Mennerotodus mackayi, Cicimurri & Ebersole & Martin, 2020

Cicimurri, David J., Ebersole, Jun A. & Martin, George, 2020, Two new species of Mennerotodus Zhelezko, 1994 (Chondrichthyes: Lamniformes: Odontaspididae), from the Paleogene of the southeastern United States, Fossil Record 23 (2), pp. 117-140 : 121-126

Mennerotodus mackayi sp. nov.

LSID urn:lsid:zoobank.org:act:2F9AABDC-5E09-4962-9D81-2EC13302109B

2011 Carcharias cf. whitei, Becker et al. , p. 19, figs. 4.5, 4.6. 2019 Carcharias sp. , Maisch et al., fig. 7g.

Etymology

The species is named for John L. Mackay, initial President and CEO at McWane Science Center, Birmingham, Alabama, USA, in honor of his distinguished career in informal education.

Hypodigm

MSC 42411 (paratype), first upper right anterior tooth ( Fig. 3a–e ); MSC 42408 (paratype), second upper right anterior tooth ( Fig. 3f–j ); MSC 42413 (paratype), third upper left anterior tooth ( Fig. 3k–o ); MSC 42412 (holotype), first upper right lateral tooth ( Fig. 3p–t ); MSC 42416 (paratype), upper posterior tooth ( Fig. 3u–y ); MSC 42407 (paratype), first lower left anterior tooth ( Fig. 4a–e ); MSC 42405 (paratype), second lower right anterior tooth ( Fig. 4f–j ); MSC 42410 (paratype), third lower right anterior tooth ( Fig. 4k–o ); MSC 42406 (paratype), fourth lower left anterior tooth ( Fig. 4p–t ); MSC 42409 (paratype), lower right lateral tooth ( Fig. 4u–y ); MSC 42632 (paratype), lower right posterior tooth ( Fig. 4z – ad).

Referred specimens

MSC 42414, second upper anterior teeth (16 specimens); MSC 42415, second upper anterior teeth (nine specimens); MSC 42417, lower posterior teeth (two specimens); MSC 42418, third upper anterior tooth; MSC 42419, third upper anterior teeth (seven specimens); MSC 42420, third upper anterior teeth (eight specimens); MSC 42421, upper right lateral tooth; MSC 42422, upper right lateral tooth; MSC 42423, upper right lateral teeth (nine specimens); MSC 42424, upper right lateral teeth (35 specimens); MSC 42425, lower left lateral teeth (11 specimens); MSC 42426, lower right lateral teeth (nine specimens); MSC 42427, first lower anterior tooth; MSC 42428, first lower anterior tooth; MSC 42429, first lower anterior teeth (seven specimens); MSC 42430, third lower anterior teeth (11 specimens); MSC 42431, third lower anterior teeth (nine specimens); MSC 42432, lower lateral teeth (17 specimens); MSC 42433, lower lateral teeth (43 specimens); MSC 42434, first upper anterior teeth (six specimens); MSC 42435, first upper anterior teeth (20 specimens); MSC 42436, second lower anterior tooth; MSC 42437, second lower anterior tooth; MSC 42438, second lower anterior tooth; MSC 42439, second lower anterior teeth (four specimens); MSC 42440, second lower anterior teeth (15 specimens); MSC 42441, fourth lower anterior tooth; MSC 42442, fourth lower anterior teeth (three specimens); MSC 42443, anterior teeth (seven specimens); MSC 42444, upper left lateral teeth (43 specimens); MSC 42445, unassigned tooth positions (21 specimens); MSC 42446, unassigned tooth positions (72 specimens); MSC 42493, second upper right lateral tooth; MSC 42494, sixth? upper right lateral tooth; MSC 42495, first upper right lateral tooth; MSC 42496, fifth? upper right lateral tooth; MSC 42497, seventh? upper right lateral tooth; MSC 42498, first lower left lateral tooth; MSC 42499, third lower left lateral tooth; MSC 42500, fourth? lower left lateral tooth; MSC 42501, fifth? lower left lateral tooth; MSC 42502, sixth? lower left lateral tooth; MSC 42633, second lower anterior tooth; MSC 42634, second lower anterior tooth; MSC 42635, fourth lower anterior tooth; MSC 42636, fourth lower anterior tooth; MSC 42718, second upper right lateral tooth; MSC 42719, first lower left lateral tooth.

