2. Fossil Record and Origin of Squaliform Sharks
Jürgen Kriwet and Stefanie Klug
Abstract.—Squaliform sharks constitute a monophyletic group of predominantly deep-water neoselachians. Their fossil record mainly consists of isolated teeth; complete skeletons or skeletal remains are very rare. The quality of the fossil record of squaliform sharks is analyzed using a phylogenetic hypothesis based on a supertree to establish the timing of cladogenetic events, those related to descent from a common ancestor, and gaps in the fossil record. The supertree is the most inclusive estimate of squaliform interrelationships that has been proposed to date and contains 23 fossil and extant members of all major groups. In addition, the simple completeness metric is used to examine the quality of the fossil record of squaliforms as an independent measure. Although different (48% and 61%, respectively), both measures indicate that the fossil record of squaliforms is very incomplete considering that most living and extinct squaliforms are deep-water sharks and corresponding sediments are very scarce. Gaps in the fossil record range from 5 to 100 million years. The most basal and stratigraphically oldest group within Squaliformes consists of Squalus and †Protosqualus1. The phylogenetic hypothesis indicates a gap in the fossil record of Squalus spp. of about 25–30 million years. Our results show a postJurassic origination of squaliforms in the shallow waters of the northern Tethyal margin. The hypothetical ancestor of squaliforms is characterized by two dorsal fin spines and absence of dignathic heterodonty (the morphology of upper and lower teeth differs significantly). Lower teeth are characterized by a slightly oblique basal root face and overlapping upper teeth. Although disappearance and appearance of organisms is a fact of life, the very long geologic range and success of Squalus highlights the need for very careful management of its current population crisis, which is due to causes that never occurred before in Earth’s history–the anthropogenetic impact.