GIK: State Darwin Museum, Moscow, Russia. GSA: Geological Survey of Alabama, Tuscaloosa, USA. IRSNB: Royal Belgian Institute of Natural Sciences, Brussels, Belgium. MMNS: MMNS, Jackson, USA MSC: McWane Science Center, Birmingham, Alabama, USA. SC: South Carolina State Museum, Columbia, USA.

Palaeohypotodus was previously comprised of three valid species, including Cretaceous P. bronni (Agassiz, 1843) and Paleogene P. rutoti (Winkler, 1874) and P. volgensis Zhelezko & Kozlov, 1999. Multiple other species have been named, like Palaeohypotodus houzeaui (Woodward, 1891) and P. lerichei Glückman, 1964, but these (and some other) taxa were subsequently synonymized with P. rutoti (Cappetta & Nolf, 2005). Still other species, including P. speyeri (Darteville & Casier, 1943) and P. striatula (Dalinkevicius, 1935), have been referred to other genera (see Cappetta 2012). Several morphological features have been utilized to identify teeth of Palaeohypotodus, including; the variable occurrence of one to three pairs of lateral cusplets; upper lateral teeth having a strongly distally inclined to hooked main cusp; upper and lower anterior teeth with tall, triangular and generally symmetrical main cusp; lower lateral teeth with a tall and relatively erect main cusp; upper teeth with complete mesial and distal cutting edges; lower teeth with incomplete cutting edges; a pronounced lingual root protuberance with nutritive groove; and a U-shaped interlobe area. Although many of these features are present on teeth of other taxa, like Jaekelotodus Glückman, 1964 and Odontaspis winkleri Leriche, 1905, all teeth assigned to Palaeohypotodus bear plications across the labial crown foot (particularly on lateral and posterior teeth). Additionally, upper and lower anterior teeth of Jaekelotodus have complete cutting edges, and all those of O. winkleri have incomplete edges. The teeth included with GSA–V447 all exhibit these characteristics, and their assignment to Palaeohypotodus is appropriate.

Of the recognized species, Palaeohypotodus rutoti (Winkler, 1874) is the one most often reported in the literature. This species would seem to have a rather long temporal distribution, being variously reported from Danian to Priabonian (late Eocene) strata, as well as an extensive geographic distribution (see Cappetta 2012; Kriwet et al. 2016). Although P. bronni (Agassiz, 1843) is predominantly known from the Maastrichtian (Late Cretaceous) of Europe, at least two reports suggest this species may have survived the K/Pg extinction event into the Paleocene (see Leriche 1906; Adolfssen and Ward 2015). To our knowledge, P. volgensis has not been identified beyond the type discussion (Zhelezko and Kozlov 1999). Many of the identifications to these species appear to be tentative at best and we believe that these occurrences need further evaluation. Although such a thorough reassessment of Palaeohypotodus is beyond the scope of this paper, we present a limited discussion of the morphological criteria that have been used to identify these species so they can be adequately compared to P. bizzocoi sp. nov.

This taxon was originally named by Agassiz (1843) based on two teeth collected from Upper Cretaceous (Maastrichtian) deposits in the Mount St. Peter area in Maastricht, Netherlands (pl. 37, figs 8–9), and a third tooth from an unspecified unit in Delaware, USA (fig. 10). Only the outline of the Delaware specimen was provided, and it is difficult to evaluate due to the lack of details. Also, considering that the tooth was collected from a different continent and the age of the source stratum is unknown, we suggest it be excluded from Agassiz’s (1843) P. bronni type suite. Based on our interpretation of the dentition of P. bizzocoi sp. nov., the two teeth from Maastricht appear to be from the lower dentition, with that shown in Agassiz’s (1843) figure 8 representing a lower lateral tooth and the one in figure 9 likely being a lower anterior tooth. Agassiz’s (1843) brief description of these teeth stated that they have a symmetrical main cusp, two pairs of lateral cusplets, and plications along the labial crown foot. Unfortunately, these two teeth are embedded in limestone and only the labial face is visible.