Type stratum

Lower Danian (Paleocene) Pine Barren Member of the Clayton Formation, approximately 10 m above lower contact with the late Maastrichtian (Upper Cretaceous) Prairie Bluff Chalk Formation, zones NP1 to NP3 ( Fig. 1b ).

Type locality

Site ALn-13, Lowndes County, Alabama, USA ( Fig. 1a ). Due to the sensitive nature of the site, the precise location is not provided here, but detailed locality information is on file at MSC and available to qualified researchers .

Description

First upper anterior teeth. These teeth, not exceeding 25 mm in total height, are slightly asymmetrical in labial view. The main cusp is very narrow, is slightly distally curving ( Fig. 3b–d ), and has a weakly sigmoidal profile ( Fig. 3d ). Mesial and distal cutting edges are sharp, smooth, and subparallel but never reach the base of the main cusp ( Fig. 3d ). There may be a minuscule tubercle or very short ridge near the base of the main cusp, well separated from the main cutting edge, on one or both sides of the crown. A single pair of diminutive triangular cusplets is located at the crown foot. When present, the short ridge is connected to the lateral cusplet ( Fig. 3e ). The labial face of the main cusp is smooth and flat except for where basal convexity occurs at each side of the labial crown foot. The lingual face is very convex and generally smooth, although faint longitudinal ridges on the lower half were occasionally observed. The root is bilobate and has a large lingual boss that is bisected by an elongate and deep nutritive groove ( Fig. 3a ). A conspicuous dental band (often deeply impressed) occurs at the lingual crown foot. Root lobes are rather short and may be cylindrical or mesiodistally compressed, and the mesial lobe is slightly longer than the distal lobe ( Fig. 3b, c ).

Second upper anterior teeth. In our sample, the largest complete tooth from this file measures 20 mm in total height, which is smaller than the largest first upper anterior tooth available to us. Larger second upper anterior teeth are present in our sample, but they are incomplete and represented fragmentary specimens. The main cusp is tall and narrow, sharply tapering ( Fig. 3g, h ), weakly sigmoidal in profile ( Fig. 3i ), and more distally inclined than the main cusp of the first anterior tooth ( Fig. 3h ). Cutting edges are subparallel, and although they do not reach the cusp base, the edges extend further basally than on teeth from the first anterior file. The labial face is smooth and flat to very weakly convex, but the lingual face is very convex and may bear faint vertical ridges on the lower half. The main cusp is flanked by a single pair of small cusplets that are triangular, are sharply pointed, and have distinct cutting edges. A short cutting edge or small denticle, well separated from the main cutting edge, may occur on the medial side of one or both lateral cusplets ( Fig. 3j ). The lingual dental band is conspicuous and may be impressed ( Fig. 3h ), and the robust lingual root boss bears a thin nutritive groove ( Fig. 3f, h ). The root is bilobate, and although the lobes are of nearly the same length, the mesial lobe is thin and pointed basally, whereas the distal lobe is wide and rounded basally ( Fig. 3g, h ).

Third upper anterior teeth. The largest specimens do not measure more than 20 mm in total height. The main cusp is broad-based but sharply tapering, is strongly distally inclined but with weak mesial curvature ( Fig. 3l, m ), and has only a very weak sigmoidal profile ( Fig. 3n ). The labial face is flat and smooth, but the lingual face is moderately convex and usually smooth. The smooth cutting edge is continuous across the entire main cusp. A single pair of broadly triangular cusplets flanks the main cusp, and these cusplets bear sharp cutting edges ( Fig. 3o ). A small denticle may be located medially to the lateral cusplet, on one or both sides of the main cusp. The lingual dental band is conspicuous and may be impressed ( Fig. 3m ). The root is bilobate with highly divergent lobes, with the mesial lobe being much more elongated than the distal one ( Fig. 3l ). The lingual boss is thin and shelflike, bisected by a thin nutritive groove ( Fig. 3k, n ).

Intermediate teeth. No intermediate teeth have been identified in the available sample.

