Wooster Geologists

This brachiopod, identified as Glyptorthis insculpta (Hall, 1847), was shared with me by its collector, Diane from New York State. She found it in a muddy horizon of the Bull Fork Formation (Upper Ordovician) in southeastern Indiana. She immediately noted the distorted plicae (radiating ribs) on the left side of this dorsal valve, along with the invagination along the corresponding margin. (Thanks for showing this to me, Diane, and allowing me to include it in this blog.)
Above  is a closer view of the unusual plicae. Note that they radiate from the top center of the brachiopod, extending as the shell grew outward along its margins. Something happened, though, when the brachiopod was growing. The shell was seriously damaged by a puncturing object. The brachiopod repaired the hole by closing it up with additional shell material coming from either side. The inwardly-curved plicae show the pattern of shell regrowth.
This is a view of the same brachiopod from the other side, showing that the ventral valve was damaged in the same event, but with slightly less destruction.

So how did such damage occur on that Ordovician seafloor? Some predator likely took a bite out of the brachiopod as it lay in its living position with the valves extended upwards into the seawater. Most brachiopods do not survive such events, but this one did.

Who was the probable predator? For that we turn to the work of the late Richard Alexander (1946-2006). He did the definitive study of pre mortem damage to brachiopods in the Cincinnatian Group in 1986, concluding that the most likely predators on these brachiopods were nautiloid cephalopods. Some of this figures show nearly identical healed scars on similar orthid brachiopods.
Richard Alexander was an accomplished paleontologist who lost his life in a swimming accident off the coast of St. Lucia just over nine years ago. He was born in Covington, Kentucky, right across the river from Cincinnati. As is so common with children in that part of the world, he developed a passion for fossils. He attended the University of Cincinnati, majoring in geology, He then went to Indiana University, completing a PhD dissertation titled: “Autecological Studies of the Brachiopod Rafinesquina (Upper Ordovician), the Bivalve Anadara (Pliocene), and the Echinoid Dendraster (Pliocene).” (We don’t see such diverse projects very much these days.) He taught at Utah State University from 1972 to 1980, and then at Rider University in New Jersey from 1981 until his death. He served as an administrator at several levels at Rider, and was known as an excellent teacher. His research interests changed when he moved to the East Coast, becoming increasingly focused on modern mollusks. No doubt he would still be contributing to paleontology but for the randomness of a freak wave in the Caribbean.

References:

Alexander, R.R. 1981. Predation scars preserved in Chesterian brachiopods: probable culprits and evolutionary consequences for the articulates. Journal of Paleontology 55: 192-203.

Alexander, R.R. 1986. Resistance to and repair of shell breakage induced by durophages in Late Ordovician brachiopods. Journal of Paleontology 60: 273-285.

Dodd, J.R. 2008. Memorial to Richard Alexander (1946-2006). Geological Society of America Memorials 37: 5-7.

Yes, the above image doesn’t look much like a brachiopod, but just wait. We see a trepostome bryozoan with extended knobs and a few borings. Flip it over, though …
… and we see that the bryozoan almost entirely covers a brachiopod. So far, so common among Ordovician fossils. However, look closely at the margin of the brachiopod valve and how clearly it is delineated from the bryozoan. It is apparent that the bryozoan had encrusted a living brachiopod, and the brachiopod stayed alive, keeping the essential commissure (the gap between the valves) open for feeding. We are looking at the valve that was in contact with the substrate (the underside of the living brachiopod). The bryozoan occupied the upper exposed surface, growing across that valve (which is invisible to us now), past its edge, but not closing the gap with the other valve. The same bryozoan species is found on the above visible valve, but only as two thin films unconnected to the colony on the upper side.
A closer view of the brachiopod hinge shows additional evidence that the bryozoan and brachiopod were living together. The red arrow on the left points to where the fleshy pedicle (attaching stalk) of the brachiopod extended from the shell to meet the substrate. The bryozoan here curves around the now-vanished pedicle. The yellow arrow on the right shows how the bryozoan growth surface folded to accommodate the opening valves at the hinge. Pretty cool.

I can’t identify the bryozoan beyond Order Trepostomata without cutting it open. The brachiopod, though, appears to be Hebertella richmondensis Foerste, 1909. This specimen is from the Whitewater Formation (Upper Ordovician, upper Katian) exposed near Richmond, Indiana. It was collected on one of my field trips in 2003.
What do we learn from this little assemblage? We first see a relatively uncommon example of a clear living relationship between a sclerobiont and its host. We also learn that the brachiopod could continue to open its valves for feeding despite the heavy calcitic bryozoan weighing it down. We even can see that this brachiopod was not living on a soft muddy substrate because only a small triangular-shaped area (see above) in the center was clear of encrusters; the thin bryozoan (and maybe a bit of the stromatoporid sponge Dermatostroma) had enough space between the valve and the substrate to feed and respire. None of this is surprising, but it is nice to see our models of how these organisms lived are congruent with the evidence.

References:

Alexander, R.R. and Scharpf, C.D. 1990. Epizoans on Late Ordovician brachiopods from southeastern Indiana. Historical Biology 4: 179-202.

Foerste, A.F. 1909. Preliminary notes on Cincinnati fossils. Bulletin of the Scientific Laboratory of Denison University 14: 208–232.

Walker, L.G. 1982. The brachiopod genera Hebertella, Dalmanella, and Heterorthina from the Ordovician of Kentucky. USGS Professional Paper 1066-M.

Wright, D.F. and Stigall, A.L. 2013. Phylogenetic revision of the Late Ordovician orthid brachiopod genera Plaesiomys and Hebertella from Laurentia. Journal of Paleontology 87: 1107-1128.

This is a follow-up post to our entry on Christmas Day two weeks ago. Above is a trepostome bryozoan (the long porous piece) with specimens of the atrypid brachiopod Zygospira modesta clustered around it. They are positioned with their ventral valves outward because in life they were attached to this bryozoan with tiny fleshy stalks called pedicles. They were buried quickly enough that this spatial relationship was preserved. Cool. This assemblage was found in the Liberty Formation (Upper Ordovician) exposed in a roadcut in southern Indiana.
This is a view of the dorsal side of Zygospira modesta showing the pedicle opening in the ventral valve at the apex of the shell.

References:

Copper, P. 1977. Zygospira and some related Ordovician and Silurian atrypoid brachiopods. Palaeontology 20: 295-335.

Sandy, M.R. 1996. Oldest record of peduncular attachment of brachiopods to crinoid stems, Upper Ordovician, Ohio, USA (Brachiopoda; Atrypida: Echinodermata; Crinoidea). Journal of Paleontology 70: 532-534.