Rhyncolites cf. simplex Fritsch, 1872

Tajika, Amane, Landman, Neil H., Koyasu, Hiromichi, Broussard, Amy, Anderson, Lian, Ikuno, Kenji & Ikuno, Kenji, 2023, New discovery of rhyncholites and conchorhynchs (cephalopod jaw elements) from the Upper Cretaceous Mount Laurel Formation of Delaware, American Museum Novitates 2023 (3998), pp. 1-20 : 5-9

Rhyncolites cf. simplex Fritsch, 1872

Figures 3–5

Rhyncholithus simplex Fritsch inFritsch and Schlönbach, 1872: 25, pl. 111, figs. 4, 5.

Rhyncolites simplex (Fritsch and Scholenbach, 1872) , Riegraf and Schmitt-Riegraf, 1995; 82.

Nautilorhynchus simplex ( Fritsch, 1872) , Košťák et al. 2010; 421, pl. 1, figs. 1–5, 8–13, text-fig. 4 (with additional synonymy).

TYPE: Rhyncholithus simplex Fritsch in Fritsch and Schlönbach (1872: 25, pl. 111, figs. 4, 5) from the Turonian of the Czech Republic .

MATERIAL: Twenty-four specimens ( AMNH 137104–137122, AMNH 137124–137128) from the Mount Laurel Formation, Delaware.

DESCRIPTION: All specimens are arrow-shaped. The rhomb-shaped hood possesses a vertical ventral ridge in the middle. The bottom part of the hood is often eroded. The shaft is much narrower than the hood. The bottom part of the shaft is often eroded. The dorsal part is generally flat with a vertical ridge in the middle (dorsal ridge) that is either eroded or covered with sediments and thus not apparent in some specimens. Length (L) ranges from 6.7 to 9.8 mm, width (W) 4.4 to 7.4 mm, height (H) 3.2 to 5.2 mm, length of shaft (Ls) 3.0 to 5.5, width of shaft (Ws) 1.9 to 3.5 mm, length of hood 4.0 to 7.5 mm. The angle formed by the median keel and the ventral plane (α) ranges from 42° to 60°. The angle formed by the left and right anterior hood margins (β) ranges from 59° to 90°. The angle formed between the left and right shaft edges, which delineates the median shaft area (δ), ranges from 88° to 153°. Most specimens probably underwent a certain degree of erosion/abrasion/ corrosion. We did not measure some morphological characters in certain specimens when they were too poorly preserved. The measurements of all specimens are available in the online supplement (https://doi.org/10.5531/sd.sp.57).

DISCUSSION: Our specimens are smaller than the holotype of Ryncholithus simplex described by Fritsch (in Fritsch and Schlönbach, 1872; 6.7–9.8 mm vs. 15 mm in length). The ratio W/L is similar (0.61–0.91 vs. 0.67). However, the ratio H/L seems slightly higher in our specimens (0.39–0.63 vs. 0.40). Our specimens are also similar to those from the same region as the holotype documented by Košťák et al. (2010; figs. 9, 10). Some of the specimens documented by Košťák et al. (2010) are similar to our specimens in size. However, those specimens tend to have lower ratios of H/L and W/L. The other ratios (Ws, Ls, and Lh vs. L) of Košťák et al. (2010) and the holotype are more similar to those of our specimens (fig. 9). It is worth noting that the ontogeny of rhyncholites is poorly known. However, in the plot of species from different geological time periods shown in figure 9, there seems to be a positive linear correlation between size and some morphological parameters. The angle formed by the median keel and the ventral plane (α) is much higher in our specimens than that in the holotype of R. simplex (42°–60° vs. 38°). The angle formed by the left and right anterior hood margins (β) of the holotype is within the range of our specimens (59°–90° vs. 78°). The principal component analysis using six parameters (H, L, Lh, Ls, W, Ws, and β) reveals that our specimens are comparable to R. debeyi illustrated by Riegraf and Schmitt-Riegraf (1995), R.simplex documented by Košťák et al. (2010), R. lhommei , and R. sagittarius , documented by Pacaud (2010), and R. aethioparion documented by Ward and Cooper (1972). The locality and age of the reported specimens of these species include the Cretaceous and Eocene of Europe and possibly North America.

Assuming that our specimens from the Mount Laurel Formation belong to a single species of Eutrephoceras (see Eutrephoceras sp. below), the intraspecific variation within them is high. Yet, it is likely that the intraspecific variation of our specimens may have slightly increased due to somewhat poor preservation. The similarity of the abovementioned rhyncholite species may be explained by high morphological variation within a single parataxon. Our morphometric analysis using the linear measurements, however, is not sensitive enough to detect subtle morphological differences and, thus, methods such as geometric morphometric analysis should be applied in future studies to better understand the evolution of rhyncholites.

OCCURRENCE: R. simplex has been widely reported from the Cenomanian–Turonian of the Czech Republic, Germany, and Poland. Košťák et al. (2010) synonymized R. curvatus reported by Till (1907) into R. simplex , which extends the record to the Albian of the United Kingdom. If our specimens belong to R. simplex , this is the first record from the Upper Cretaceous of North America.

Genus Conchorhynchus de Blainville, 1827