The infauna included specialized borers from many varied animal groups; all of them were suspension feeders, each animal with its own particular form of boring. The bivalve Lithophaga created a flask-shaped 'crypt' with a narrow entrance at the top; it spent its whole life in the same place, gradually enlarging its crypt as it grew by secreting a chemical to dissolve the surrounding rock. Another bivalve (Pholas) had strong anterior ribs which ground the rock when the animal rotated. Some cirripedes, sponges, algae and bryozoans also bored, each with a different pattern closely comparable to those made by their modern descend-ents, from which these inferences have been made.
Most hardgrounds formed in quite shallow clear water, but some occurred in deeper, more turbid water where the faunal diversity was much reduced.
Liassic clays often contain calcareous concretions which formed a few centimetres below the sea floor. Normally, once formed, these were never re-exposed, but on rare occasions they were exhumed and planed off by later erosion; their flat upper surfaces later provided a suitable medium for borers like Lithophaga and encrusting oysters whereas serpulids selected less exposed sites beneath and between the nodules (Hallam, 1969). Very occasionally these surfaces supported solitary corals.
Hardgrounds are present in Dorset, central England and Normandy.
Fig. 72 Carbonate Mud Community a Uptonia (Mollusca:
Zeilleria (Brachiopoda: Articulata: Terebratulida) pliosaur tooth (Vertebrata: Reptilia: Euryapsida) Chlamys (Mollusca: Bivalvia: Pterioida — pectinid Entolium (Mollusca: Bivalvia Pterioida — pectinid)
72 Carbonate Mud Community
Carbonate mudstones (or calcilutites) are fine grained limestones, and their fauna is comparable to that of restricted clays (Fig. 64), but trace fossils appear to be more diverse. Slow deposition sometimes led to the development of a hardground or near-hardground, with a resulting increase in diversity of the epifauna.
In the Lias, calcilutites often alternate with clays to form rhythms 1 to 1.5m thick. The lime mud of the calcilutites partly originated from early coccolithophorids, sub-microscopic calcareous algae which floated near to the surface of the sea. When they died their skeletons dropped to the sea floor to be mixed with mud laid down from bottom currents. This coccolith 'rain' was probably fairly continuous. Besides the coccoliths there were other floating and swimming organisms (ammonites, belemnites, ostracodes), fish and reptiles. Ichthyosaur and plesiosaur bones are rare. Plio-saurs (Fig. 72, inset q) evolved from the long-necked plesiosaurs in the Middle Jurassic.
The epifauna consisted of suspension feeders, most of which were adapted to living on a soft substrate. Bivalves were represented k
Inoceramus (Mollusca: Bivalvia: Pterioida)
Diplocraterion (trace-fossil -crustacean)
Chondrites (trace-fossil — annelid)
l1 living Plagiostoma (Mollusca:
Bivalvia: Pterioida) I* fossil Plagiostoma with tooth marks (Mollusca: Bivalvia: Pterioida) m serpulid (Annelida — poly-
chaete) n Gryphaea (Mollusca:
Bivalvia: Pterioida — oyster) o Thalassinoides (trace-fossils -crustacean)
Schizospharella (Coccolithophorida) — inset pliosaur (Vertebrata: Reptilia: Euryapsida) — inse p q
Was this article helpful?