulation of squalene is almost exclusive to members of five speciose deep-sea squaloid families, which all evolved from the ancestral family Squalidae (Klug and Kriwet, 2010), a shallow group lacking squalene accumulation ( Fig. 3B; Table S3).…”

Node age estimations and the origin of angel sharks, Squatiniformes (Neoselachii, Squalomorphii)

“…Thies 1983), whereas the oldest fossil record of Squaliformes is from the Early Cretaceous (e.g. Kriwet & Klug 2008;Klug & Kriwet 2010) and the oldest representative of Pristiophoriformes occurs in the Cenomanian (Cappetta 2006). Thus, identification of the basal sister group of Squatiniformes is not trivial but central if diversification events are used for setting an evolutionary framework.…”