Is Dentalium A Living Fossil

— nuculoid)

mucus-lined tubes in the soft sea bed, down which they suck detritus. Allen (1958) noted that modern lucinoids are able to live in environments with a low food content and with restricted supplies of oxygen, but that they cannot compete with other burrowing bivalves in more normal environments.

The commonest suspension-feeding epifaunal elements are pectinids, bivalves which could keep their mantles free of mud by valve-clapping. The thin-shelled pectinids could create jet-propulsion by the same activity, and were therefore capable of limited swimming. Inoceramus was sedentary and byssally attached to shell fragments or to floating wood.

Ammonites and belemnites were the commonest nekton and plankton; some ammonites were encrusted by oysters and serpulid worms. If the whole shell was encrusted on both sides, the oysters must have grown while the ammonite was alive and swimming above the sea floor, the oysters taking advantage of the abundant food material where the ammonite was living. More often, however, encrustation only occurred on one side. Some free-swimming bivalves {Bositra) are also present, their separated valves occurring rarely in the sediment. This bivalve has been considered a swimmer (as depicted in Fig. 63) but may well have been an extremely tolerant benthic form.

Like the bituminous shales, these impermeable clays usually show excellent preservation of calcite and aragonite shells, and many cavities within the shells are filled with pyrite. Some shells are preserved in calcite nodules which formed before compaction when the sediment lost water.

The Restricted Clay Community can be seen in England in parts of the Black Ven Marls (Lower Lias) of the Dorset coast, and parts of the Alum Shales (Upper Lias) of Yorkshire.

65 Silty Clay Community

Both the fauna and the trace fossils (burrows) were more diverse in silty clays than in clays without silt particles, partly because the substrates were more stable, and partly because more abundant suspended food was available. In addition to the benthic detritus-feeding organisms (including protobranchs, lucinoids and the worms which made the Chondrites burrows), there were many benthic filter feeders; some of these lived on the sea floor (epifauna) and others burrowed at various depths within the substrate (infauna).

The epifaunal filter feeders included Gryphaea, pectinids and crinoids. Most modern oysters live cemented by the left valve to some hard substrate, but Gryphaea was free-living for most of its life; the spat settled on a hard substrate (often a shell fragment), but, as its shell grew, the area of attachment did not increase and the left valve became curved; eventually the shell tipped over to rest on the convex left valve, and the animal lay free on the sea floor. The pectinids were widespread in silty clays, particularly genera with thick, robust shells. Rippled silts and storm scours filled with cross-bedded sand suggest the influence of waves. In similar modern environments the shell thickness in bivalves often increases with wave turbulence, and the thick shells of the pectinids of the Lower Jurassic suggest a similar environmental response. The suspension-feeding crinoids lived in clusters that were destroyed by storms and most of their remains occur as isolated ossicles and plates. Brachiopods were rare in these silty clays, the most common form being the spire-bearer Spiriferina; this had a more efficient feeding mechanism than the other groups of Mesozoic brachiopods.

With the presence of silt, the most notable increase in diversity was in the infaunal and semi-infaunal bivalves (Pholadomya, Pleuromya, Astarte, Cardinia and Pinna). Modern representatives of these genera feed on matter suspended in the seawater. Pinna had a large shell; in life this stood vertical, the lower part being buried in the sediment and attached to particles in the sediment by byssal threads (seldom preserved fossil). The upper part of the shell is often encrusted with serpulids and small bivalves (Lios-trea), which took advantage of the food-carrying inhalent currents of the Pinna. Many Pinna are totally encrusted; these were shells exhumed by storm action and often laid down pointing in the direction of the depositing current. The infauna also included crustacea and worms, seldom preserved as fossils, but represented by their burrows. In particular, the Thalassinoides and Rhizo cor allium crustacean burrows were characteristic of this environment.

The nekton and plankton included the belemnites, ammonites and vertebrates seen in the clay communities, but, in addition, the silty clays contain larger ammonites with tubercles (e.g. Eodero-ceras), which probably scavenged near the sea floor.

The diverse Silty Clay Community described above is present in the Lower Lias beds of Robin Hood's Bay on the Yorkshire coast in England (Sellwood, 1972). A sparser fauna is seen in some silty Middle Lias clays of Dorset, where sedimentation rates were higher. Intermediate communities occur in the Lias of east and west Scotland. In other silty clays the infaunal suspension feeders, which were very sensitive to turbidity, may be rare or absent.

