This is Mazon Monday post #348. What’s your favorite Mazon Creek fossil? Tell us at esconi.info@gmail.com.

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Grammysioidea hayi is a very rare Mazon Creek bivalve clam. The holotype specimen was found by Andy Hay in the early 1980’s, on the channel side of ESCONI Hill, where the hill projects out into the water. In life, the animal was likely sedentary.

Grammysioidea hayi was described by Jack B. Bailey of Western Illinois University in Chapter 8A “Bivalvia” of the Richardson’s Guide to the Fossil Fauna of Mazon Creek. That chapter was written by Jack Bailey and Steven Sroka. The animal was named for Andrew Hay, long-time ESCONI member and co-author of three Mazon Creek fossil books, all published by ESCONI.

Andy with Karen Nordquist in 2001.

Andy Hay wrote a Creature Corner article for the October 2001 ESCONI newsletter.

Creature Corner

The hunt for ironstone concretions containing plant or animal fossils has been going on for more than 150 years in northern Illinois. Papers, pamphlets and books have been published describing new species of flora and fauna. After all this time one might harbor the thought that nothing new can be found. Not so, as the creature illustrated here demonstrates. More than 20 species of pelecypods (clams) found in the Mazon Creek biota had been classified prior to its discovery, but there it was. A new member of the biota had been discovered and described. Keep checking your finds!!!!!

Grammysioidea hayi (Holotype)

(Scale 10 mm; actual size around 1 7/8 inches or 47-48 mm)

This Bivalve was found at Pit 11 in the early 1980’s on the channel side of ESCONI Hill where the hill projects out into the water.

“This rare Essex species is represented by a single specimen from Pit 11. In life, this species was likely a partially buried sedentary bivalve, attached by a byssus.” Excerpt from Section 8A Bivalvia of Richardson’s Guide to The Fossil Fauna of Mazon Creek. Andrew A. Hay and Charles W. Shabica were the Editors. Donald Auler was the Principal Artist. Specimen described by Jack B. Bailey, Western Illinois University on page 113-114 (picture on page 110).

Andrew A. Hay

Grammysioidea hayi description from the Richardson’s Guide, pages 113-114.

Grammysioidea hayi new species
Figure 8A.50

Diagnosis. Similar to G. omaliana (de Kon-inck, 1844), but with well-defined anterior radial groove. Comarginal rugae bifurcate posteri-orly as they cross the groove.

Holotype. NEIU MCP 604.

Etymology. Named in honor of Andrew A. Hay, who donated the holotype.

Description. Shell medium to large, equivalve, thin, swollen, posteriorly elongated, and somewhat trapezoidal in outline with moderately elevated umbos; inequilateral, but not strongly so; dorsal margin marked by a broad lunule. Outline of posterior margin of valve is arcuate below but straightens and extends obliquely above, where it is marked by a weak sinus. Sculpture consists of widely spaced rounded comarginal rugae that markedly bifurcate behind a broad radial fold or groove extending from beaks to ventral margin. Holotyp measures 30+ mm (incomplete) in length and about 25 mm in height.

Remarks. This rare Essex species is represented by a single specimen from Pit 11. It is easily distinguished from other Mazon Creek clams by the bifurcating rounded comarginal ru-gae and broad radial sulcus. Present limitations of material do not allow the range of variation, which was probably broad, to be investigated.

Except for the stronger radial groove, this species is basically similar in shape (including the posteriodorsal margin with its weak sinus) and ornament to G. omaliana (de Koninck, 1844). The presence of this highly variable European Carboniferous species in North America was noticed by Driscoll (1965), who considered it to be identical to the American species “Sanguinolites” hannibalensis Winchell (1865) (= “Grammysia” hannibalensis Walcott, 1884), from the Mississippian of the Midwest.

This species resembles two Carboniferous British species: “Sanguinolites” omalianus (de Koninck) (Hind, 1899, pl. 40, fig. 17) and “S.” variabilis M’Coy (Hind, 1899, pl. 44, fig. 8). Driscoll (1965) reported the first species in the Mississippian of the midwestern United States under the name of Grammysia omaliana. We place the same species in Grammysioidea due to the lack of a double radial groove. Driscoll’s (1965, text-figs. 4 and esp. 5) study is particularly significant because he documented the strong degree of variation in shell shape and ornament inherent in that species. A few of the variants possess a radial groove; most lack it. Rugae bifurcate in some but do not in others. Hoare et al. (1989) believed that Driscoll’s understanding of the species was too broad, but we consider their understanding too narrow (several of the other taxa in their study are oversplit). In fact, their Ectogrammysia crassatis from the Imo Formation (Late Mississippian) of Arkansas is indistinguishable from “S.” (=Grammysioidea) omaliana as figured by Hind (1899, pl. 40, fig. 8). Among Imo samples of E. crassatis in the Western Illinois University collections, both the distinctness of the broad radial groove and the frequency of rib bifurcation are remarkably variable (but less pronounced than in G. hayi). This variation tends to confirm Driscoll’s original understanding of the breadth of shell variation among gram-mysiids.

In life, this species was likely a partially buried sedentary bivalve, attached by a byssus. Grammysioidea, like other pholadomyaceans, probably had short siphons. Among uncrushed Imo G. omaliana specimens in the Western Illinois University collections, a moderately wide, permanent siphonal gape is seen along the posterior margin; there is also a small anterior gape, probably for the byssal apparatus. These features have not been previously described.

Specimen Photos

Holotype. From Jack Wittry’s “The Fossil Fauna of Mazon Creek.”