Inoceramus kleini Müller, 1888

Landman, Neil H., Plint, A. Guy & Walaszczyk, Ireneusz, 2017, Allostratigraphy And Biostratigraphy Of The Upper Cretaceous (Coniacian-Santonian) Western Canada Foreland Basin, Bulletin of the American Museum of Natural History 2017 (414), pp. 1-173 : 65-69

Inoceramus kleini Müller, 1888

Figure 6

1888. Inoceramus kleini , sp. nov. G. Müller: 415, pl. 18, fig. 1a-b.

? pars 1911. Inoceramus kleini Müller. Andert : 48–50, pl. 2, fig. 6 [non pl. 1, fig. 7; pl. 2, figs. 3, 7–8].

non 1934. Inoceramus kleini Müller. Andert : 115– 117, text-figs. 10–12. pl. 4, figs. 9–10; pl. 5, figs. 1–2.

1929. Inoceramus kleini Müller. Heine : 44–46, pl. 2, figs. 10–11; pl. 3, figs. 12–13.

1969. Inoceramus kleini Müller. Radwanska : 709.

1979. Inoceramus kleini Müller. Ivannikov : 62, pl. 14, fig. 4; pl. 15, figs. 1–2; pl. 16, fig. 1; pl. 17, fig. 1.

pars 1991. Inoceramus kleini Müller. Tarkowski : 109–110, pl. 13, fig. 7; pl. 14, fig. 2 [non pl. 14, fig. 3].

1992. Inoceramus kleini Müller. Walaszczyk : 37; pl. 38, fig. 3.

1992. Inoceramus kleini Müller. Cech and Svabenicka , pl. 2, fig. 2.

TYPE: By monotypy, the holotype is the specimen figured by Müller (1888, pl. 18, fig. 1a-b) from the Spiegelsberge , south of Halberstadt (Subhercynian Basin), Germany ; from the mid- Inoceramus undabundus Meek and Hayden , dle Coniacian. 1862

MATERIAL: Two specimens in total. Double-

Figure 7A–D valve TM P 2016.041.0407, from Bighorn Dam, and one uncataloged, juvenile fragment of a sin- 1862. Inoceramus undabundus Meek and Hayden : gle valve from West Thistle Creek. 26.

MEASUREMENTS: See table 1. 1876. Inoceramus undabundus Meek and Hayden.

DESCRIPTION: The double-valve specimen, Meek: 60, pl. 3, fig. 2. TMP 2016.041.0407 (fig. 6A–C), is weakly 1894. Inoceramus undabundus Meek and Hayden. deformed, almost complete, with posteroventral Stanton: 84, pl. 16, figs. 1–2 [illustration parts of valves missing. The specimen is equi- and description after Meek, 1876]. valve, inequilateral, small sized, and prosocline, 1898. Inoceramus undabundus Meek and Hayden. with the disc subtriangular in outline and mod- Logan: 455, pl. 105, figs. 1–2. [illustration erately inflated. The posterior auricle is small, and description after Meek, 1876] moderately well separated from the disc. The?1901. Inoceramus undabundus Meek and beak is pointed, curved anterodorsally. Anterior Hayden. Sturm: 92, pl. 10, fig. 4. margin is slightly concave below the umbo, pars 1929. Inoceramus undabundus Meek and then slightly convex; with the anterior margin Hayden. Heine: 100, pl. 11, fig. 50; pl. 13, steep. The ornament is composed of regularly fig. 57 [non pl. 11, fig. 51; pl. 19, fig. 71, spaced, symmetrical rugae, with interspaces which are Volviceramus exogyroides Meek growing gradually ventralward. The edges of the and Hayden] rugae are sharp. The rugae weaken on the pos- pars 2006. Inoceramus undabundus Meek and terior auricle and on the anterior wall, but are Hayden, 1862. Walaszczyk and Cobban: still visible. 260; text-figs. 9.2, 9.4, 27.2, 28.2, 38.6

REMARKS: The species is interpreted herein as [only] the evolutionary precursor of Inoceramus undabundus Meek and Hayden. Early representatives TYPE: The lectotype, designated by Walaszcof the latter species are similar in valve outline zyk and Cobban (2006: 260), is USNM 1909, and type of ornament, however, the interspaces which is the original of Meek (1876: pl. 3, fig. 2), are much larger at equivalent axial length. from the Marias River Shale of Chippewa Point

