Lower Cretaceous

91 Lower Cretaceous Terrestrial Communities

In parts of northern Europe the end of the Jurassic witnessed a phase of marked earth movements which caused some of the old subsiding marine basins to become swamps and marshlands. These early Cretaceous flood-plain regions were periodically inundated by seasonal rivers. In shallow pools along the watercourses and in the waterlogged parts of the flood plains horsetails (Equisetales) grew. One genus of this group of pteridophytes (Equisetum) still survives, and like its early Cretaceous ancestors, possesses a ramifying underground stem and tuber system which binds the sediment. It is not particularly tolerant of salt water and generally grows in fresh water on acid soils, in water depths of less than about lm. In places, perhaps where the early Cretaceous soils were better drained, tree-ferns like Tempskya grew. The horsetails with their coarse foliage seem to have been the staple diet of the larger herbivores like Iguanodon. In Belgium, fissure fillings have been discovered containing a number of Iguanodon that were trapped as a group, suggesting that perhaps the animals were gregarious. These slow, lumbering dinosaurs contrasted with the smaller and more agile bipedal dinosaurs like Hypsilophodon, which were also herbivores and may have been able to climb trees. Flesh-eating dinosaurs were represented by Megalosaurus which were probably slow moving carrion feeders.

The passage of Iguanodon herds and of solitary megalosaurs was recorded on soft surfaces by footprints and occasionally by the impressions of their hides. In the skies above there were

Cretaceous Ecology

Fig. 91 Lower Cretaceous Terrestrial Communities a Iguanodon (Vertebrata: Reptilia: Archosaur — dinosaur) b Megalosaurus (Vertebrata: Reptilia: Archosaur — dinosaur) c Hypsilophodon (Vertebrata: Reptilia: Archosaur — dinosaur) d Acanthopholis (Vertebrata: Reptilia: Archosaur — dinosaur) e Equisetites (Pteridophyta: Calamites — horsetails)

Fig. 91 Lower Cretaceous Terrestrial Communities a Iguanodon (Vertebrata: Reptilia: Archosaur — dinosaur) b Megalosaurus (Vertebrata: Reptilia: Archosaur — dinosaur) c Hypsilophodon (Vertebrata: Reptilia: Archosaur — dinosaur) d Acanthopholis (Vertebrata: Reptilia: Archosaur — dinosaur) e Equisetites (Pteridophyta: Calamites — horsetails)

pterosaurs and birds. The terrestrial invertebrates included insects, other arthropods and worms; they were a source of food for mammals similar to those of the latest Jurassic. The pools sometimes supported unionid bivalves, ostracodes and rarer pulmonate gastropods.

Early Cretaceous sediments in this 'Wealden' Facies are found in south-east England (the Weald), the Isle of Wight, the Isle of Purbeck and under the Celtic Sea. On the continent of Europe a 'Wealden' facies flanks the southern margin of the Ardennes massif and runs south from Belgium. Current ideas about the 'Wealden' depositional environment have been published by Allen (1976).

92 Lower Cretaceous Lake Communities

The waters of the lakes of the Wealden basins of southern England lapped against shorelines of low-lying coastal plains like those represented in the previous figure. These lakes fluctuated markedly in salinity, with indications of fresh, brackish-marine and possibly coastal salt marsh conditions at times. Kilenyi and Allen (1968) noted two broad molluscan assemblages: a freshwater one dominated by the gastropod Viviparus but also including unionids, and a much less common assemblage with Filosina, Cassiope, Melanopsis, Nemocardium, Ostrea and other forms. Locally, as at the top of the Wealden shales (Barremian) echinoid debris is recorded (Casey, 1961) whilst some of the subordinate sandstones in the Weald Clay have yielded the trace fossil Ophiomorpha, the burrow of arthropods (normally considered to be marine). The former freshwater assemblage is usually associated with plants known to be intolerant of saline waters, including stoneworts (Cirtonitella), liverworts (Hepaticites) and horsetails (Equisetites), which are sometimes found with in situ rootlets and rhizomes, (Batten, 1975), whilst work on oxygen and carbon isotopes (Allen et al. 1973) and on microfaunas, notably ostracodes, has confirmed that this was a freshwater environment. Freshwater Wealden microfaunas are dominated by the weakly ornamented genus Cypridea, and the brackish water faunas by a 'non-C/ypn'afea' assemblage of Theriosynoecum, Mantelliana, Rhinocypris, darwin-ulids, and Dicrorygma (Anderson, 1963, 1967, Kilenyi and Allen, 1968). Salinity fluctuations are recorded in the lake clays of the Wealden;but in the sandier Hastings Beds these fluctuations appear to have been lower. Definite salinity cycles are indicated by the following:

Cassiope (a gastropod; most marine) — Filosina (bivalve) — Paludina (gastropod) — Cypridea (ostracode) — Equisetites (horsetails; freshwater).

Lower Cretaceous Bivalves

Fig. 92 Lower Cretaceous Lake Communities a Filosina gregaria (Mollusca: Bivalvia: Veneroida)

b Iguanodon fragments (Vertebrata: Reptilia:

Archosaur — dinosaur) c Equisetites lyelli (Pteridophyta: Calamites — horsetail)

d Onychiopsis mantelli (Pteridophyta: Fern) e Cypridea valdensis (Arthropoda: Crustacea: Ostracoda)

f Cypridea spinigera (Arthropoda: Crustacea: Ostracoda)

Fig. 92 Lower Cretaceous Lake Communities a Filosina gregaria (Mollusca: Bivalvia: Veneroida)

b Iguanodon fragments (Vertebrata: Reptilia:

Archosaur — dinosaur) c Equisetites lyelli (Pteridophyta: Calamites — horsetail)

d Onychiopsis mantelli (Pteridophyta: Fern) e Cypridea valdensis (Arthropoda: Crustacea: Ostracoda)

f Cypridea spinigera (Arthropoda: Crustacea: Ostracoda)

g Cypridea rotundata (Arthropoda: Crustacea: Ostracoda)

h Viviparus sussexiensis (Mollusca: Gastropoda:

Mesogastropoda) i Pseudunio valdensis (Mollusca: Bivalvia: Unionoida)

j Lepidotes mantelli (Vertebrata: Osteichthyes) k Sternbergella cornígera (Arthropoda:

Crustacea: Ostracoda) 1 Mantelliana mantelli (Arthropoda: Crustacea: Ostracoda)

m Theriosynoecum fittoni (Arthropoda: Crustacea: Ostracoda)

The illustration summarizes two such faunal associations. At the base of the sequence are ironstone nodules developed at the level of a shell bed rich in Filosina, a burrowing, suspension-feeding heterodont bivalve. The lower inset illustration shows the ostracodes occurring in shales at this level as a 'novL-Cypridea? association with Sternbergella, Mantelliana and Theriosynoecium.

In contrast, the bedding plane shown in the middle of the sketch shows a drifted association of entirely freshwater and terrestrial elements. The semi-infaunal suspension-feeding bivalve Pseudunio, drifted remains of the semi-aquatic horsetail Equise-tites, and some dinosaur vertebra and bones (Iguanodon) perhaps from a decayed drifted corpse. The ostracodes in the shale at this level (shown in the upper inset illustration) are again freshwater, being entirely species of Cypridea. A similar surface assemblage is shown, with unionids such as Pseudunio here accompanied by the freshwater gastropod Viviparus, either live, or drifted after death into shell heaps, strands of Equisetites and the freshwater holo-stean fish Lepidotes, whose rhomboidal scales occur frequently in many Wealden shales and bone beds. This broad association thus differs little from the fresh-brackish association of the Upper and Middle Jurassic already described. Wealden lake deposits are best exposed in Britain in south-eastern England, the Isle of Wight and eastern Dorset.