Fortunately, additional P. bronni specimens from the vicinity of the type locality were subsequently reported by Preim (1897, pl. 1, figs 11–14), Van de Geyn (1937, figs 117–123), Herman (1977, pl. 10, fig. 2a–f), and Cappetta and Corral (1999, fig. 4). Additionally, we examined three teeth from Maastrichtian deposits exposed at the ENCI Quarry in Limburg, Netherlands (MMNS VP–9616, MMNS VP–10577.1–2). This assortment of teeth allows us to gain a better understanding of the dental variation within this species, as it includes anterior and lateral tooth groups from both the palatoquadrate and Meckel’s cartilage (see Table 4). Critical examination of these teeth demonstrated that they are morphologically similar to one another by their having a triangular main cusp that is erect in the anterior and lower lateral files but distally hooked in the upper lateral files, labial plications occur along the crown foot, one to two pairs of lateral cusplets (with the larger pair being positioned closer to the main cusp), a robust root with a pronounced lingual protuberance and nutritive groove, and a relatively deep and U-shaped interlobe area.

The historical descriptions of P. bronni teeth support our observations. For example, Giebel (1848) reiterated that the teeth had one or two pairs of lateral cusplets. Sauvage (1898) also noted the presence of two lateral cusplets on P. bronni teeth and he mentioned that Cretaceous specimens from Portugal have conspicuous plications at the labial crown foot. Leriche (1902) described the teeth as having two pairs of lateral cusplets, with the second pair smaller, and noted that labial folds were small and obsolete on Paleocene specimens he examined (suggesting these latter specimens need to be reevaluated as they may not belong to this genus). From these descriptions, it seems clear that Late Cretaceous teeth from Europe assigned to P. bronni have two pairs of lateral cusplets, with the second pair being smaller than the first. Adolfssen and Ward (2015) tentatively reported a Danian record of this taxon, but they specifically noted that the material lacked labial ornamentation and only had a single pair of lateral cusplets. These differences indicate that Paleogene records of P. bronni need to be reevaluated, as they may not belong to this genus.

The P. bronni teeth we examined are morphologically very similar to those of P. bizzocoi sp. nov., but several distinct features distinguish the two species. Adult teeth of P. bronni generally have two pairs of rather wide lateral cusplets, whereas P. bizzocoi sp. nov. teeth typically have a single pair of relatively narrow cusplets. Overall, teeth of P. bizzocoi sp. nov. have one to three pairs of cusplets (adults generally with one pair and juveniles with one to three pairs) that, compared to P. bronni, are much shorter, narrower, more needle-like, and diminutive with respect to the size of the main cusp. In addition, the secondary cusplets (i.e., the more labial pair) on P. bronni teeth are much larger than those that occur on any P. bizzocoi sp. nov. teeth, where on the latter they are generally minute and are largely united to the larger medial cusplet (while they are more clearly separated on P. bronni). Furthermore, the root lobes of P. bronni teeth are much more robust than those of P. bizzocoi sp. nov. On the latter, the root lobes are thin and have consistent width across their length, whereas they widen towards the crown base on P. bronni teeth. Lastly, the root lobes are significantly longer on P. bizzocoi sp. nov. teeth, resulting in a much deeper interlobe area. This is readily apparent in Table 4, which shows the depth of the interlobe area of P. bizzocoi sp. nov. teeth consistently constitutes roughly 25–50% (mean = 31%) of the overall tooth height. This value rarely exceeds 23% on P. bronni teeth (mean = 20%). Although we identified the depth of the interlobe area as a characteristic that increases through ontogeny on P. bizzocoi sp. nov. teeth, many of the P. bronni teeth figured in the literature appear to approach and even exceed the size of the largest P. bizzocoi sp. nov. teeth in our sample. This indicates that the extremely elongated root lobes and deep interlobe area are characteristic of P. bizzocoi sp. nov. teeth.

