The Spurdogs are a highly successful genus of sharks that have a worldwide distribution. According to Compagno et al (2005) the genus is made up of 10 described and possibly another six undescribed species. The genus generally inhabits continental and insular shelves and slopes. Often traveling in large groups, the Spurdogs are bottom dwellers that feed mostly on bony fishes.
Although two species (Squalus acanthias LINNAEUS, 1758 & S. mitsukurii JORDAN & SNYDER, 1903) can be found circumglobally, others have strangely disjunct ranges. Squalus cubensis HOWELL-RIVERO, 1936 for example, ranges from North Carolina through the Gulf of Mexico and from southern Brazil through Argentina. S. megalops (MACLEAY, 1881) is known from south and western Africa, the China Sea and southern Australia. S. blainvillei (RISSO, 1826), if a valid species, can be found in the Mediterranean, eastern South Atlantic and Japan.
From a paleontological perspective, this suggests two things to keep in mind. First, there is little reason to consider location (other than water temperature & depth) as might be the case with a genus such as Squatina. Secondly, various Squalus species appear to be very stable over time. There would appear to be no valid reason (if there ever is) to use time as a relevant factor when determining a tooth's identification. For example, Squalus teeth from the Middle Atlantic's Oligocene, Miocene and Pliocene which compare well with S. acanthias are more likely to be from that species than some paleontological species erected on an ad hoc basis.
Squalus acanthias The Piked dogfish
The most widely distributed species, S. acanthias inhabits boreal to warm temperate waters of both hemispheres. It can be found inshore or offshore from intertidal to 900 M. This slender-bodied shark may reach 1.5 meters and has been described as a powerful and voracious predator that preys largely on bony fishes, but also feeds on cephalopods, arthropods and other marine animals.
The distally directed cusps and tightly interlocked teeth create a long, serrate, knife-like edge in each jaw quadrant which may have from 20 to 30 teeth. Considering the strength of the jaws and feeding habits, Compagno (1984) referred to the dentition as being of a clipping design. The dentition has weak monognathic heterodonty. The angle of the cusp is slightly different between jaws, the crowns grow lower distally and upper teeth are more mesio-distally compressed than the lowers. Despite their small size, the Squalus tooth is quite robust. Viewed laterally, the base of the crown may be 60% as wide as the crown is high and the root is relatively massive. The crown is made-up of a blade-like cusp and a high distal heel. A smooth cutting-edge extends the length of the crown. Compagno (1984) notes that the cutting edge in all extant species is smooth. The base of the labial face bears a long strong apron.
The Fossil Record
Cappetta (1987) notes that this genus has been reported from the Upper Cretaceous of Belgium, Lower Eocene of the USSR, Oligocene of Oregon & Czechoslovakia and Miocene of Japan. He went on to list five fossil species.
S. almeidae ANTUNES & JONET 1970 (Middle Miocene of Portugal & France),
S. alsaticus ANDREAE, 1892 (Lower Oligocene of France),
S. crenatidens ARAMBOURG 1952 (Lower Palaeocene to Lower Eocene of Morocco),
S. minor (DAIMERIES, 1888) (Palaeocene - Lower Eocene western Europe) and
S. serriculus JORDAN & HANNIBAL, 1923 (Miocene Southern California).
Welton & Farish (1993) found this genus in Campanian-Maastrichtian deposits of Texas.
Ward & Wiest (1990) reported S. minor from the Early & Late Palaeocene of Maryland (USA).
Kent (1999) included Megasqualus was erected by Herman (1982) for over-sized Squalus teeth. This genus is listed separately although it shows little difference from that of Squalus design other than size.
Case & Cappetta (1997: 135) erected S. huntensis for small Squalus teeth from the Late Maastrichtian of Texas; in doing so, they synonymized the Squalus sp of Welton & Farish (1993: 74, fig 2).
The Aquia Formation Squalus minor
Although uncommon, the Aquia formation yields Squalus teeth that generally agree with the S. minor tooth-design. Note the serrate cutting edge and V-shaped apron. Bill Heim (pers. com. 2002) points out that in S. minor, the cutting edge is usually very finely serrated. The mesial shoulder consists of one large, rounded cusplet which may also possess very fine serrations. The root is usually fairly thick and almost flat basally.
The Nanjemoy Formation Squalus crenatidens
In Squalus crenatidens, the blade is more erect than other members of the genus and possesses strong serrations. The mesial shoulder has a prominent cusplet(s) and strong serrations.
Miocene - Lee Creek Mine and Sharktooth Hill
Purdy et al (2001) included a single Squalus tooth (USNM207546) from the lower Yorktown (Unit 2). They found it to resemble S. alemeidae but failed to ascribe it to that species. This author has recovered two dozen Squalus teeth (Figs.
& ) from Lee Creek, most from Yorktown Unit 2. None of these teeth compare well with the Smithsonian specimen. They do compare very favorably with the extant species S. acanthias.
The Round Mountain Silt of Sharktooth Hill (Kern Co., CA) commonly yields the relatively large teeth of Squalus occidentalis (AGASSIZ, 1856) (Fig. ). Older literature attributed this tooth-design to S. serriculus which was shown to be an junior synonym by Welton in his 1979 PhD thesis,
Cappetta, H., 1987. Chondrichthyes II. Mesozoic and Cenozoic Elasmobranchii. In: Handbook of Paleoichthyologie, vol. 3b, Gustav Fischer Verleg, Stuttgart, 193 pp.
Case, G. and Cappetta, H.. 1997. A new selachian fauna from the late Maastrichtian of Texas. Münchener Geowissenschaften Abhandungen 34:131-189.
Compagno, L.,1984. FAO Species Catalogue, Vol 4, parts 1 & 2 Sharks of the World. United Nations Development Program.
Compagno, L,, Dando & M., Fowler, S., 2005. Sharks of the World. HarperCollins, 368 p.
Kent, B., 1999. Sharks from the Fisher/Sullivan site. In: Weems and Grimsley (eds) Early Eocene Vertebrates and Plants from the Fisher/Sullivan Site (Nanjemoy Formation) Stafford County, Virginia. Virginia Division of Mineral Resources, Pub 152. pp 11-51.
Purdy, R., Schneider, V., Appelgate, S., McLellan, J., Meyer, R. & Slaughter, R., 2001. The Neogene Sharks, Rays, and Bony Fishes from Lee Creek Mine, Aurora, North Carolina. In: Geology and Paleontology of the Lee Creek Mine, North Carolina, III. C. E. Ray & D. J. Bohaska eds. Smithsonian Contributions to Paleobiology, No 90. Smithsonian Institution Press, Washington D.C. pp. 71-202.
Ward, D. and Wiest, R., 1990. A checklist of Paleocene and Eocene sharks and rays (Chondrichthyes) from the Pamunkey Group, Maryland and Virginia, USA. Tertiary Res., 12(2) p 81-88.
Welton, B., 1979 Late Cretaceous and Cenozoic Squalomorphii of the Northwest Pacific region. - Unpubl. Ph. D. thesis, Univ. Calif.Berkley, 553 pp.
Welton, B. J. and R. F. Farish 1993. The Collector's Guide to Fossil Sharks and Rays from the Cretaceous of Texas. Before Time, Texas. 204 pp.