Upper lateral teeth. Upper lateral teeth can be differentiated from the anterior teeth in that the main cusp is labiolingually thinner, flat to only weakly sigmoidal in profile ( Fig. 3s ), and distally inclined, and the base is broader. The smooth cutting edge is continuous across the entire main cusp, reaching the crown foot. The mesial and distal cutting edges may be straight, but more often the main cusp appears distally curving because the mesial edge is slightly convex and the distal edge straight to concave. One or two tiny denticles may occur on one or both sides of the crown foot, adjacent to lateral cusplets ( Fig. 3t ). A single pair of broadly triangular, tall, sharply pointed cusplets flanks the main cusp ( Fig. 3q ). The lingual dental band is impressed. The bilobate root bears a small lingual boss that is bisected by a deep nutritive groove ( Fig. 3p, r ). Root lobes are rather short, wide, and divergent ( Fig. 3r ). Within the first few lateral files, the mesial root lobe is more elongated and thinner than the distal lobe, but other lateral teeth have more equidimensional root lobes. Total tooth height exceeds root width. Within our sample of upper lateral teeth, it appears that the crown decreases in size but becomes more strongly distally inclined towards the commissure.

Upper posterior teeth. No upper posterior teeth have been identified in the sample.

First lower anterior teeth. Teeth from this file are not known to exceed 15 mm in total height. The main cusp is very narrow and sharply inclined mesially, may be straight to distally curving, and has a strong lingual curve in profile but is not sigmoidal ( Fig. 4a–c ). The labial face is smooth and nearly flat, whereas the lingual face is very convex and smooth. Cutting edges are smooth, sharp, and subparallel and extend to the crown foot ( Fig. 4e ). A single pair of lateral cusplets flanks the main cusp, and in labial view the mesial cusplet appears to be located higher on the tooth than the distal cusplet ( Fig. 4c ). The cusplets are small, triangular, sharply pointed, and divergent. The root is laterally compressed, weakly bilobate (mesial lobe is shorter and narrower than the distal lobe), and the large lingual boss ( Fig. 4a, d ) is bisected by an elongate nutritive groove ( Fig. 4b ). Crown height slightly exceeds root height.

Second lower anterior teeth. Teeth in this position are the largest in the available sample, reaching 24.4 mm in total height. The main cusp is tall, narrow, and erect and has a sigmoidal profile ( Fig. 4f–h ). The labial face is smooth and flat, whereas the lingual face is very convex and may bear fine longitudinal ridges on the lower half. The cutting edges are smooth and appear biconvex due to medial curvature ( Fig. 4g ), and the edges end well before the cusp base. A small node or short ridge may occur near the crown base, well separated from the main cutting edge, on one or both sides of the main cusp. A single pair of very small, triangular lateral cusplets flanks the main cusp ( Fig. 4h ). When present, the short basal ridge is closely connected with the lateral cusplet ( Fig. 4j ). The root is bilobate with a large lingual boss that is bisected by a deep nutritive groove, and the thin dental band is impressed ( Fig. 4f, g, i ). Root lobes are elongated but of equal length, although the distal lobe may be slightly wider. Root height is roughly one-third (30 %) of the total tooth height.

Third lower anterior teeth. These teeth ( Fig. 4k–n ) are very similar to those of the second lower anterior file. They can be distinguished by their less convex cutting edges and root with a more elongated and narrower mesial lobe compared to the distal lobe ( Fig. 4l, m ).

Fourth lower anterior teeth. The main cusp is erect and slightly curved distally and has a sigmoidal profile ( Fig. 4p– r ). The cutting edges are smooth and convex apically but otherwise subparallel ( Fig. 4r ), nearly reaching the crown foot. The labial face is weakly convex and smooth, whereas the lingual face is very convex and only occasionally bears faint longitudinal ridges. A very convex ridge or small denticle, well separated from the main cutting edge, may occur on one or both sides of the cusp base ( Fig. 4t ). A single pair of broadly triangular cusplets flanks the main cusp. The lingual dental band is impressed. The lingual boss is indistinct, bisected by a thin nutritive groove ( Fig. 4s, q ). The bilobate root has a very elongated and narrow mesial lobe and very short, rounded distal lobe ( Fig. 4q, r ). Root height represents 30 % (one-third) of the total tooth height. Teeth from this file are somewhat similar to the third upper anterior tooth, but they differ in that the main cusp is rather erect and has an obvious distal curvature ( Fig. 4r ). In contrast, the third upper anterior tooth has a highly distally inclined cusp that exhibits distinctive mesial curvature (i.e., Fig. 3l, m ).