Interestingly, Van de Geyn (1937, fig. 118) illustrated a distinct upper anterior tooth with an elongated mesial root lobe and a main cusp that has a distal cutting edge that is more convex than the mesial edge. This tooth is morphologically very similar to the upper third anterior teeth within extant lamniform taxa like Carcharias taurus and Mitsukurina owstoni and is comparable to an upper third anterior tooth of P. bizzocoi sp. nov. we identified in our sample (MSC 49452, Fig. 7z–cc). A comparison of these teeth clearly illustrates the morphological differences between these two species, as the P. bronni tooth figured by Van de Geyn (1937, fig. 118) has root lobes that are wider, the interlobe area is shallower and more V-shaped, and the lateral cusplets are wider, more triangular, and more divergent compared to P. bizzocoi sp. nov.

This taxon was named for two teeth (IRSNB P 123 and IRSNB P 124) derived from the Selandian (middle Paleocene) Orp Member of the Heers Formation in Orp-le-Grand (Maret), Belgium (Hovestadt and Steurbaut 2023). Winkler (1874, pl. 1, figs 3–4) illustrated only the lingual view of these two teeth, but Hovestadt and Steurbaut (2023, p. 51) provided high-resolution images of these specimens in labial, lingual, and mesial views. These images show that the type specimens have a distally hooked main cusp, which is indicative of upper lateral teeth. These teeth have two to three pairs of tall and triangular lateral cusplets that are divergent and well separated from one another and from the main cusp. Faint plications occur along the labial crown base on both teeth. Unfortunately, the roots on both type specimens are incomplete, with only the distal lobe preserved on IRSNB P 123 and both lobes are missing from IRSNB P 124. Because the P. rutoti type specimens represent only the upper lateral tooth morphology, we expanded our investigation into dental variation in this species by examining published descriptions and illustrations of specimens derived from, or close to, the type locality.

In his type description, Winkler (1874) noted that the teeth have a distally curved main cusp, ‘creases’ along the labial crown base, and two to three pairs of cusplets that decrease in size laterally and are well-delineated from one another. Leriche (1902) described Selandian P. rutoti (pl. 1, figs 37–40) teeth as having two or three pairs of slender, sharply pointed lateral cusplets, with the first pair being quite large, and the labial crown foot having numerous short, very closely spaced vertical plications that produced an unbroken line of sharp spines. Vincent (1876) provided a similar description of teeth with a straight to distally curved main cusp, at least one pair of lateral cusplets on anterior teeth and additional pairs on lateral teeth, and small enameloid folds along the base of the labial face. Casier (1942) provided the most thorough description of P. rutoti from the Selandian of Belgium based on an associated skeleton consisting of 31 teeth, 58 vertebrae, and three pieces of cartilage that were likely derived from the Meckel’s cartilage or palatoquadrate. Casier (1942) described the teeth as having a slender crown with conspicuous labial folds at the crown base that were “spiniform” apically. He also noted that the teeth had two to three pairs of distinctly separated and lingually angled lateral cusplets (with most having three pairs), and a robust lingual protuberance. Casier’s (1942) associated specimen included teeth from both the upper and lower jaws, with those in the lower files being described as having a straighter main cusp. The similarity of these descriptions indicates that Selandian P. rutoti from Belgium generally includes: 1) anterior and lower teeth with an erect main cusp; 2) upper lateral teeth with a distally hooked main cusp; 3) teeth with two to three pairs of lateral cusplets; 4) enameloid folds (and at times forming spinose ornamentation) along the labial crown base.

