Wooster’s Fossils of the Week: Abundant borings in Early Cretaceous cobbles from south-central England

Faringdon cobble in matrix 071714Last week I described a cyclostome bryozoan on the outside of a quartz cobble from the Faringdon Sponge Gravels (Lower Cretaceous, Upper Aptian) of south-central England near the town of Faringdon. This week I’m featuring a variety of heavily-bored calcareous cobbles from the same unit. One is shown above in its matrix of coarse gravel. The holes are bivalve borings known as Gastrochaenolites. As a reminder, these gravels are very fossiliferous and were deposited in deep channels under considerable tidal current influence (see Wells et al., 2010).

Faringdon cobble 1 071714The large and medium-sized flask-shaped borings are all Gastrochaenolites. In the suite of cobbles described in Wilson (1986), there are three ichnospecies of bivalve borings: G. lapidicus, G. cluniformis and G. turbinatus. It is thus likely, although not necessarily, an indication that at least three bivalve species were boring the soft calcareous claystone to make secure homes for their filter-feeding. The thin, worm-like borings are Maeandropolydora, which were probably made by polychaete “worms”.

Faringdon cobble 3 071714Some of the Gastrochaenolites lapidicus borings have remarkably spherical chambers, a testament to the uniform lithological character of the rock.

Faringdon cobble 5 071714Occasionally bivalve shells are found still preserved in their crypts, along with nestling brachiopods. Some shell bits are visible in the borings above.

FaringdonCobble 585 071714Some of the cobbles are so heavily bored that they fall apart quickly on removal from the matrix. On the Cretaceous seafloor this intensity of boring must have reduced many cobbles to bits before burial — a classic example of bioerosion.

Diagram 071714What is very cool about these Faringdon cobbles is that the borings often overlapped inside, creating a network of tunnels and small cavities that hosted dozens of bryozoan, foraminiferan, sponge, annelid worm, and brachiopod species. This is a diagram from Wilson (1986) showing the combination of external encrusters in a high energy, abrasive world, and coelobites (cavity dwellers) in the protected enclosures. A diverse community can be found on each cobble, inside and out. In a future post I will describe some of these coelobite fossils.

References:

Pitt L.J. and Taylor P.D. 1990. Cretaceous Bryozoa from the Faringdon Sponge Gravel (Aptian) of Oxfordshire. Bulletin of the British Museum (Natural History), Geology Series, 46: 61–152.

Wells, M.R., Allison, P.A., Piggott, M.D., Hampson, G.J., Pain, C.C. and Gorman, G.J. 2010. Tidal modeling of an ancient tide-dominated seaway, part 2: the Aptian Lower Greensand Seaway of Northwest Europe. Journal of Sedimentary Research 80: 411-439.

Wilson, M.A. 1986. Coelobites and spatial refuges in a Lower Cretaceous cobble-dwelling hardground fauna. Palaeontology 29: 691-703.

About Mark Wilson

Mark Wilson is a Professor of Geology at The College of Wooster. He specializes in invertebrate paleontology, carbonate sedimentology, and stratigraphy. He also is an expert on pseudoscience, especially creationism.
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