Lower lateral teeth. In general, the main cusp is broad-based but sharply tapering, vertical to very slightly distally inclined, and flat to weakly sigmoidal in profile ( Fig. 4u–w ). The labial face is smooth and flat to very weakly convex, but the lingual face is convex and may bear faint vertical ridges on the lower half. The cutting edges are smooth and sharp and extend to the very base of the main cusp ( Fig. 4w ). The mesial and distal cutting edges are usually straight, but some teeth exhibit a mesial edge that is somewhat concave. The base of the cutting edge may be smooth and continuous or punctuated by one (rarely two) rounded-to-pointed denticle (or denticles; Fig. 4y ). The main cusp is generally flanked by a single pair of tall, triangular, sharply pointed lateral cusplets ( Fig. 4v, w ), but an inconspicuous second cusplet may occur on one or both sides of the crown. The dental band is thin and impressed. The bilobate root bears a conspicuous boss that is bisected by a deep nutritive groove ( Fig. 4v, x ). The lobes are divergent, roughly of equal length, and separated by a V-shaped interlobe area, and their ends may be rounded or pointed ( Fig. 4v, w ). Root width is approximately two-thirds of the total tooth height.

Teeth within the first few lateral files have a slightly wider mesial lobe compared to the distal lobe ( Fig. 4v, w ). Other lateral teeth have more equidimensional root lobes and are difficult to place into a specific file. Within our sample of lower lateral teeth, it appears that the crown decreases in size and becomes slightly distally inclined towards the commissure. Lower lateral teeth are distinguished from upper lateral teeth in having erect main cusps as opposed to conspicuously distally inclined ones, and root lobes are less robust.

Lower posterior teeth. The lower posterior teeth are similar to the lower lateral teeth, but they are significantly smaller in size and have a much shorter main cusp ( Fig. 4z –ad). The total root width is greater than the overall tooth height, the latter of which does not exceed 7 mm ( Fig. 4 aa, ab). No denticulations are present on any of the lower posterior teeth observed.

Remarks

Mennerotodus mackayi sp. nov. differs from the type species, Mennerotodus glueckmani (including the subspecies M. g. glueckmani and M. g. usunbassi), by anterior teeth having a maximum height of only 2.4 cm and there being only a single pair of lateral cusplets on upper and lower lateral teeth. In contrast, M. glueckmani anterior teeth can reach up to 7 cm in height, and upper and lower lateral teeth have two pairs of lateral cusplets. Of the two M. glueckmani boktensis teeth illustrated by Zhelezko and Kozlov (1999), the lower lateral tooth (pl. 6.3) has two pairs of lateral cusplets, and the mesial and distal cusplets of the upper left lateral tooth (pl. 6.4) are broad and appear to be serrated. These features contrast with the single pair of cusplets on M. mackayi sp. nov. lower and upper lateral teeth. The upper right lateral Mennerotodus sp. tooth illustrated by Dutheil et al. (2006, pl. 2.3) differs from all available M. mackayi sp. nov. upper lateral teeth in that the mesial cusplet appears serrated (rather than denticulation occurring at the base of the main cusp, medial to the lateral cusplet). The crown of M. mackayi sp. nov. appears to be more gracile and root lobes more elongated than on M. glueckmani , The crown and root of M. mackayi sp. nov. are more gracile than those of M. glueckmani usunbassi , and the crown is also straighter.

Three other lamniform taxa occur within the Clayton Formation that could be confused with Mennerotodus mackayi sp. nov., namely Palaeohypotodus rutoti ( Winkler, 1874) , Odontaspis substriata ( Stromer, 1910) , and Striatolamia cederstroemi Siverson, 1995 . One specimen of P. rutoti (MMNS 8578) observed from Arkansas bears lateral denticulation similar to that on Mennerotodus mackayi sp. nov. teeth. However, Palaeohypotodus is easily distinguished from Mennerotodus by having teeth that are generally broader; lateral teeth with a distinctively distally curved main cusp; cutting edges that are continuous to the crown foot on all jaw positions; and larger lateral cusplets, of which there can be up to three pairs. Additionally, the secondary/tertiary cusplets on Palaeohypotodus teeth are conspicuously located on the labial face of the crown, not adjacent to the primary/secondary pair, and there are short longitudinal ridges at the labial crown foot (particularly obvious on smaller specimens). Lastly, M. mackayi sp. nov. teeth apparently do not reach the large size of Palaeohypotodus teeth (up to 3 cm in total tooth height).