When the previously figured Selandian P. rutoti teeth from Belgium (i.e., Winkler 1874, pl. 6, figs 3–4; Vincent 1876, pl. 6, fig. 1; Leriche 1902, pl. 1, figs 37–44; Casier 1942, pl. 1, figs 1–12; Casier 1950, pl. 2, fig. 1; Leriche 1951, pl. 42, figs 8–11; Herman 1972, pl. 2, figs 1–5; Herman 1977, pl. 10, fig. 3; Cappetta 2012, fig. 192e; and Hovestadt and Steurbaut 2023, pg. 51) are compared to those of P. bizzocoi sp. nov., several distinct differences become apparent. First, P. rutoti teeth generally have two to three pairs of lateral cusplets, with most having three pairs, and at least one figured specimen appears to have four pairs (see Vincent 1876, pl. 6, fig. 1a). On P. bizzocoi sp. nov. teeth, the number of pairs of lateral cusplets rarely exceeds two (with three pairs occurring only on small, presumably juvenile, upper lateral teeth), with most teeth having only a single pair. Second, the more medial pair of lateral cusplets on P. rutoti teeth, particularly those on the type specimens (see Winkler 1974, pl. 6, figs 3–4, Hovestadt and Steurbaut 2023, pg. 51), are more robust and taller in relation to the height of the main cusp than they are on P. bizzocoi sp. nov. teeth. The lateral cusplets on P. rutoti teeth are also more widely separated from one another than they are on P. bizzocoi sp. nov., with the secondary pair of cusplets on the latter being more closely united to the base of the larger, more medial pair of cusplets. Lastly, the root lobes are generally longer on P. bizzocoi sp. nov. teeth, resulting in a deeper interlobe area than on P. rutoti specimens. Table 4 shows the ratio of interlobe depth to overall tooth height for P. bizzocoi sp. nov., P. bronni, and P. rutoti for teeth examined first-hand or published in the literature. On P. bizzocoi sp. nov. teeth, the depth of the interlobe area constitutes 25–50% (mean = 31%) the overall tooth height, whereas this value rarely exceeds 23% on P. bronni teeth (mean = 20%). Although the ratio for P. rutoti teeth ranges between 14–34%, which overlaps the low end of that measured on P. bizzocoi sp. nov. teeth, they have a significantly lower mean (mean = 24%).

Interestingly, both Casier (1942, pl. 1, fig. 2) and Cappetta (2012, fig. 192e–f) illustrated P. rutoti upper third anterior teeth that can be directly compared to specimen MSC 49452 (Fig. 7z–cc), which is herein referred to P. bizzocoi sp. nov. Although this tooth position in both taxa has two pairs of lateral cusplets, on P. rutoti the outer pair of cusplets is larger and more separated from the medial pair of cusplets than it is on MSC 49452. In contrast, the outer pair of cusplets on MSC 49452 is minute and mostly united to the larger medial pair of cusplets. In addition, in mesial and distal views, the lingual root protuberance on P. rutoti teeth is substantially more developed, although this may indicate that specimen MSC 49452 was derived from a replacement row (where the root may be incompletely developed) or the root shows some signs of erosion. Nevertheless, the labial ornamentation is extremely faint and visible only under magnification on MSC 49452. However, it is visible but inconspicuous on the tooth illustrated in Casier (1942) and is very pronounced on the specimen illustrated by Cappetta (2012).

Existence of this taxon appears to be limited to the type description, and we are unaware of other published occurrences of this species. The type specimens were derived from the Selandian “lower Saratov beds” exposed along the Volga River near the town of Kamyshin in the Volgograd Oblast of Russia. The type suite consists of a holotype (GIK No. 8057/87) and three paratypes (GIK No. 8057/88–91) that are reposited at the State Darwin Museum in Moscow, Russia. The holotype is an upper left lateral tooth (GIK No. 8057/87; Zhelezko and Kozlov 1999, pl. 1, fig. 5), and the paratypes consist of two upper anterior teeth (pl. 1, fig. 1, 4), an upper lateral tooth (pl. 1, fig. 2), and a lower anterior tooth (pl. 1, fig. 3). Zhelezko and Kozlov (1999) also mentioned the presence of 100 additional teeth belonging to this taxon that were collected from the type locality, but unfortunately none of these specimens were figured and it is unclear whether they also reside within the State Darwin Museum collections.

