Encombe Bay, Swyre Head–Chapman's Pool, Dorset

([SY 937 773]–[SY 955 771])

Highlights

Swyre Head to Chapman's Pool includes an important array of late Kimmeridgian reptile sites.

These have produced various species of turtle, pterosaur, dinosaur, plesiosaur and ichthyosaur, including the plesiosaur Kimmerosaurus and a new theropod dinosaur.

Introduction

The Upper Kimmeridge Clay exposed between Swyre Head and Chapman's Pool also known as Encombe Bay or Egmont Bight (Figure 7.6) has produced a range of fossil reptiles, ichthyosaurs, plesiosaurs, crocodilians and turtles. Many of the specimens have been collected recently. The cliffs are subject to continuing erosion and the section has good potential for future finds. The geology has been described in detail by Cope (1967; in Torrens, 1969a; 1978; in Cope et al., 1980b), and the occurrence of the reptiles has been reviewed by Taylor and Benton (1986). Reptiles from these sections have been described by Brown (1981), Delair (1986) and Clarke and Etches (1992).

Description

The Kimmeridge Clay in this section covers the upper part of the Late Kimmeridgian. It consists of a sequence of grey and bituminous shales and clays with stone bands towards the base ((Figure 7.6)A and B). The sequence according to Cope (1967; in Torrens, 1969a; 1978; in Cope et al., 1980b) is:

The beds have an apparent easterly dip, and the ammonite zones occur in sequence from north-west to south-east (Figure 7.6)A: pectinatus Zone (cliff below Swyre Head–Egmont Bight; [SY 937 773]–[SY 947 772]), pallasioides Zone (Freshwater Steps–Chapman's Pool; [SY 942 772]–[SY 955 771]) and the rotunda Zone, above this, between Egmont Bight and continuing past Chapman's Pool. The succeeding fittoni Zone and the Portlandian occur higher in Hounstout Cliff. The White Stone Band and Middle White Stone Band come to beach level below Swyre Head and 250 m east of that, respectively. The Freshwater Steps Stone Band reaches beach level at Freshwater Steps. The shales and clays of the pallasioides and lower rotunda Zones (i.e. between the top of the pectinatus Zone and the rotunda Nodule Bed) are sometimes known as the Crushed Ammonoid Shales.

The reptile specimens have apparently been collected largely at beach level either in the wave-cut platform west of Freshwater Steps, or in Chapman's Pool (Figure 7.6)A. Although Hounstout Cliff is accessible, no reptile remains have been reported above the lower parts of the rotunda Zone.

Specific localities in the pectinatus Zone include the 'ledges below Swyre Head, [SY 939 773]' for a partial Teleosaurus specimen (DORCM G.347, label), thus eastlecottensis Subzone. Arkell (1947c, p. 78) noted that the White Stone Band occasionally contains 'saurian vertebrae and bones'. A plesiosaur centrum (DORCM G.172) is noted as pectinatus Zone, above highest White Stone Band, west of Freshwater Steps on shore ledge', thus above the Freshwater Steps Stone Band in shales of the paravirgatus Subzone.

Recent finds confirm the occurrence of reptiles in the pectinatus Zone. A vertebra and ribs of a crocodilian were found by R.A. Langham in the White Stone Band, thus base of the eastlecottensis Subzone, to the west of Freshwater Steps. The same collector also found some limb bones of a pterosaur just above the Freshwater Steps Stone Band at Freshwater Steps, thus paravirgatus Subzone. Finally, P.A. Langham found some turtle remains (BMNH R8699) from a horizon in the shales just above the Freshwater Steps Stone Band, about 300 m west of Freshwater Steps, thus pectinatus Zone also.

Brown (1981, p. 301) reported a skull and isolated teeth of the plesiosaur Kimmerosaurus langhami (BMNH R8431) from 'Endcombe Bay' (also known as Egmont Bay) in the Crushed Ammonoid Shales (Figure 7.7). R.A. Langham (pers. comm. to M.J.B., 1982) gave further information on the find stating that it came from a location 'in situ in shale at the base of the cliff approximately 270 m west of Freshwater Steps', thus perhaps shales of the eastlecottensis Subzone at [SY 924 773]. If the find site is generally in the vicinity of Encombe Bay then the specimen could, in fact, come from the upper pectinatus Zone, the pallasioides Zone, or the rotunda Zone; Brown (1981, p. 301) suggested the rotunda Zone. However, Brown et al. (1986) revise the horizon as 'about 2 m above the Middle White Stone Band' in the upper part of the eastlecottensis Subzone of the pectinatus Zone (Cope et al., 1980b; Cox and Gallois, 1981). A second partial skull and mandible with some associated postcranial remains (BMNH R10042) belonging to K. langhami was reported by Brown et al. (1986, pp. 225–34) from the type locality and horizon, in situ about 3 m east of the site of R8431. This was collected by P.A. Langham in 1976.