The teeth of Odontaspis substriata differ from M. mackayi sp. nov. in having anterior teeth with two pairs of very tall and needlelike lateral cusplets and lateral teeth with three pairs of cusplets. In addition, the lingual faces of the main cusp and lateral cusplets of the substriata morphology bear very strong lingual ornamentation. The ornamentation on the main cusp varies depending on tooth size, with smaller teeth bearing ridges nearly to the lingual apex, whereas on larger teeth the ornamentation is only located on the lower half of the crown. On those teeth where ridges are not obvious on the main cusp, ornamentation is visible on the lingual face of the lateral cusplets. With very few exceptions, crown enameloid of Mennerotodus mackayi sp. nov. is smooth, and only rarely were extremely weak longitudinal ridges observed. In addition, the lateral cusplets of M. mackayi sp. nov. are diminutive compared to those of O. substriata , and development of a second lateral cusplet (albeit diminutive) was very rarely observed. Lastly, the Odontaspis teeth never have the denticulation as observed on Mennerotodus .

Although tooth crowns of Striatolamia cederstroemi are smooth, this taxon is easily distinguished from M. mackayi sp. nov. in that the mesial and distal sides of the crown are drawn out into elongated shoulders or, more often, bear a very low, broad, weakly convex cusplet that is poorly differentiated from the main cusp. Our sample size of S. cederstroemi was relatively small (n = 35), but none of the specimens we examined bear denticulation as observed on M. mackayi sp. nov.

Figures

Figure 1. Generalized location and surface stratigraphy of site ALn-13, Lowndes County, AL, USA. (a) Geographic maps showing the location of site ALn-13 in country, state, and county contexts. (b) Danian surface stratigraphy in Alabama, USA. Shaded areas on the stratigraphic chart represent unconformities.

Figure 3. Mennerotodus mackayi sp. nov. upper-dentition hypodigm, Paleocene (Danian) Pine Barren Member of the Clayton Formation, site ALn-13, Lowndes County, Alabama, USA. (a–e) MSC 42411, first upper right anterior tooth (paratype). (a) Basal view. (b) Labial view. (c) Lingual view. (d) Mesial view. (e) Close-up of distal cusplet. (f–j) MSC 42408, second upper right anterior tooth (paratype). (f) Basal view. (g) Labial view. (h) Lingual view. (i) Mesial view. (j) Close-up of distal cusplet and denticle. (k–o) MSC 42413, third upper left anterior tooth (paratype). (k) Basal view. (l) Labial view. (m) Lingual view. (n) Mesial view. (o) Close-up of distal cusplet. (p–t) MSC 42412, upper lateral tooth (holotype). (p) Basal view. (q) Labial view. (r) Lingual view. (s) Mesial view. (t) Close-up of distal cusplet and denticle. Labial at top in (a), (f), (k), and (p). Scale bars = 5 mm.

Figure 4. Mennerotodus mackayi sp. nov. lower-dentition hypodigm, Paleocene (Danian) Pine Barren Member of the Clayton Formation, site ALn-13, Lowndes County, Alabama, USA. (a–d) MSC 42407, first lower left anterior tooth (paratype). (a) Distal view. (b) Lingual view. (c) Labial view. (d) Basal view. (e) Close-up of distal cusplet. (f–i) MSC 42405, second lower right anterior tooth (paratype). (f) Distal view. (g) Lingual view. (h) Labial view. (i) Basal view. (j) Close-up of mesial cusplet. (k–n) MSC 42410, third lower right anterior tooth (paratype). (k) Mesial view. (l) Lingual view. (m) Labial view. (n) Basal view. (o) Close-up of mesial denticle and cusplet. (p–s) MSC 42406, fourth lower left anterior tooth (paratype; reversed). (p) Mesial view. (q) Lingual view. (r) Labial view. (s) Basal view. (t) Close-up of mesial denticle and cusplet. (u–x) MSC 42409, lower right lateral tooth (paratype). (u) Mesial view. (v) Lingual view. (w) Labial view. (x) Basal view. (y) Close-up of mesial denticle. (z–ad) MSC 42632, lower right posterior tooth. (z) Close-up of distal cusplet. (aa) Lingual view. (ab) Labial view. (ac) Mesial view. (ad) Basal view. Labial at bottom in (d), (i), (n), (s), (x), and (ad). Scale bars = 5 mm.