The Lives of the Strophs

As mentioned a few years ago, strophomenid brachiopods must have had a different lifestyle than your typical brachiopod. With no pedicle to attach to anything, they would have been loose on the seafloor. Their strongly concave-convex shell anatomy seem likely to have been inconvenient in several ways. If you place them convex-up, the opening between the valves is liable to be in the sediment, which doesn't help a filter-feeder. If you place them concave-up, the shell is liable to be flipped over if it is not partially sunk into the sediment, and even if it's clear, the narrow gape would make the intake prone to fouling. Clearly, though, they must have been doing something right, at least for a few million years in the Late Ordovician if the rocks in the Twin Cities have anything to say about it.

A stroph in the Magnolia Member of the Platteville. See, I can always find ways to use even more photos from Uŋčí Makhá Park!

A new publication by Dattilo et al. (2024) offers a lifestyle reconstruction of the stroph Rafinesquina that may resolve these issues: in brief, rather than a narrow valve gape, these brachiopods may have lived with their valves wide-open.

Perhaps like this, as restored by Kyle Hartshorn for Figure 15, not unlike some modern brachiopods. CC BY 4.0.

Dattilo et al. present several lines of evidence leading to the conclusions that Rafinesquina had a typical gape around 45 degrees and could potentially open wider. The major area of focus was the anatomy of the hinge, including both the hard structures and the inferred musculature. The assembly, as it turns out, is rather more complex than one might suspect just looking at Stroph #46893 in a random Platteville surface. (It's also rather more complex than can be explained in a couple of sentences, so fortunately there is the paper to refer to.)

Although to be fair, Stroph #46893 has its charms.

The possibility of widely gaped strophs leads to some other potential implications. For instance, once the ability to make a respectable gape is admitted, there is also the possibility for mobility. A stroph buried by sediment could have used valve clapping to escape. (And if we're doing that, why not a bit of clap-swimming, like modern scallops?) Clapping is also a great way to quickly clear sediment from the feeding organs, and we may have evidence of this in the form of "moats" around some stroph fossils. A natural wide gape also helps explain how stroph internal anatomy worked: a stroph with a narrow gape has a cramped area for its lophophore to function in, while a stroph with a wide gape doesn't have to worry about this as much, as long as there's enough space to stow the lophophore when the valves are closed. Finally, a wide gape does not settle the concave-up or convex-up question, as the brachiopod can function in either mode. Dattilo et al. suggest that convex-up is more stable and protective for the stroph's soft parts.

References

Dattilo, B. F., R. L. Freeman, K. Hartshorn, D. Peterman, A. Morse, D. L. Meyer, L. G. Dougan, and J. W. Hagadorn. 2024. Paradox lost: wide gape in the Ordovician brachiopod Rafinesquina explains how unattached filter-feeding strophomenoids thrived on muddy substrates. Palaeontology 67(2): e12697. doi:https://doi.org/10.1111/pala.12697.