The permeability of silty clays is higher than that of pure muds. Water moving through the sediment may have dissolved the aragonite shells of many bivalves and ammonites, leaving only the moulds preserved. Calcite shells (like oysters and belemnites) are less soluble and were normally preserved with little alteration.

Pholadomya

b belemnite (Mollusca: Cephalopoda: Coleoidea)

c crinoids (Echinodermata: Crinozoa)

d Spiriferina (Brachiopoda: Articulata: Spiriferida)

e Pholadomya (Mollusca: Bivalvia: Anomalodesmata)

f Pleuromya (Mollusca: Bivalvia: Anomalodesmata)

g Gryphaea (Mollusca: Bivalvia: Pterioida — oyster)

h Astarte (Mollusca: Bivalvia: Veneroida)

i Cardinia (Mollusca: Bivalvia: Veneroida)

j Pinna (Mollusca: Bivalvia: Mytiloida)

k serpulids (Annelida — polychaete)

1 Liostrea (Mollusca: Bivalvia: Pterioida — oyster)

m Chondrites (trace-fossil — Annelida)

n Thalassinoides (trace-fossil — Crustacea)

o pectinids (Mollusca: Bivalvia: Pterioida)

66 Muddy Sand Community

There was a complete gradation between the communities that lived in mud and muddy sand. As the proportion of silt and sand increased, so there was an increase in diversity of both epifauna and infauna. The sand provided a more stable substrate than silt for those animals living on the sea floor, and the greater turbulence in the shallower water caused an increase in the amount of organic-material in suspension and provided a richer food supply for suspension feeders and deposit feeders. The Muddy Sand Community includes many infaunal suspension-feeding bivalves such as the siphonate pholadomyoids (Pholadomya and Pleuromya) and the veneroids (Cardinia, Astarte, Hippopodium and Protocardia). The semi-infaunal Pinna was also present, and the fissure-dwelling pectinid Camptonectes, which was normally epifaunal, may sometimes be found attached to Pinna shells partially below the surface. Many of the infaunal and semi-infaunal bivalves occur with the valves vertical, in their positions of growth; other valves lie parallel with the bedding, the shells having been drifted by periodic fast currents in this shallow marine environment.

The infauna was also represented by many burrowers including Chondrites while suspension-feeding crustaceans formed the trace fossils Thalassinoides, Rhizo cor allium and Diplocraterion. The 'LP burrows of the latter two forms sometimes show an upward migration of the burrow system in response to the periodically high rates of sedimentation (Goldring, 1964; Sellwood, 1970).

The epifauna was also more diverse in these more sandy beds. In addition to Gryphaea and thicker-shelled pectinids (Pseudo-pecten, Camptonectes and Chlamys), brachiopods (rhynchonellids and terebratulids) are locally common. They often occur in 'nests' representing colonies; close inspection of many of the brachiopods may show the pedicular scars of other brachiopods (Bromley and Surlyk, 1973). Brachiopods evidently anchored themselves on each other. Bivalves with a similar mode of life to the brachiopods include the byssally attached pterioids (Oxytoma and Gervillia), which probably swung free over the sea floor (Kauffman, 1969) in the turbulent water.

Swimming and floating scavengers were represented by ammonites, belemnites, reptiles (ichthyosaurs and plesiosaurs) and fish. Sea floor scavengers included gastropods (Procerithium and the thicker-shelled Amberleya) and large spinose eoderoceratid ammonites.

The diverse faunas occur in the Lower Lias of Yorkshire, intermediate diversity faunas occur in Scotland and fossils are sparser in many of the more rapidly deposited beds of Dorset and central England. The Dorset muddy sands also show some reduction in the numbers of bivalves in most beds.

Free Pictures Fossils

Fig. 66 Muddy Sand Community a DactyHoceras (Mollusca: Cephalopoda: Ammonoidea)

b belemnite (Mollusca: Cephalopoda: Coleoidea)

c Isocrinus (Echinodermata: Crinozoa)

d pectinids (Mollusca: Bivalvia: Pterioida)

e Oxytoma (Mollusca: Bivalvia: Pterioida)

f Amberleya (Mollusca: Gastropoda: Mesogastropoda)

g Gryphaea (Mollusca: Bivalvia: Pterioida — oyster)

h Astarte (Mollusca: Bivalvia: Veneroida)

i Cardinia (Mollusca: Bivalvia: Veneroida)

j Pholadomya (Mollusca: Bivalvia:

Anomalodesmata) k Hippopodium (Mollusca: Bivalvia: Veneroida)

1 Thalassinoides (trace-fossil — Crustacea) m Diplocraterion (trace-fossil — Crustacea or Annelida) n Pinna (Mollusca: Bivalvia: Mytiloida) o Liostrea (Mollusca: Bivalvia: Pterioida — oyster)

p Camptonectes (Mollusca: Bivalvia:

Pterioida — pectinid) q serpulids (Annelida — polychaete) r Chondrites (trace-fossil — Annelida) s Eoderoceras (Mollusca: Cephalopoda: Ammonoidea)

Cephalopoda

Fig. 67 Sand Community a belemnite (Mollusca: Cephalopoda: Coleoidea)

b ammonite (Mollusca: Cephalopoda: Ammonoidea)

c Gryphaea (Mollusca: Bivalvia: Pterioida — oysters)

d Diplocraterion (trace-fossil)

e Tetrarhynchia (Brachiopoda: Articulata: Rhynchonellida)

f Skolithos (trace-fossil — Annelida)

g Dentalium (Mollusca: Scaphopoda)

h Pinna (Mollusca: Bivalvia: Mytiloida)

i serpulids (Annelida — polychaete)

j Thalassinoides (trace-fossil — Crustacea)

Fig. 67 Sand Community a belemnite (Mollusca: Cephalopoda: Coleoidea)

b ammonite (Mollusca: Cephalopoda: Ammonoidea)

c Gryphaea (Mollusca: Bivalvia: Pterioida — oysters)

d Diplocraterion (trace-fossil)

e Tetrarhynchia (Brachiopoda: Articulata: Rhynchonellida)

f Skolithos (trace-fossil — Annelida)

g Dentalium (Mollusca: Scaphopoda)

h Pinna (Mollusca: Bivalvia: Mytiloida)

i serpulids (Annelida — polychaete)

j Thalassinoides (trace-fossil — Crustacea)

67 Sand Community

The marine Lower Jurassic sands contain oysters (Gryphaea and Liostrea), thick-shelled pectinids, the scaphopod Dentalium, and serpulids. Occasional 'nests' of brachiopods are present, and the semi-infaunal bivalve Pinna may occur. The substrate appears to have been too unstable and, at frequent intervals, deposition too rapid to have supported large populations of siphonate infaunal suspension-feeding bivalves. But these sands are extremely porous and many shells, especially those of aragonite, may have been dissolved (most of the fossils listed above are of calcite).

Burrows within the sands made by crustaceans and worms are similar to those seen in muddy sands (Fig. 66). Diplocraterion, Rhizo cor allium, Thalassinoides and Chondrites were all present, and in addition there were vertical tubes (Tigillites, Skolithos or Monocraterion). We have based our interpretation of these assem-lages on the work of Seilacher (1967) who suggested that a general dominance of simple vertical burrows in marine sediments indicates shallow turbulent water with a high concentration of suspended food. The addition of 'U' burrows and Thalassinoides indicates slightly less turbulent water, and meandering 'U' burrows and the complex spiral burrows of detritus feeders reflects quiet water with low concentrations of suspended food.

The oysters and serpulids lived attached to hard materials, usually other shells, but after death the oyster shells were normally redeposited by currents as disarticulated valves. Dentalium belongs to a small group of molluscs whose shell is in the form of an open cone. It moved through the sediment with its head downwards and its posterior extending into the water above. Dentalium can sort out the finer grained material in the sediment, digest the organic matter, and eject the rest up into the seawater. A bed worked over in this way by Dentalium has a low content of fine grained particles. Dentalium was often transported after death, and the shells aligned in accordance with the prevailing current direction.

Nektonic and planktonic organisms (ammonites and belem-nites) were present in this environment, but the aragonite shells of the ammonites were often dissolved, except where the sands were calcified shortly after sedimentation. Calcified sandstone bands are therefore the most fruitful beds in which to search for fossils.

Sand Communities occurred in the Scalpa Sandstone of west Scotland, and in England in the Lower to Middle Lias Sands of the Yorkshire coast; in the Cotswold Sands of the Midlands and the Bridport, Yeovil, Thorncombe and Downcliff Sands of Dorset and Somerset.

Jurassic

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  • Veli-Matti
    Is dentalium a living fossil?
    9 months ago

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