Zachos—Paleocene echinoids
1017
Description.—Outline cuneate, slightly emarginate anteriorly, truncated posteriorly, apex behind apical disc, test gently sloping towards anterior; oral surface slightly convex, no keel. Apical disc nearly central, ethmophract, four genital pores. Anterior ambulacrum shallowly depressed, zygopore pairs equal size, separated by single tubercle or bead, becoming unipores just above peripetalous fasciole on anterior margin. Petals shallowly depressed, closed, nearly equal in length, anterior pair slightly flexed toward anterior, diverge at 130º, posterior pair more strongly flexed towards posterior and approaching final divergence at 40º; pores elongated, interior pores larger than exterior, interporiferous zone narrow, width equal to poriferous zones. Periproct small, taller than wide, inclined, visible from above. Peristome in anterior quarter of test, small, reniform to subpentagonal, lipped. Oral plating uncertain as a result of damage to sternal area. Peripetalous fasciole apparent only at the tips of the petals, probably com- plete, no lateral fasciole.
Figure 10. Hemiaster stella (Morton, 1830), MMNS 5095, oral plate pattern, from the McBryde Member, Clayton Formation, Furman, Wilcox County, AL. Scale bar 10mm.
Ridgeville, Butler County, Alabama (UTM Zone 16, E 525,183m, N 3,539,105m, NAD83); West of County Rd 59, 2.3 miles south of Furman, Wilcox County, Alabama (UTM Zone 16, E 502,537m, N 3,538,048m, NAD83).
Remarks.—The description is expanded based on new material from Alabama. This species is locally common in the Clayton Formation (Danian). Although superficially similar to small specimens of Linthia alabamensis, the two species are not found together in the Clayton Formation and apparently occupied different habitats. For this study, no comparison was made directly with European material attributed by Kroh (2001) or Smith and Jeffery (2000) to this species.
Family Micrasteridae Lambert, 1920 Genus Diplodetus Schlüter, 1900
Diplodetus moscovensis (Cooke, 1959) Figure 9.3, 9.6
1959 Hemiaster moscovensis Cooke, p. 68, pl. 28, figs. 5–9. 2000 Diplodetus? sp. indet. Smith and Jeffery, p. 310.
Type.—Holotype USNM 76285, probably from Prairie Bluff Chalk (Upper Cretaceous), Moscow Landing, Sumter County, Alabama.
Occurrence.—Probably Prairie Bluff Chalk (Upper Cretaceous), Moscow Landing, Sumter County, Alabama (UTM Zone 16, E 402,209m, N 3,587,818m, NAD83).
Remarks.—Description is based on the original description. As noted by Smith and Jeffery (2000), there is no indication of a subanal fasciole on the type, but this portion of the test is badly worn and damaged. Reported from “Bed 3, Porters Creek Formation”, but internal matrix is unlike any of the Porters Creek (late Danian) at Moscow Landing and appears to be identical with the Prairie Bluff Chalk (Maastrichtian) lithology seen in the lower part of the section below the exposed Cretaceous/Paleogene contact. This specimen is most likely from the Prairie Bluff Chalk, either by collection or as a reworked specimen. The station identifier (USGS 5658) on the original label suggests the specimen was collected by T.W. Vaughan prior to the description of the section by Stephenson (1915). The only stratigraphic notation on the label is “Bed 3”. Smith et al. (1894) described the section at Moscow Landing, and here Bed 3 was described as a ledge containing the mollusk Gryphaea vomer, which was corrected by Smith (1910) to Ostrea pulaskensis ( = Pycnodonte pulaskensis), a guide fossil to the lower Clayton Formation (early Danian). Large numbers of P. pulaskensis are found just above the Cretaceous/ Paleogene contact at Moscow Landing. The type of Hemiaster moscovensis is distinguishable from
the type of Micraster ( = Diplodetus) americanus (Stephenson, 1941), from the Corsicana Marl (Maastrichtian), Bexar County, Texas, primarily by slight differences in the depth of the ambulacral depressions and the flexure of the petals. Otherwise, the similarities are so striking that even with lack of information regarding the presence or absence of a subanal fasciole this
Figure 9. Hemiaster parastatus (Morton, 1833) from the Vincentown Formation, Vincentown, Burlington County, NJ: (1) USNM 29472, aboral. Hemiaster stella (Morton, 1830) from the McBryde Member, Clayton Formation, Furman, Wilcox County, AL: (2) MMNS 5095, aboral; (4) MMNS 5095, oral. Diplodetus moscovensis (Cooke, 1959) from the Prairie Bluff Formation (?), Moscow Landing, Sumter County, AL: (3) USNM 562457, holotype, aboral; (6) USNM 562457, detail of apical disc. Linthia alabamensis Clark, 1915: (5) MMNS 7164, aboral, from the McBryde Member, Clayton Formation, Palmyra tract, Lowndes County, AL; (7) MMNS 7354, aboral, from the Clayton Formation, Possum Grape, Jackson County, AR; (8) UF 278479, aboral, from the Pine Barren Member, Clayton Formation, Mussel Creek, Lowndes County, AL; (9) USNM 470904, holotype of Linthia maverickensis, aboral, from the Kincaid Formation, Maverick County, TX. Linthia tumidula Clark, 1891, from the Vincentown Formation, New Jersey: (10) AMNH 8954, lectotyope, aboral; (11) AMNH 8954, lectotype, posterior; (12) AMNH 8954, lectotype, profile. Scale bars are (1, 5, 7, 10–12) 20mm; (2–4) 10mm; (6) 2mm; (8, 9) 5mm.
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