Cardiaster Fossarius

(Mollusca: Bivalvia: Pterioida)

t

Neohibolites minimus (Mollusca: Cephalopoda: Coleoidea — belemnite)

bivalves by boring small circular holes in the valves with its specialized radula. Byssally attached inoceramids are common in clusters, as in the Lower Gault, but the ribbed Inoceramus concen-tricus is replaced by Actinoceramus sulcatus, a descendant with coarse radial ribs. Coelenterates are represented by the tiny, solitary, ahermatypic cup coral Trochocyathus.

Deposit feeders dominate the infauna, and the sediment is bioturbated, with traces of arthropods (Thalassinoides) and the ubiquitous Chrondrites and Planolites. The deposit-feeding proto-branchs are represented by Nuculana, and the scaphopods by Dentalium, which probably hunted through the sediment for Foraminiferida.

Much of the Gault fauna retains its original aragonitic shell material; although often crushed, the ammonites show nacreous lustre. In other cases, early cementation of clay infillings resulted in the preservation of the original shape; ammonite phragmocones are commonly filled with iron sulphides as they are in many Jurassic clays. Exposures with this fauna may be found in southeast England and the south Midlands.

ALBIAN COMMUNITIES OF SILTY-SANDY AND GLAUCONITIC SEDIMENTS (98-100)

These three faunas are all drawn from the Upper Greensand formation of south-east and south-west England, a terrigenous and glauconitic equivalent of the Gault Clay, which presumably accumulated in shallower water higher energy environments. Some or all of these faunas can be found in every part of the outcrop of the Upper Greensand, and comparable faunas occur in some of the lithologically similar facies of the Cenomanian which are to be found in south-west England, in Scotland and in Northern Ireland.

98 Exogyra-pectinid Community

The Exogyra Rock is a quartrose glauconitic and calcarenitic sand crowded with interlocking calcareous nodules which occurs over much of Dorset and south-east Devon. It takes its name from the abundance of the coiled oyster Exogyra, and was broadly contemporaneous with the Upper Albian Gault Clay shown in the previous illustration. Nektonic elements are represented chiefly by rare ammonites; a specimen of Mortoniceras is shown here.

Benthos was of rather low diversity, the commonest animals being clusters of Exogyra cemented to each other or on to dead

Cardiaster Fossarius

a Mortoniceras inflatum (Mollusca: Cephalopoda: Ammonoidea)

b Cardiaster fossarius (Mollusca: Cephalopoda: Echinoidea)

c Chlamys aspera (Mollusca: Bivalvia: Pterioida — pectinid)

d Rotularia concava (Annelida — polychaete)

e Entolium orbiculare (Mollusca: Bivalvia: Pterioida — pectinid)

f Exogyra obliquata (Mollusca: Bivalvia: Pterioida — oyster)

g Torquesia granulata. (Mollusca: Gastropoda: Mesogastropoda)

h Neithea gibbosa (Mollusca: Bivalvia: Pterioida — pectinid)

i 'Spongeliomorpha'annulatum (Burrow)

shells, often turritellid gastropods. Epifaunal pectinid bivalves are also present with the free-living, perhaps occasionally swimming, smooth, orbicular Entolium, the radially ribbed and spinose Chlamys aspera, and a large species of coarsely ribbed Neithea. Small coiled serpulids Rotularia concava are also part of the benthos, commonly occurring in clusters. The infauna includes turritellid gastropods and the heart-shaped shallow-burrowing Cardiaster. Trace fossils indicate other animal groups, including arthropods and polychaetes; a trace common in sediments of this type is the burrow 'Spongeliomorpha' annulatum, a branching, clay-lined cylinder, with ridges on the outer surface, produced either by arthropods or polychaetes.

The reconstruction of these faunas is based on specific occurrences on the Dorset coast; the Exogyra Rock association can be found in Britain throughout Dorset and Devon, and broadly similar associations occur in sandy Albian and Cenomanian facies elsewhere in south-west England, Scotland and Northern Ireland.