APTIAN SAND BOTTOM COMMUNITIES (93-95)

These three examples are taken from different horizons in the Lower Greensand Group of the Isle of Wight, Hampshire in England where marine faunas of Aptian age are most easily studied (Casey, 1961). The Lower Greensand is a complex sequence of clays, silts, sands and sandstones, sometimes glauconi-tic, with occasional conglomerates, limestone and chert in some areas of southern England, whilst in the Midlands there are calcareous sponge gravels. Much of the sequence consists of cross-bedded sands (for instance the Folkestone Beds, Woburn Sands and much of the Sandrock of the Isle of Wight) which represents the build-up of submarine dune complexes. This facies, now widely exploited for glass and building sand, is generally barren of fossils, owing to the mobile substrates and also to post-depositional leaching out of all originally calcareous shells. It is thus only at certain levels in the sequence, notably in clays, limestones, silty and argillaceous burrowed sands, or in calcareous concretions which formed at an early stage, that fossils are common.

Although the examples shown are taken from the coastal outcrops in the Isle of Wight, the faunas described below can also be found in Britain in the Weald and to a lesser degree in eastern Dorset. Broadly similar, although not identical, types of fauna can be found in similar facies of the Ryazanian-Aptian of Lincolnshire and Norfolk.

93 Aptian Sand Community (1)

The marine sands and clays of the Lower Greensand (Aptian) of southern England begin with a unit known as the Perna Bed, which is notable for the occurrence of hermatypic corals; indeed this is the only level in the British Lower Cretaceous where hermatypic corals are numerous. Only one genus, Holocystis, is common, occurring in the form of hemispherical masses, sometimes attached to shell debris. Associated epifaunal elements are byssate bivalves such as the large digitate genus Mulletia, and Isognomon, the latter living attached to other shells. Shells also acted as a base for the development of large 'nests' of brachiopods, both rhynchonellids (Sulcirhynchia) and terebratulids (Sellithyris), which may contain hundreds of individuals, some distorted by crowding. Other elements of the epifauna are oysters, notably the large exogyrid Aetostreon, which lived cemented to shell debris or to others of its species, or lying loose. Byssate semi-infaunal bivalves are represented by elongate gervillellids, two species of which are shown here, partly buried in sediment. The preserved infaunal elements are mainly thick-shelled siphonate suspension-feeding bivalves of which the genera Sphaera, Protocardia and Venilicardia are illustrated. Indications of other infaunal elements are given by various other burrow systems; indeed the sands at this level have generally been homogenized by burrowing infauna. Decapod burrows (Thal-assinoides) are particularly conspicuous.

Nektonic elements of the Perna Bed fauna are rather limited; fish are common, and are usually represented by bone debris and teeth. Ammonites are scarce, but the coarsely-ribbed nautiloid Cymtoceras is fairly common. This fauna is found in southern England in the Isle of Wight and the Weald.

Siphonate Bivalve

Fig. 93 Aptian Sand Community (1) a Mulletia mulleti (Mollusca: Bivalvia:

Pterioida) b Isognomon ricordeana (Mollusca:

Bivalvia: Pterioida) c Gervillella sublanceolata (Mollusca:

Bivalvia: Pterioida) d Gervillella alaeformis (Mollusca:

Bivalvia: Pterioida) e Aetostreon latissima (Mollusca:

Bivalvia: Pterioida — oyster) f Sphaera corrugata (Mollusca: Bivalvia: Veneroida)

Protocardia sphaeroidea (Mollusca: Bivalvia: Veneroida)

Venilicardia protensa (Mollusca: Bivalvia: Veneroida) Holocystis elegans (Coelenterata: Anthozoa: Scleractinia) Sellithyris sella (Brachiopoda: Articulata: Terebratullda) Sulcirhynchia hythensis (Brachiopoda: Articulata: Rhynchonellida) Cymatoceras radiatum (Mollusca: Cephalopoda: Nautiloidea) Thalassinoides (Trace-fossil — Crustacea)

94 Aptian Sand Community (2)

The Crackers and Lobster Beds are one of the most fossiliferous units in the Lower Greensand. The former consists of several lines of calcareous concretions in fine grained, bioturbated clay sands (Casey, 1961), the latter of silts and silty clays with small calcareous nodules. The exceptional fossil preservation in these beds allows us to reconstruct a diverse assemblage of organisms, although only a fraction of it is illustrated.

