Rhynchonella inconstans & Lopha gregarea

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Rhynchonella inconstans and Lopha gregarea - two Jurassic Coast fossils, View 1

Two Jurassic Coast fossils from Ringstead Bay in Dorset, UK. Although there are superficial similarities between these marine shell fossils, they are in fact the preserved remains of two very different kinds of organism. One is a marine bivalved Mollusc but the other is a Brachiopod. The two intact specimens are stuck together in a stony matrix which contains other smaller and fragmentary shell fossils. The clump of fossils was found in the Ringstead Bay area which is part of  the Jurassic Coast World Heritage Site.

Externally, they both look fairly similar with their possession of two shell valves, and distinct natural sculpturing of sharply angled ribs radiating out over their surface. However, the two valves in a Mollusc represent the coverings for the left and right sides of the animal contained within. In Brachiopods, on the other hand, the shell halves represent the external protection for the dorsal (upper) and ventral (lower) parts of the animal’s body. The animals in the two separate kinds of organism are therefore always orientated in different ways within their shells – although the shells are formed by much the same sort of biological mechanism.

Brachiopods are, strangely enough, related to the minute, colonial animals known as Bryozoa (Sea Mats or Moss Animals). Bryozoa and Brachiopods uniquely possess a structure called the lophophore with which the animals filter food particles from the sea water flowing through their shells. In Brachiopods, the lophore is supported by long internal extensions of the hard shell – and therefore Brachipods can be said to possess an internal as well as and external skeleton, compared with the Bivalves with just an external shell.

We are much more familiar with the many species of bivalve Molluscs (like clams, cockles, and mussels) than with Brachiopods. Brachiopods still exist today but there are only a couple of hundred species of these ‘lamp shells’ now remaining – compared with the many thousands of now extinct species from the long distant past.

The Brachiopod is Rhynchonella inconstans – seen on the left in the picture above and from other perspectives in the photographs below. It has other synonyms. It is also called Torquirhynchia inconstans and Rhactorhynchia inconstans. It is found in Lower Kimmeridge Clay from Upper Jurassic deposits (beneath the Kimmeridge Clays from which Liostrea delta is frequently recovered). The Jurassic Period lasted from about 195 to 136 million years ago.

The most easily recognised feature of R. inconstans is the way that one half of the shell is stepped down from the other in a peculiar form of asymmetry which is thought to be an adaptation to life in tidal environments.

The bivalved mollusc is probably Lopha gregarea – seen on the right in the top picture (with other views in the lower photographs). It is also known as Ostrea gregarea, Ostrea gregaria, and Alectryonia gregarea. I say ‘probably’ because it would perhaps be more likely to be Lopha marshi from its association with R. inconstans but it just does not look like it to me. Maybe someone out there could advise me?

Rhynchonella inconstans and Lopha gregarea - two Jurassic Coast fossils, View 2

Rhynchonella inconstans and Lopha gregarea - two Jurassic Coast fossils, View 3

Rhynchonella inconstans and Lopha gregarea - two Jurassic Coast fossils, View 4


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3 Replies to “Rhynchonella inconstans & Lopha gregarea”

  1. Lovely examples and well captured. That asymmetrical brach is a rare creature indeed. I think there is only one other species from the Late Jurassic worldwide and it occurs in one spot in NZ. Many thanks for the post.

    Liked by 1 person

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