Cappetta erected this genus for a dentition displaying a combination of characteristics. The lower anteriors lack cusplets and have thick, but mesio-distally compressed, roots with nearly vertical lateral margins and a straight cusp. The lingual protuberance bears an elliptical foramen. The upper anteriors have roots that are more slender & elongated, and may bear a reduced mesial cusplet. The lateral teeth have a pair of triangular, labio-lingually compressed cusplets and root lobes, which are elongated but thick, and obtusely divergent.

The history of these various tooth-designs is a story in itself.

Isurolamna inflata (LERICHE 1905)

Lamna vincenti var inflata was described by Leriche (1905:76) from late Ypresian, and from the early Middle Eocene of Belgium (1905:125). As it was only a variety of "Lamna vincenti = "Lamna" lerichei (CASIER 1946) it was not figured. It was first figured (as Lamna vincenti var inflata) in Leriche (1908:11, Pl. 1, fig. 9) for a tooth in the Bories collection from the Early Lutetian of Fontas, near Fabrezan, in the Corbières region SW France. The tooth, Leriche suggested, was a first upper right anterior. This tooth may actually be the second upper and the horizon may prove to be Late Ypresian (NP 12, not Lutetian). The name was used as a species, as opposed to a variety, by Leriche in 1936 (Leriche 1936a: 296) in a paper on reworked basal Oligocene teeth from NE Holland. This was the same year as he again published on teeth from the Corbières (Leriche 1936b: 398, 399).

Odontaspis hopei form affinis CASIER 1946 was based on lower anterior teeth from the late Ypresian, in the region of the NP12 of Belgium. "affinis" was first used as a species, "Lamna affinis", by Casier in 1950. Cappetta (1976: 555) erected the genus Isurolamna, and made Casier's Odontaspis hopei form affinis the type without mention of the species "inflata". However, in 1987, in the Handbook of Paleoichthyology, Cappetta referred "inflata" to Isurolamna, suggesting that it (inflata) ranged from Thanetian (Palaeocene) to the Ypresian (Early Eocene).

As noted earlier, Leriche originally described inflata from the Early and Middle Eocene. As both "species" were described from more or less strata of the same age and geographic region, inflata must be the senior synonym, having been described 41 years earlier. Cappetta suggested that affinis probably branched from inflata (Cappetta 1987: 95). This implies that both affinis and inflata co-existed in the Palaeocene or Early Eocene -- there is no published or personally observed evidence of this.

Isurolamna bajarunasi (GLIKMAN & ZHELEZKO 1985)

Glikman and Zhelezko (1985) unaware of Isurolamna, described two new species, Lamiostoma bajarunasi and Lamiostoma menneri. These they suggested, gave rise to the early Oligocene species Lamiostoma gracilis (LE HON 1871) (more commonly known as "Lamna" rupeliensis LE HON 1871 and Isurus gracilis LE HON 1871). Lamiostoma bajarunasi was described from the lower Schorym Formation (Mid-Late Eocene) of Mangyschlak, NW Kazakhstan and L. menneri from the middle and upper parts of the same formation. Both species were later referred to Isurolamna by Zhelezko & Kozlov (1999: 224-5).

Both Lamiostoma bajarunasi and menneri appear to be identical and if so, Isurolamna bajarunasi would be the senior synonym by page priority. On the basis of contour-analysis, Tanya Malyshkina, a student of the late Victor Zhelezko, is convinced (pers. com. to DW) that I. bajarunasi is a distinct species from I. inflata.

Lamiostoma GLIKMAN 1964 is a junior synonym of Isurus RAFINESQUE 1810, but Glikman's type species, L. belyaevi GLIKMAN 1964 may well be the senior synonym of Isurus paucus GUITART, 1966. Used in a palaeontological sense, it recognises the likelihood that the mako sharks were derived from Isurolamna.

Isurolamna gracilis (LE HON 1871)

Glikman 1964 was the first to point out that Isurus gracilis LE HON 1871 and Lamna rupeliensis LE HON 1871 are the anterior and lateral teeth respectively of the same species of lamniform shark, which currently is generally regarded as an Isurolamna.

Baut & Génault (1999: 21) in a paper on early Oligocene teeth from Belgium, referred the lateral teeth of Isurolamna gracilis (LE HON 1871) to a new genus Rhizoquadrangulus. Their discussion hinges around whether the species (rupeliensis) is a true Lamna or not, and omits Glikman's (1964) key observation that rupeliensis is a species comprising only lateral teeth. Reinecke et al (2001: 21) give an exhaustive synonomy of Isurolamna gracilis. They rejected the genus Rhizoquadrangulus in favour of Isurolamna.

It would appear that either the concept of Rhizoquadrangulus should be modified to include anterior teeth, those described, in part by Leriche (1910: 275) as Oxyrhina (= Isurus) desori, or (better) be consigned to a junior synonym of Isurolamna.

Isurolamna vandenbroecki (WINKLER 1880)

This is an unusual and fairly rare species, only currently known from the Oligocene of NW Europe and Kazakhstan. It was originally described as Odontaspis Van den Broecki in Winkler (1880), changed to Lanma Van den Broecki by Leriche (1910). Nolf (1988) was the first to ascribe it to Isurolamna. It is typically Isurolamna in shape, similar to juvenile specimens from the Palaeocene, but with rather more separated lateral cusplets. Baut & Génault (1999: 23) erected the monospecific genus Lethenia for it. It is unclear how this helps our understanding of its relationships between it and similar species of Isurolamna, and we reject its usage at this time.

Species Summary

Isurolamna inflata (LERICHE 1905) [Lamna vincenti var inflata LERICHE 1905, Odontaspis hopei form affinis CASIER 1946]. Reported from the Late Palaeocene (Selandian/Thanetian) and Early Eocene (Ypresian) of North Africa, Europe and North America.

Isurolamna bajarunasi (GLIKMAN & ZHELEZKO 1985) [Lamiostoma bajarunasi GLIKMAN & ZHELEZKO 1985 and L. menneri GLIKMAN & ZHELEZKO 1985]. From the Mid-Late Eocene (Bartonan-Priabonian) of Kazakhstan.

Isurolamna gracilis (LE HON 1871) [Isurus gracilis LE HON 1871, Lamna rupeliensis LE HON 1871, Rhizoquadrangulus rupeliensis (LE HON 1871)]. From the Early Oligocene of Western Europe.

Isurolamna vandenbroecki (WINKLER 1880) [Odontaspis Van den Broecki WINKLER 1880, Lanma Van den Broecki LERICHE 1910, Isurolamna vandenbroecki NOLF 1988]. From the Early Oligocene of NW Europe and Kazakhstan.

References