In their brief description of P. volgensis, Zhelezko and Kozlov (1999) stated that the teeth have one to two pairs of conical lateral cusplets, a pronounced lingual root protuberance, pointed root lobes, a deep lingual nutritive groove, and a tall and triangular main cusp. An examination of their figured type specimens corroborates these remarks and also confirms that the P. volgensis morphology lacks enameloid plications along the labial crown foot. We considered the possibility that this characteristic was overlooked by the authors or simply not described or visible on the type specimens (pl. 1, figs 1–5), but Zhelezko and Kozlov (1999) prominently mentioned the occurrence of plications in their description of P. rutoti, and they are clearly visible on some of the P. rutoti teeth that they illustrated (pl. 36, figs 3b, 8b) alongside their P. volgensis specimens (pl. 36, figs 12–16). This leads us to believe that labial crown ornamentation is indeed absent on P. volgensis teeth, as the authors were clearly aware of this characteristic. Furthermore, the conical lateral cusplets of P. volgensis teeth as described by Zhelezko and Kozlov (1999) are a difficult characteristic to evaluate based on the figures they provided. However, their description, in combination with the lack of labial ornamentation, suggests that the specimens require further evaluation because they might not represent Palaeohypotodus.

If we consider P. volgensis a valid taxon, the figured type specimens are considerably different from teeth of P. bizzocoi sp. nov. Not only do P. volgensis teeth appear to lack labial plications along the crown base, but the main cusp on the upper lateral teeth has a much wider base than any of the upper lateral teeth of P. bizzocoi sp. nov. In addition, P. volgensis lateral teeth have wider and more triangular root lobes, and the root lobes are more pointed on the anterior teeth. In contrast, the root lobes on P. bizzocoi sp. nov. teeth are more even in width across their length, and they almost always have rounded basal extremities. Zhelezko and Kozlov (1999) also noted that the root lobes on P. volgensis teeth transition from rounded to flattened, whereas they are broadly rounded on all teeth of P. bizzocoi sp. nov. Furthermore, Zhelezko and Kozlov (1999) described the lateral cusplets on P. volgensis teeth as conical, which could mean that cutting edges are absent. If true, this feature provides additional evidence that P. volgensis and P. bizzocoi sp. nov. are not conspecific. Lastly, Zhelezko and Kozlov (1999, pl. 36, fig. 16e) showed an interesting characteristic on P. volgensis teeth where, in apical view, the crown exhibits conspicuous mesial and distal constrictions between the main cusp and lateral cusplets. This subtle characteristic is absent on all P. bizzocoi sp. nov. teeth.

Although not exhaustive, our review of Palaeohypotodus has provided us with a number of morphological characteristics that can be used to differentiate the various species, as well as the means to determine the stratigraphic and paleogeographic range of P. bizzocoi sp. nov. The 34 P. bizzocoi sp. nov. teeth in our sample (i.e., 17 associated with GSA–V447 and 17 isolated teeth) were collected from four counties in Alabama (Butler, Dallas, Lowndes, and Wilcox counties) and one in Arkansas (Hot Spring County). To determine whether the range of P. bizzocoi sp. nov. extends beyond what our sample can elucidate, we examined the published literature for other Paleocene occurrences of Palaeohypotodus in Alabama, the Gulf Coastal Plain, and elsewhere in the USA.

