Backreef Community

The picture shows part of the reef top near its Iagoonal margin during the first Zechstein cycle at a later time than that shown in the previous picture. Laminated algal sheets bound and encompassed pockets of shell and bryozoan debris. On and between the upper surfaces of these sheets the benthos still included many brachiopods, but more bivalves and still more gastropods. All the species represented were survivors from the earlier reef but some of them had increased in numbers because rivals, less tolerant of the increasing salinity, had been eliminated. The stenohaline

Streblochondria? pusilla had disappeared and the only common nektonic species was the nautiloid Peripetoceras. In addition to the sessile benthic organisms shown, the strophalosiid brachiopod Orthothrix and the bivalve Pseudomonotis were also common in the reef at this time. Among the micro-organisms, foraminifera were more rare than in the earlier reef but ostracodes were abundant.

This kind of reef community can be found preserved near Sunderland.

59 Hypersaline Landlocked Basin Community

A number of basins west of the Zechstein sea were also inundated during the first Zechstein cycle but they were almost land-locked and the water in them was generally hypersaline. They supported faunas which were largely molluscan and included very few species. Of these species much the most common was Bakevellia binneyi, indeed to such an extent that the sea in these western basins has been called the Bakevellia Sea (Smith, 1970).

In one of the basins in north-west England was deposited the Manchester Marl,the lower part of which contains marine fossils. The substratum in and on which they lived was soft, consisting of mixed silt and mud. The fauna in this area is notable for the presence of the infaunal bivalve Schizodus obscurus which is large in size and abundant. The small Bakevellia binneyi lived in clumps wherever there were suitable points of attachment; these were possibly algal fronds which are no longer preserved, or empty Schizodus valves. Other bivalve species such as Permophorus were much more rare. The small gastropods probably grazed on algae. They are much more common in the thin limestones which occur in the Manchester Marl than in the intervening mudstones and siltstones, and the limestones were probably at least partly algal in origin. No attempt has been made here to illustrate the algae, as their form is unknown.

Micro-organisms in this hypersaline basin include foraminiferida and ostracodes, but no bryozoans or articulate brachiopods have been recorded. Their apparent absence was probably due to the hypersalinity.

Fig. 59 Hypersaline Landlocked Basin Community a Coelostylina? obtusa (Mollusca: Gastropoda: Mesogastropoda) b Naticopsis minima (Mollusca: Gastropoda: Archaeogastropoda) c Bakevellia binneyi (Mollusca: Bivalvia: Pterioida) d Permophorus costatus (Mollusca: Bivalvia: Veneroida) e Schizodus obscurus (Mollusca: Bivalvia: Trigonioida)

Fig. 60 Hypersaline Sea Community a Coelostylina?permiana (Mollusca: Gastropoda: Mesogastropoda)

b Yunnania helicina (Mollusca: Gastropoda: Archaeogastropoda)

c Liebea squamosa (Mollusca: Bivalvia: Pterioida)

d Permophorus costatus (Mollusca: Bivalvia: Veneroida)

e Phestia speluncaria (Mollusca: Bivalvia: Palaeotaxodonta)

f Schizodus obscurus (Mollusca: Bivalvia: Trigonioida)

Fig. 60 Hypersaline Sea Community a Coelostylina?permiana (Mollusca: Gastropoda: Mesogastropoda)

b Yunnania helicina (Mollusca: Gastropoda: Archaeogastropoda)

c Liebea squamosa (Mollusca: Bivalvia: Pterioida)

d Permophorus costatus (Mollusca: Bivalvia: Veneroida)

e Phestia speluncaria (Mollusca: Bivalvia: Palaeotaxodonta)

f Schizodus obscurus (Mollusca: Bivalvia: Trigonioida)

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