This is the "Fossil Friday" post #51. Expect this to be a somewhat regular feature of the website. We will post any fossil pictures you send in to [email protected]. Please include a short description or story. Check the #FossilFriday Twitter hash tag for contributions from around the world!
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Today, we have a couple specimens of Mazonomya mazonensis or colloquially known as clam-clam. We covered Mazonomya back in Mazon Monday #25. For a long time, this species was misidentified as Edmondia. In 2011, it was redescribed by Jack Bowman Bailey of Western Illinois University.
Abstract
The most abundant bivalve of the Essex biofacies (Mazon Creek fauna, Pennsylvanian of Illinois), misidentified by past authors as the marine pholadomyoid Edmondia de Koninck, 1841, is herein named Mazonomya mazonensis n. gen., n. sp., and assigned to the family Solemyidae, based on: (1) anterior elongation of the shell as deduced from brevidorsal placement of the hinge-axis, preserved traces of the external ligament, and supporting structures; (2) preserved traces of a longidorsal extension of the ligamental outer layer and periostracum; and (3) sedimentary backfill marks left by the large foot near the longiterminus of the shell. e second most abundant Essex solemyid (Solemya radiata of past authors), showing traces of the periostracal frill and external ligament, is emended as Acharax radiata (Meek & Worthen, 1860) n. comb. Other Essex solemyids previously unreported include two probable solemyids left in open nomenclature, and Acharax (Nacrosolemya) trapezoides (Meek, 1874), for which Meek's original, non-Essex specimen is designated as lectotype.Systematic revisions herein challenge open-marine and open-estuary depositional models of the Essex biofacies. Unlike coeval euhaline oxic communities in which solemyids are rare, the Essex bivalve community is dominated by solemyids, a recurrent phenomenon in carbonaceous roof-strata immediately overlying Pennsylvanian coal seams. Extant solemyids are common in shallow euryhaline waters, forming dense chemoautotrophic populations in organic-rich dysoxic/anoxic muds. Within the Essex, the prevalence of solemyids along with an admixture of thin-shelled euryhaline bivalves and growth-inhibited stenohaline bivalves is suggestive of a transitional paleoenvironment, such as a drowned coal-swamp or restricted estuary, in which superabundance of organics and nutrient pollution had induced eutrophication.Arguably, a persistent suite of traits (amphidetic ligament, edentulous hinge, periostracal frill, mantle fusion, reduced gut, and enlarged gills hosting bacterial chemosymbionts) has characterized the Solemyidae since the Early Ordovician. Whereas the diagnostic internal ligament of Solemya Lamarck, 1818, is apparently a post-Paleozic trait, the prevalence of external ligaments among Paleozoic solemyids requires that species previously placed in Solemya be transferred to Acharax Dall, 1908, or other genera. Emended examples herein are: S. [Janeia] primaeva Phillips, 1836, sensu Hind (1900) (Carboniferous, United Kingdom) is emended as Acharax primaeva n. comb., a probable senior synonym of S. parallela Beede & Rogers, 1899 (Pennsylvanian, Kansas) (non S. parallela Ryckholt, 1853 [1854]); Carydium elongatum Clarke, 1907 (Lower Devonian, New Brunswick) is emended as Dystactella elongata n. comb. Additionally, several European Carboniferous species of "Solemya" (e.g., S. puzosiana de Koninck, 1842, S. saginata Ryckholt, 1853 [1854], S. costellata M’Coy, 1844, and S. excisa de Koninck, 1885) should be reassigned to Acharax.
Our first specimen is from Braceville and was collected in September 2020 by ESCONI member Andrew Young. It opened via freeze/thaw in December 2020. If you look closely, you'll notice some soft tissue preservation... the flexible hinge ligament. It's more noticeable on the left.
This specimen was found open at Braceville by ESCONI member Phil Anderson.
These next 2 are textbook examples of the clams found at Chowder Flatts. A locality known and named for the clams found there. Bunches were very common. Chowder Flatts was located on the west side of Morris, IL and is now a housing development. These fossils were also contributed by Phil Anderson.
Nice fossils! Thanks for the contributions guys!