OCCURRENCE: The species spans the lower to near Fort Benton, Choutea County, Montana. middle Coniacian boundary. Although we have MATERIAL: Seven specimens; TMP only two specimens (one from the topmost lower 2016.041.0402, TMP 2016.041.0403, TMP Coniacian in West Thistle Creek; and the second 2016.041.0426, TMP 2016.041.0427, TMP from the basal middle Coniacian at Bighorn 2016.041.0434; all from the Bighorn Dam sec- Dam), this stratigraphic occurrence corresponds tion; TMP 2016.041.0244 from West Thistle well with its occurrence in other areas. The spe- Creek; and TMP 2016.041.0196 from Chungo cies is known from Germany, Poland, Russia, Creek. Romania, and the Czech Republic. MEASUREMENTS: See table 1. FIG. 5. A, B , Inoceramus ex gr. lamarcki Parkinson, 1819 , sulcate form, TMP 2016.041.0097, Wapiabi Formation, Wapiabi Creek, 66.2 m: A, anterior view, B, left lateral view. C, Inoceramus gibbosus Schlüter, 1877 , Wapiabi Formation, Sheep River, 76.0 m, right lateral view. D, Inoceramus gibbosus Schlüter, 1877 , TMP 2016.041.0110, Wapiabi Formation, Blackstone River, 43.5 m, right lateral view. E, F, Inoceramus ex gr. lamarcki Parkinson, 1819 , sulcate form, TMP 2016.041.0233, Wapiabi Formation, West Thistle Creek, 34.4 m: E, left lateral view, F, anterior view. G, Inoceramus gibbosus Schlüter, 1877 , TMP 2016.041.0172, Wapiabi Formation, Chungo Creek, 42.0 m, right lateral view. All photographs are ×1.

DESCRIPTION: The typical specimens of the species are TMP 2016.041.0426 (fig. 7A, D), TMP 2016.041.0427, TMP 2016.041.0434, and TMP 2016.041.0244. Based on these four specimens, the characteristic of the species is as follows. The shell is inequivalve, with LV larger and more inflated than the right valve ( RV), with a general architecture typical for Volviceramus . Both valves grow obliquely, with δ angle ranging between 30° and 45°. The beaks in both valves pointed, only slightly coiled, not projecting above the hinge line. The oblique growth continues till ca. 10 cm in axial length, the obliquity is then markedly reduced. Both valves are ornamented with strong commarginal rugae, with distinct ventralward increase of interspaces. The details of the shell are not known ; the rugae on internal molds are symmetrical to moderately asymmetrical, with leading edges steeper. The interspaces are distinctly wider than the rugae edges. The edges of the rugae are rounded.

The smallest specimen, which comes from the basal middle Coniacian and apparently phylogenetically the oldest (2016.041.0402; fig. 7B, C), is only slightly inequivalve. It already possesses, however, the typical ornament. TMP 2016.041.0403 from Bighorn Dam and TMP 2016.041.0196 from Chungo Creek, are fragments of LVs.

DISCUSSION: The material studied includes the first well-documented RVs of the species ever reported.

Although in our material the species is represented by only seven specimens, they demonstrate the evolution of the species from a relatively small-sized, weakly inequivalve morphotype (fig. 7B, C), in the earliest middle Coniacian, to moderate size, markedly inequivalve more advanced morphotype stratigraphically higher (fig. 7A, D). The phylogenetically oldest, weakly inequivalve specimens closely resemble Inoceramus kleini , from which they differ in having more robust ornament. The phylogenetically more advanced specimens possess features typical for volviceramids; this is particularly well seen on the RVs, in which the beak moves ontogenetically from its marginal position gradually inward (fig. 7A), as a consequence of inequivalve growth of the valve. Based on its evolutionary interpretation, the species (as a successor of Inoceramus kleini ) is left in the genus Inoceramus , in spite of its close morphological resemblance to Volviceramus .

Walaszczyk and Cobban (2006) included into the synonymy of Inoceramus undabundus also Inoceramus stantoni (= I. acuteplicatus of Stanton, 1894) Sokolov, 1914. Based on the Canadian material, however, it seems that phylogenetic changes of both species differ. In contrast to I. undabundus , I. stantoni remained only slightly inequivalve and preserved its more subtle ornament. Consequently we keep both species separate here. Both I. undabundus and I. stantoni seem to originate from I. kleini .

OCCURRENCE: Inoceramus undabundus is most abundant in the Bighorn Dam section, where it was found at levels 41.0, 60.0, and 73.0 m. The single specimen from West Thistle Creek is from 72.4 m, and the specimen from Chungo Creek is from 54.0 m. The species is known from the U.S. Western Interior and Gulf Coast of North America. It is convincingly documented from Poland ( Sturm, 1901) and Germany ( Heine, 1929).

Figures

FIG. 6. Inoceramus kleini Müller, 1888, double-valve specimen, TMP 2016.041.0407, Wapiabi Formation, Bighorn Dam, 47.0 m: A, posterior view, B, anterior view, C, right lateral, view; the specimen is ×1.

FIG. 7. Scaphites (S.) preventricosus Cobban, 1952, macroconch, TMP2016.041.0038, 115 m, Wapiabi Formation, Mill Creek, Alberta. A. Right lateral; B. apertural; C. ventral; D. left lateral.

FIG. 5. Stratigraphic ranges of Scaphites (S.) preventricosus, S. (S.) ventricosus, S. (S.) depressus, and Clioscaphites saxitonianus (plotted on the left side) next to a curve of the changes in relative sea level during the Turonian-Santonian based on analysis of stratigraphic sections in the Western Foreland Basin (Plint et al., this issue). The evolutionary relationships proposed by Cobban (1952) are presented as a cladogram showing the trend toward recoiling at maturity.