Of Alabama occurrences, White (1956), who was later referenced by Thurmond and Jones (1981), reported a tooth identified as Odontaspis cf. rutoti from the Bartonian Gosport Sand in Clarke County, Alabama. White (1956, p. 148) described this tooth as resembling a lower posterolateral tooth of Otodus rutoti (=Palaeohypotodus rutoti), but his assignment to the species was tentative because the tooth appeared atypical when compared to those figured by Winkler (1874). Although White (1956) mentioned the presence of fine “puckering” along the labial crown base, he unfortunately did not figure his specimen and we therefore could not confirm its identity. However, no Palaeohypotodus specimens were identified by Ebersole et al. (2019) during their extensive study of lower-to-middle Eocene fishes of Alabama, which included the examination of more than 6,000 teeth derived from the Gosport Sand. This leads us to believe that the tooth reported by White (1956), and Thurmond and Jones (1981) was misidentified, and the only confirmed occurrence of Palaeohypotodus in Alabama is that of P. bizzocoi sp. nov. reported herein.

Within the Gulf Coastal Plain of the USA, Maisch et al. (2020, figs 8f–m, 12m) figured nine teeth as “Palaeohypotodus rutori [sic]” that were derived from the lower Clayton Formation in Hot Spring County, Arkansas. Of these teeth, the morphology of the teeth illustrated in figs 8h–i and 12m appear better aligned with Odontaspis rather than Palaeohypotodus, and those figured in fig. 8f, j–n are not well enough preserved to be properly evaluated. Furthermore, the posterior teeth figured in fig. 8j–n have wider and fewer cusplets than those illustrated for P. rutoti (see Herman 1972, pl. 2, figs 1–3, 5; Herman 1977, pl. 10, fig. 3e; Cappetta 2012, fig. 192h–j), and the main cusp on the anterior tooth figured in 8f is too narrow and gracile compared to the wide and robust main cusp of typical Palaeohypotodus teeth. It is likely these teeth belong to a genus, or genera, other than Palaeohypotodus. However, one tooth having a robust main cusp and two pairs of lateral cusplets was figured by these authors (fig. 8g) and morphologically it falls within the range of P. bizzocoi sp. nov. The small size of this tooth (1.5 cm), coupled with the extended mesial root lobe and double pair of cusplets, suggests it is an upper third anterior tooth of a juvenile P. bizzocoi sp. nov. individual. This tooth, along with specimen MMNS VP–8578, demonstrates the occurrence of this taxon in the Danian of Arkansas. As far as we are aware, the Alabama and Arkansas occurrences of Palaeohypotodus represent the only verified accounts of this genus within the entirety of the Gulf Coastal Plain of the USA.

Within Paleocene deposits from elsewhere in the USA, Ward and Wiest (1990) included P. rutoti in their list of elasmobranch taxa occurring in Maryland and Virgina. The authors did not figure these specimens and we could not confirm their identity. Cvancara and Hoganson (1993) reported 13 teeth derived from the Danian Cannonball Formation in North Dakota that they referred to P. rutoti. They noted the teeth approach nearly 3 cm in height, have two (sometimes three) pairs of large lateral cusplets, and labial plications occur at the crown foot. The cusplets were described as being “conical” but complete cutting edges were clearly visible. In their discussion of the material, Cvancara and Hoganson (1993) expressed their opinion that teeth of P. bronni and P. rutoti were indistinguishable and they therefore referred the Cannonball Formation teeth to the latter taxon. However, the tooth they illustrated (fig. 3mm–nn) which appears to be an upper lateral tooth, has large, triangular lateral cusplets and shallow interlobe area (19% the height of the tooth) that morphologically falls outside of P. bizzocoi sp. nov., P. rutoti, and P. volgensis. The Cannonball Formation material should be reevaluated using the criteria highlighted herein because the teeth would represent the first verified occurrence of Palaeohypotodus in North America and potentially a new species.