Ammonites are diverse and often abundant; two genera of ribbed forms are shown here, the typically compressed Deshayesites and the inflated genus Roloboceras. Benthic elements include the crab Mithracites and the lobster-like Hoploparia longimana, which may also have been a burrower; the prawn Meyeria (the crustacean which gives its name to the Lobster Beds) is abundant particularly within small nodules at this level.

Other elements of vagrant benthos include diverse gastropods (the deposit-feeding Tessarolax and Anchura, which have long marginal digitations,'are shown here); infaunal gastropods include Turritella. Infaunal bivalves are very common, including several species of trigoniids, heterodonts (Thetironia) and the deep-burrowing Panopea, commonly found in its life position. Semi-infaunal elements are again represented by the elongate Gervillella, whilst bioturbation suggests the presence of a range of soft-bodied infauna, and Thalassinoides (perhaps produced by some of the decapods present) are common.

Fig. 94 Aptian Sand Community (2)

a Meyeria magna (Arthropoda: Crustacea: Malacostraca — decapod)

b Mithracites vectensis (Arthropoda: Crustacea: Malacostraca — decapod)

c Hoploparia longimana (Arthropoda: Crustacea: Malacostraca — decapod)

d Deshayesites forbesi (Mollusca: Cephalopoda: Ammonoidea)

e Roloboceras hambrovi (Mollusca: Cephalopoda: Ammonoidea)

f Gervillella sublanceolata (Mollusca: Bivalvia: Pterioida)

g Yaadia nodosa (Mollusca: Bivalvia: Trigonioida)

h Thetrionia minor (Mollusca: Bivalvia: Veneroida)

i Panopea gurgitis (Mollusca: Bivalvia: Myoida)

j Anchura (Mollusca: Gastropoda: Mesogastropoda)

k Tessarolax fittoni (Mollusca: Gastropoda: Mesogastropoda)

1 Turritella (Haustator) dupiniana (Mollusca: Gastropoda: Mesogastropoda)

m Thalassinoides (Trace-fossil — Crustacea)

Trace Bivalvia The Bottom

95 Aptian Sand Community (3)

This is a further example of a Lower Cretaceous sand bottom fauna, and shows many similar elements to those already discussed, including clusters of brachiopods (Sellithris), epifaunal decapods (Hoploparia) and large thick-shelled coiled oysters (Aeto-streon). Infaunal elements are represented by the ubiquitous decapod burrow Thalassinoides, and two bivalves, the deep bur-rower Panopea and the costate Pterotrigonia. The ammonites shown include two normally coiled genera: the rather compressed ribbed genus Deshayesites, and Cheloniceras. Juveniles of the Cheloniceras are spinose (they are here shown bedded in sediment) whilst the adults have strong, coarse ribs (they are shown swimming). Three loosely coiled forms are illustrated: the giant Australiceras and Epancyloceras, and the smaller Toxoceratoides. Loosely coiled hetermorph ammonites of this type are known from a few genera in late Triassic and mid-Jurassic rocks, but in the latest Jurassic and throughout the Cretaceous they are an important component of most ammonite faunas (e.g. Figs. 102, 103, 105, 107). Many genera and families have long time-ranges and show complex evolutionary patterns: some even re-coil and assume a 'normal', planispiral shell form. The Deshayesites and Cheloniceras (Fig. 95, a, b, f) are both descended from early Cretaceous heteromorph ancestors. The large Aptian heteromorphs (Fig. 95, c, d) reach sizes of nearly a metre across; they commonly form the core of concretions; other concretions may contain masses of juvenile Cheloniceras (Fig. 95, a).