99 Diverse Molluscan-sponge Community

The Blackdown Hills between Honiton and Wellington in Devon are capped by white, light yellow and red sands, with white siliceous concretions. In the last century, the siliceous concretions were worked for whetstones, and very large fossil faunas, usually silicified, were brought to light. Scores of species are known, and they make up some of the more diverse Upper Albian sandy bottom faunas.

The nekton includes ammonites, such as the Mortoniceras already illustrated (Figs. 97-98), various hoplitids, and the small ribbed Hysteroceras, shown in this diagram. Epifauna includes bivalves, serpulids and other animals similar to the examples shown in Figs. 98, 100 and 101, and also diverse gastropods, including Rostellaria and species of 'Fusus', 'Phasianella' and their relatives. Sponges,including Siphonia, are also common. The infauna includes some gastropods, notably the carnivorous naticid Gyrodes (shown here as empty shells) and the particulate feeder Turritella; the latter often occurs in vast numbers and forms shell beds. Bivalves are diverse and include shallow burrowers such as Glycimeris, Epicyprina, Eriphyla, Cyprina and Protocardia, the rostrate Pterotrigonia, and deeper burrowers such as Panopea. Intensive bioturbation, with trace-fossils such as Thalassinoides, Chondrites and Planolites, indicates the presence of diverse arthropods and polychaetes.

Fig. 99 Diverse Molluscan-sponge Community a Siphonia tulipa (Porifera: Demospongea)

b Torquesia granulata (Mollusca: Gastropoda: Mesogastropoda)

c Epicyprina angulata (Mollusca: Bivalvia: Veneroida)

d Ptero trigonia aliformis (Mollusca: Bivalvia: Trigonioida)

e Rostellaria (Mollusca: Gastropoda: Mesogastropoda)

f 'Fusus' (Mollusca: Gastropoda: Neogastropoda)

g 'Phasianella' (Mollusca: Gastropoda: Archaeogastropoda)

h Gyrodes genti (Mollusca: Gastropoda: Mesogastropoda)

i Cucullaea glabra (Mollusca: Bivalvia: Arcoida)

j Eriphyla (Mollusca: Bivalvia: Veneroida) k Cyprina cuneata (Mollusca: Bivalvia: Veneroida) 1 Glycymeris (Mollusca: Bivalvia: Arcoida)

Mollusca Bivalves

The classic Blackdown sections have long vanished, but representative elements of this fauna are widespread in the lower parts of the Upper Greensand of Dorset and Devon.

100 Sponge Community

At various localities in southern and south-west England, the glauconitic sands of the Upper Greensand of Albian and Ceno-manian age have yielded diverse sponge faunas. These are commonly dominated by lithistid sponges, a group of Demosponges characterized by lumpy, knobbly spicules which are fused into a framework which results in good preservation of the overall body form.

Siphonia is a genus with a basal root-like anchorage, a long, slender stem, and a pear- or tulip-shaped body. There are small, slightly curved incurrent canals which extend from the surface of the sponge to its centre, and larger excurrent canals, running parallel to the surface from the base to the summit, where they open into a deep central cavity. Hallirhoa is a similar genus, but the sides of the body are lobed. Doryderma is another stemmed form, rooted at the base; some species, such as Doryderma ben-netiae, have a body shaped like a tall wine glass while others, such as Doryderma dichotomum, have a distinctive dichotomously branched body. Doryderma has parallel vertical canals running from the base to the summit of the sponge, and smaller radial canals extending from the surface towards the centre.

Other elements of the fauna in this community are similar to those occurring in the Exogyra Rock and Blackdown Greensand (Figs. 97—98), such as the coiled oyster Exogyra and the pectinids Chlamys and Neithea. The associated sediment is bioturbated, and large Thalassinoides are shown here.

The best examples of rich sponge faunas of this type are found in Devon and Dorset, but elements of it occur widely in the Albian and Cenomanian sandy facies of southern England.

0 0

Post a comment