Case (1996) referred 24 teeth derived from the Danian part of the Hornerstown Formation in Monmouth County, New Jersey to P. rutoti. He illustrated six of these teeth (Case 1996, pl. 2, figs 1–6), all of which have extremely straight, narrow, conical, and needle-like lateral cusplets. The cusplets on some of these teeth (i.e., figs 2, 5, 6) are rather tall with respect to the height of the main cusp when compared to the teeth of Palaeohypotodus spp., and these particular specimens are similar to those of extant Odontaspis and likely belong to a Paleocene representative of this latter genus. Lastly, Purdy (1998) reported Odontaspis rutoti from a temporally mixed Paleocene locality in Berkley County, South Carolina. Unfortunately, the precise stratigraphic provenience of his material cannot be ascertained, as the entirety of the Williamsburg Formation (Danian to Thanetian) was exposed. However, as his figured specimens (fig. 3) possess lateral cusplets that are taller and more robust than those of P. bizzocoi sp. nov. and the interlobe area is shallower, referral of these teeth to P. rutoti appears to be appropriate.

The temporal and stratigraphic occurrences noted above establish that P. bizzocoi sp. nov. had a paleogeographic range that extended across the northern Gulf Coastal Plain of the USA, at least between Alabama and Arkansas. Future work may yield additional records of the taxon in other northern Gulf states, like Mississippi, Louisiana, and eastern Texas. The occurrence of P. bizzocoi sp. nov. is at present confined to three lithostratigraphic units that all date to the Danian Stage (zones NP2–4) of the Paleocene, including the lower Clayton Formation, Pine Barren Member of the Clayton Formation, and the Porters Creek Formation. The occurrence of this taxon within the lowermost Danian units in Alabama and Arkansas (the Pine Barren Member of the Clayton Formation and the equivalent lower beds of the Clayton Formation, respectively) establishes that this taxon was present in the Gulf Coastal Plain of the USA shortly after the K/Pg extinction event. Furthermore, the absence of the species from any Maastrichtian deposits in the region (see Ikejiri et al. 2013) indicates a first occurrence within the lower-most Paleocene. Additionally, the occurrence of P. bizzocoi sp. nov. within the upper Danian Porters Creek Formation demonstrates that this species persisted within this region throughout the entirety of the stage. The earliest stratigraphic occurrence of this taxon is well-defined, but its vertical stratigraphic extent is presently unknown due to the lack of systematic vertebrate paleontology work in local Selandian and Thanetian units like the Naheola Formation, Nanafalia Formation, and Tuscahoma Sand.

When P. bronni and P. rutoti were originally named by Agassiz (1843) and Winkler (1874), respectively, both were assigned to the genus Otodus. Vincent (1876) later contended that the rutoti morphology was similar to teeth of extant Odontaspis and utilized this generic name for the species. Although Daimeries (1888) followed Vincent (1876) in the use of Odontaspis rutoti, he also noted differences between these teeth and other species assigned to the genus at the time, namely the presence of labial vertical ridges and the greater number of lateral cusplets on the former. A review of the historical literature indicates that both the bronni and rutoti morphologies were consistently placed within Odontaspis, and by extension, within the family Odontaspididae, until Glückman (1964) erected the name Palaeohypotodus to include these species. Aside from the occasional usage of Odontaspis for these teeth (i.e., Purdy 1998), the rutoti and bronni morphologies were predominantly assigned to Palaeohypotodus, prompting Zhelezko and Kozlov (1999) to assign their volgensis morphology to this genus.

In addition to placing the rutoti and bronni morphologies within Palaeohypotodus, Glückman (1964) erected the family Jaekelotodontidae to accommodate this genus as well as Hypotodus, Jaekelotodus, and Anotodus. Glückman (1964) argued that a new family was warranted for these genera because they all had one to three pairs of lateral cusplets, mesiodistally expanded cusps on the upper lateral teeth, elongated root lobes, and they lacked the elongated anterior tooth morphology typical of other members of the Odontaspididae. However, due to dental similarities and the lack of lateral cusplets on Anotodus, this taxon was subsequently reassigned to the Alopiidae (Herman 1979; Cappetta 2012). Zhelezko (1994) later expanded the Jaekelodontidae to include Mennerotodus, but reconstructions of the dentition of the genus (Cicimurri et al. 2020) show that this taxon is more appropriately referred to the Carchariidae.