Fig. 95 Aptian Sand Community (3)

a Cheloniceras (juvenile) (Mollusca: Cephalopoda: Ammonoidea)

b Cheloniceras (Mollusca: Cephalopoda: Ammonoidea)

c Australiceras gigas (Mollusca: Cephalopoda: Ammonoidea)

d Epancyloceras (Mollusca: Cephalopoda: Ammonoidea)

e Aetostreon latissima (Mollusca: Bivalvia: Pterioida — oyster)

f Deshayesites grandis (Mollusca: Cephalopoda: Ammonoidea)

g Toxoceratoides (Mollusca: Cephalopoda: Ammonoidea)

h Panopea gurgitis (Mollusca: Bivalvia: Myoida)

i Pterotrigonia mantelli (Mollusca: Bivalvia: Trigonioida)

j Hoploparia longimana (Arthropoda: Crustacea: Malacostraca — decapod)

k Sellithyris sella (Brachiopoda: Articulata: Rhynchonellida)

1 Thalassinoides (Trace-fossil — Crustacea)

Gault Clay Fossils

Cretaceous

ALBIAN CLAY BOTTOM COMMUNITIES (96-97)

The Albian Gault Clay of south-east Britain is a sequence of black and grey clays with lines of phosphatic nodule beds and a rich and diverse marine fauna, often preserved with original aragonitic shell material. As they are traced westwards the clays pass laterally into the Upper Greensand, a complex of terrigenous silts, sands, limestones and similar facies, (see Figs. 97, 98). In Norfolk, Lincolnshire and Yorkshire they pass into a condensed red, sometimes nodular limestone, the Red Chalk. Gault faunas (Jukes-Browne and Hill, 1900, Smart et al. 1966) are rich and diverse; a series of quite distinctive associations can be recognized, reflecting variations in general depth-related factors and substrate conditions. The illustrations show two common situations: one with a soft clay bottom, the other with harder substrates, formed either by shell debris or by phosphatic nodule beds. These phosphates are a particular feature of the Gault, and indeed other, Cretaceous clays. Many nodules are moulds of whole and fragmentary fossils, or burrow fillings; others are irregular and of unknown origin. A combination of subsurface and sea floor alteration of cemented carbonate-rich mudstone seems to be the most likely origin for these nodules, and they are commonly associated with non-sequences, condensed beds and periods of slow or non-deposition. Rather similar nodule beds and faunas occur in other Lower Cretaceous clays in Britain, in Lincolnshire and Yorkshire.

96 Phosphatic Nodule Bed Community

The illustration is based on a phosphatic nodule bed and associated clays found near the base of the Gault in southern England, known as the dentatus-spathi nodule bed, after the ammonites Hoplites dentatus and Hoplites spalhi which are found there. As in Jurassic clays, deposit-feeding protobranch bivalves are particularly common and, as examples, species of Nucula and the radially ribbed Acila are shown. These are accompanied by the byssate, semi-infaunal suspension-feeding arcoid Nannoavis carin-ata and the burrowing, suspension-feeding Linotrigonia fittoni, a rather rare form. Other elements of infauna include scaphopods, carnivorous gastropods and turritellids. Trace fossils are prominent, and the sediment is bioturbated with Planolites, Chondrites and Thalassinoides.

Epifaunal organisms are of two types: those living on the hard surfaces of nodules and shells, and those living on soft or stiff mud. The former include several types of cemented bivalves, such as Atreta, Spondylus and Pycnodonte, and serpulid polychaetes.

