Isogramma

The brachiopod Isogramma Meek and Worthen (1870) is challenging to identify when found by inexperienced collectors. These fossils appear as a half circle of sharp, equal-spaced concentric growth lines. All specimens appear flattened, making them difficult to identify as dimensional brachiopod shells. The shells are concavo-convex in life, but due to a “spongy” shell, they are readily compressed after the end of life. Stojanović-Kuzenko (2003) published unflattened figures of a new species, Isogramma serbica. The figures (Plate 1, figures 1-4) show the life morphology of Isogramma well.

Brachiopod Shell Configurations — Brachiopod shell configuration—Three relationships are expected between the dorsal and ventral values in brachiopods. Isogramma is of the concavo-convex type, but preservation is usually flat, and its original morphology is non-apparent.

Isogramma, the Isogrammidae, and the Dictyonellids

The Dictyonellids are a strange group. Chen and Shi (2016) called Isogramma “one of the most morphologically characteristic genera among Late Palaeozoic brachiopods.” The pair put together an evolutionary history of the genus, claiming rapid diversification after its origination but suffering a setback during a Mid-Carboniferous extinction event.

In the Visean—or the middle of the Mississippian subperiod—authors named 16 species of Isogramma. This number gradually fell into the Pennsylvanian subperiod, with recovery in the Moscovian and reaching another peak during the Kasimovian, occupied by 16 additional species. In total, 15 species are available by the end of the Gzhelin, which marks the end of the Carboniferous. Unfortunately, only five named species occurred at the start of the Permian, a number that was never again matched until the genera disappeared during the Late Permian.

Species diversity of Isogramma. — Species diversity of Isogramma from the earliest known Mississippian subperiod species until the last species in the late Permian period. Note the second diversity peak during the Kasimovian, the age from which the specimen below is.

Reports of unusual specimens of Isogramma

Cooper (1952) reported on unusual specimens from North Central Texas. The pedicle/ventral valve of the holotype of Isograma Renfrarum Cooper 1952 (USNM 111687) measures an astonishing 183 mm at its widest point. The specimen shown below is less than 50 mm in width.

A Brush Creek specimen of Isogramma

Sturgeon and Hoare (1968) reported Isogramma millepunctata from the Lower Mercer limestone within the Pottsville Group, an older group, and the Cambridge limestone. Cambridge is the name in Ohio for the Nadine limestone in Pennsylvania.

The specimen below comes from local decalcified limestone. Weathering softens the rock and turns it into a brown color. One could reduce a representative to dust with light to moderate pressure. This phenomenon produces fossils with exceptional preservation characteristics. Without a cemented matrix to hide details, many unique examples appear. Yet, specimens can break during recovery and need special care in storage.