Protosqualus

Protosqualus Temporal range: 130–66 Ma PreꞒ Ꞓ O S D C P T J K Pg N Barremian to Maastrichtian[1][2]
Drawing of a Protosqualus argentinensis tooth
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Order:	Squaliformes
Family:	Squalidae
Genus:	†Protosqualus Cappetta, 1977
Type species
†Protosqualus sigei[4] Cappetta, 1977
Other species
†Protosqualus albertsi Thies, 1981[3] †Protosqualus glickmani Averianov, 1997 †Protosqualus pachyrhiza Underwood & Mitchell, 1999 †Protosqualus barringtonensis Guinot et al., 2013 †Protosqualus argentinensis Bogan et al., 2016

Protosqualus ("Primitive Squalus") was a genus of dogfish shark that existed during the Cretaceous. Fossils have been found in Europe (mainly in France, Russia, Germany, Lithuania, The United Kingdom and Ukraine), East Asia (mainly in Japan),^[5] Antarctica,^[6] Australia,^[7] India and South America.^[8][9] The type species is Protosqualus sigei, which was found around an Albian aged deposit in France.^[10] There are 6 species which can be differentiated by distinct features in their teeth. Some species show some level of heterodonty, for example Protosqualus barringtonensis shows a rather high level of heterodonty within its teeth. The oldest specimens are from the Speeton Clay Formation.^[11] Protosqualus teeth are quite common in the Grey Chalk deposit of England.^[12] The genus went extinct at the end of the Cretaceous during the Cretaceous–Paleogene extinction event, with the last species in the genus being Protosqualus argentinensis from southern Argentina as well as possibly being from earlier deposits in India.

The type specimen of Protosqualus was found in a deposit that had been known to contain shark teeth since the 1930's. The type specimen was finally named in 1977 by paleontologist and ichthyologist, Henri Cappetta. The deposit is the Mortoniceras inflatum ammonoid zone. The name Protosqualus literally means "Primitive Squalus" in reference to how it is an ancient genus related to the extant genus, Squalus. The type species, P.sigei, derives its species name from paleontologist, B.Siegé.^[10]

Protosqualus, like most fossil sharks, is known from mostly dental remains.^[13] Protosqualus had an undulated ventral margin of the dental crown that was distal to the apron. Unlike modern members of the genus Squalus, Protosqualus had dental crowns which did not have a great degree of labiolingual compression.^[14] The holotype of P. sigei featured a tooth with small gaps between them, each with a clearly defined slanted tip. Its front ledge was well-developed and slightly raised, culminating in a rounded shape firmly attached at the base of the crown. The back of the tooth appeared flat, with a slight depression near the ledge. The root was low and thick, angled slightly in relation to the crown, and exhibited minor protrusions on both its front and rear sides. The bottom of the tooth was flat and gently curved, with the holes on the sides either separated or, less commonly, connected by a groove.^[10] The teeth of P. pachyrhiza were more bulky.^[15] Protosqualus glickmani had a broad cusp.^[16] Protosqualus argentinensis had defined serrated cutting edges and was probably the largest member of the genus.^[14] Protosqualus barringtonensis may have shown some sexual dimorphism in the teeth, however all teeth show a short cusp.^[17] P.albertsi had a broad, labial protuberance.^[15]

Protosqualus is a member of the Squalidae within the Squaliformes group. Today, only two extant genera of squalids exist. Squalids tend to note have the teeth in their lower jaw not being any larger than the teeth seen in the upper jaw.^[18] Below is a cladogram showing where Squalidae sits in the Squaliform clade.^[19]

In 2001, Adnet & Cappetta found the following position for Protosqualus.^[20]Here, it is shown to be closer to the extant dogfishes called Spurdogs.

Protosqualus lived alongside other chondrichthyans as well as other types of fish. It also lived alongside mosasaurs and plesiosaurs.^[21] Fossils of Protosqualus have been found in the Speeton Clay Formation, Omagari Formation, Hunstanton Formation, Jiesia Formation, Lopez de Bertodano Formation, Burim Formation, Ferriby Formation, Karai Formation,^[22] Labguva Formation, Hibernian Greensand Formation and Calafate Formation among a few others. The Calafate Formation likely had estuaries and shallow seas.^{[23] [24]}