Fig. 96 Phosphatic Nodule Bed Community a Hoplites dentatus (Mollusca: Cephalopoda: Ammonoidea) b Anahoplites planus (Mollusca:

Cephalopoda: Ammonoidea) c Heteroclinus nodosus (Mollusca: Cephalopoda: Ammonoidea) d Neohibolites minimus (Mollusca: Cephalopoda: Coleoidea — belemnite) e Inoceramus concentricus

(Mollusca: Bivalvia: Pterioida) f Nucula (Pectinucula) pectinata (Mollusca: Bivalvia: Palaeotaxodonta — nuculoid) g Nucula (Leionucula) ovata (Mollusca: Bivalvia: Palaeotaxodonta — nuculoid) h Acila bivirgata (Mollusca: Bivalvia: Palaeotaxodonta — nuculoid) i Linotrigonia fittoni (Mollusca: Bivalvia: Trigonoida) j Pycnodonte (Mollusca:

Bivalvia: Pterioida) k Plicatula gurgitis (Mollusca:

Bivalvia: Pterioida) I Atreta nilssoni (Mollusca:

Bivalvia: Pterioida) m Spondylus (Mollusca:

Bivalvia: Pterioida) n Entolium orbiculare

(Mollusca: Bivalvia: Pterioida -pectinid) o Nanonavis carinata

(Mollusca: Bivalvia: Arcoida) p Ichtyosaurus (Vertebrata: Reptilia: Euryapsida — ichthyosaur tooth) q serpulids (Annelida)

Mollusca Bivalvia

Soft bottom epifauna include Plicatula, a reclining form, and Entolium, a free-living, and perhaps occasionally swimming pectinid. Inoceramus are particularly common living in clusters attached by byssal threads to each other and to shell debris. The species shown here, ornamented by fine concentric growth striae and ribs is known as Inoceramus concentricus; it ranges throughout much of the Gault.

The nekton consists chiefly of cephalopods. Of the forms with normal coiling, hoplitids are dominant: Hoplites itself, a strongly ribbed genus with a sulcate venter, and Anahoplites, a compressed, feebly ornamented genus. Heteromorphs are represented by the ribbed and spinose Heteroclinus, and coleioids by the small belemnite Neohibolites. Other nekton such as fish and saurians are usually represented by fragments only, like the ichthyosaur tooth shown here. The fauna can be found in southeast England and the south Midlands.

97 Upper Gault Clay Community

This illustration shows an entirely soft bottom faunal association from the Upper Gault Clay. Many elements are identical to, or have evolved from, those of the previous fauna. Hoplitid ammonites are still a major element of the nekton, with the long-ranging species Anahoplites planus common. It is accompanied by Epihop-lites and Dimorphoplites, coarsely ribbed and tuberculate forms with a flat venter, and Euhoplites, a ribbed tuberculate form with a distinctive deep and narrow ventral groove. Other ammonites are the diminutive Hysteroceras, a loosely coiled genus with rounded whorls and strong simple ribs and the larger, ribbed, tuberculate and keeled Mortoniceras. The specimen (f) in the shell bed in the lower part of the figure is an adult Mortoniceras and shows a hornlike extension or rostrum to the ventral part of the aperture, a structure associated with sexual maturity, possibly housing some accessory reproductive organ.

Other elements of nekton are the long ranging belemnite Neohibolites, and various vertebrates such as sharks, here represented by teeth. A diverse epifauna is shown: the small crab Notopocorystes stokesi; the crinoid Nielsenicrinus, which presumably lived rooted in mud; and various gastropods, including the aphorrhaid Anchura, which perhaps fed on plant debris, Pleuro-tomaria, a sponge feeder, and the superficially similar but spinose Nummocalcar. Gyrodes, here shown on the surface, was a carnivore which probably spent much of its time burrowing, and fed on

Fig. 97 Upper Gault Clay Community a Hysteroceras orbignyi (Mollusca: Cephalopoda: Ammonoidea) b Anahoplites planus

(Mollusca: Cephalopoda: Ammonoidea) c Epihoplites (Mollusca:

Cephalopoda: Ammonoidea)

Cretaceous Ecology

d

Euhoplites (Mollusca:

j

Nuculana (Pectinucula) pectinata (Mollusca: Bivalvia: Palaeotaxodonta — nuculoid)

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