Although Glückman (1964) erected the family Jaekelotodontidae to include Palaeohypotodus, many subsequent authors continued to place the genus within Odontaspididae (i.e., Cappetta 1987; Cappetta and Nolf 2005; Iserbyt and De Schutter 2012). However, assignment of the genus to Odontaspididae has recently become problematical because of the reintroduction of the family Carchariidae by Shimada et al. (2015) and Stone and Shimada (2019). The latter authors used skeletal data to conduct a phylogenetic analysis of extant “sandtiger” sharks that ultimately corroborated paraphyly within Odontaspididae, which classically included Odontaspis and Carcharias taurus. The use of Carchariidae is supported to include extant C. taurus, whereas Odontaspididae contains only the genus Odontaspis. Unfortunately, the application of these family names to fossil species was not addressed by Stone and Shimada (2019), and determining the familial placement of an extinct species is tentative without the aid of associated skeletal material, which is largely lacking for extinct taxa. For example, the Cretaceous tooth morphology amonensis was variously assigned to Odontaspis (Cappetta & Case, 1975) and Carcharias (Cicimurri, 2001), but discovery of a partial skeleton allowed researchers to assign the species to a new genus and determine that it belonged to a new family, Haimrichiidae Vullo et al., 2016.

Although the familial placement of Palaeohypotodus has heretofore remained unresolved, it may be elucidated when the suite of teeth we herein assign to P. bizzocoi sp. nov., along with other Palaeohypotodus teeth reported elsewhere in the literature, are taken into account. For example, Van de Geyn (1937, fig. 118) and Casier (1942, pl. 1, fig. 2) figured upper third anterior teeth for P. bronni and P. rutoti, respectively, that are very similar to a tooth we assign herein to P. bizzocoi sp. nov. (MSC 49452, Fig. 7z–cc). All of these teeth have a distinct combination of an elongated mesial root lobe and a distal cutting edge that is more convex than the mesial edge, which compares more favorably to some teeth in dentitions of extant C. taurus rather than Odontaspis ferox, suggesting Palaeohypotodus is more closely allied with Carchariidae than to Odontaspididae. However, the occurrence of up to three pairs of lateral cusplets on Palaeohypotodus teeth is more consistent with teeth of extant Odontaspis ferox teeth (which can have one to three pairs), as opposed to C. taurus teeth that generally have only a single pair.

Our comparison of P. bizzocoi sp. nov., P. bronni, and P. rutoti teeth to those of extant lamniform sharks revealed similarities between the fossil taxa and both C. taurus and O. ferox. However, these extant taxa lack both the mesiodistally wide and laterally hooked upper lateral tooth morphology and enameloid plications along the labial crown base, features that have been regarded as characteristic of Palaeohypotodus (Herman 1977: 299). Furthermore, the distinct upper lateral tooth crown morphology of Palaeohypotodus is comparable to the condition of Hypotodus and Jaekelotodus (as is the dentition as a whole), suggesting that these taxa are likely closely related (as was suggested by Glückman 1964). The teeth of Palaeohypotodus, Hypotodus, and Jaekelotodus appear to exhibit a combination of features occurring in both Carchariidae and Odontaspididae, and there is no unequivocal modern familial analogue to assign these genera. The dentition of Palaeohypotodus spp. also appears to have a condition not present in extant lamniforms, where the upper teeth have complete cutting edges, whereas those in the lower files are incomplete. This characteristic, along with the evidence stated for the other genera indicates that teeth of Palaeohypotodus, Hypotodus, and Jaekelotodus represent an extinct type of lamniform dentition, and we find it appropriate to assign these genera to their own family and herein follow Glückman (1964) by utilizing Jaekelodontidae.