Other records include phalanges of a pliosauroid (DORCM G639) from 'below Encomb(e) House at… [SY 942 772]', thus just west of Freshwater Steps, and probably the pectinatus Zone. Some plesiosaur vertebrae and a rib (DORCM G5093; BGS(GSM)) came from around [SY 940 773], also presumably pectinatus Zone. A partial ichthyosaur skeleton (BMNH R8693) came from a water-worn platform exposed at low tide, 400 m east of the Yellow Ledge, thus scitulus Zone, much lower down. Clarke and Etches (1992) note a plesiosaur limb bone from a higher horizon, the rotunda Zone, at Chapman's Pool. Other plesiosaur and ichthyosaur specimens are not so well localized (Taylor and Benton, 1986).

All of the finds, as at Kinuneridge Bay, appear to have been made in the shales; Brown (1981, p. 304) notes that BMNH R8431 was preserved in a clay matrix. The preservation of this skull was generally good, and surface ornament was visible. Parts of the skull were slightly crushed and the dentary somewhat 'eroded'. Other specimens from this area are generally isolated postcranial elements (vertebrae and limb bones) or slightly disturbed partial skeletons. Fuller details are given by Taylor and Benton (1986).

Fauna

Testudines: Cryptodira: Thalassemyidae

Pelobatochelys sp. BMNH R8699

Archosauria: Crocodylia: Thalattosuchia

'Teleosaurus sp.' DORCM G.347

Dakosaurus/Metriorhynchus R.A. Langham collection

Archosauria: Pterosauria

Unnamed R.A. Langharn collection

Archosauria: Dinosauria: Theropoda

Gracile theropod (OUM)

Sauropterygia: Plesiosauria: Cryptoclididae

Kimmerosaurus langhami Brown, 1981 Type specimen: BMNH R8431; also BMNH R10042

Sauropterygia: Plesiosauria: Elasmosauridae

'Colymbosaurus sp.' DORCM G.172, G.184

Sauropterygia: Plesiosauria: Pliosauridae

Pliosaurus sp. DORCM G.186, G.639; Etches collection

Ichthyopterygia: Ichthyosauria

Grendelius sp. BRSMG

'Ophthalmosaurus sp.' DORCM G.8, BMNH R8693

Interpretation

The turtle Pelobatochelys is represented by a partial carapace, about 0.4 m long, with remains of limbs (BMNH R8699). The genus is known only from Dorset and was founded on carapace platesfrom Weymouth. If this undescribed specimen from Encombe Bay belongs to Pelobatochelys, the remains include the first record of its limbs.

The partial skeleton of 'Teleosaurus' (vertebrae, ribs, jaws; DORCM G.347) may belong to one of several Kimmeridgian crocodile genera (e.g. Dakosaurus, Machimosaurus, Steneosaurus, Teleosaurus). Exact identification depends on snout length and features of the skull roof which is not preserved.

A gracile theropod dinosaur is represented by a partial skeleton of the hip region in the OUM.

The only fossil reptile from Encombe Bay that has been described is Kimmerosaurus langhami (Brown, 1981, pp. 300–14; Brown et al., 1986). The type specimen (BMNH R8431; (Figure 7.7)) consists of the posterior part of a skull roof, an occiput, partial braincase, partial lower jaws and 11 isolated teeth. The referred material from Freshwater Steps (BMNH R10042) consists of a braincase, mandible, atlas-axis complex and five cervical vertebrae. The skull is 0.3 m long. Kimmerosaurus differs from all other plesiosaurs by the nature of the teeth, which lack the usual longitudinal ridges, and are greatly recurved and elliptical rather than circular in cross-section. The skull is the most lightly built of all species known from the Late Jurassic and there is no sagittal crest on the parietals, a clear difference from all other plesiosaurs. Kimmerosaurus is one of only five genera of Late Jurassic plesiosauroids recognized as valid by Brown (1981), and one of only two species from the Kimmeridgian. The other, Colymbosaurus trochanterius Owen, known from five postcranial skeletons and a number of isolated propodials, is the longest and heaviest English plesiosauroid, measuring 6 m from the tip of the snout to the end of the tail. Brown (1981) referred the two genera to different families, tentatively placing Kimmerosaurus with Cryptoclidus in the Cryptoclididae and Colymbosaurus in the Elasmosauridae. The possibility that Kimmerosaurus might be synonymous with Cryptoclidus was discussed by Brown et al. (1986). Among the available material, the only elements shared by both forms are the anterior cervical vertebrae (in Kimmerosaurus, only in specimen R10042), and these appear closely comparable. Should this be the case, the wider taxonomic status of Elasmosauridae and Cryptoclididae would need to be reviewed.

The elasmosaurids have very long necks, produced by increases both in the number of cervical vertebrae and in the lengths of centra, particularly among the anterior cervicals. The anterior cervicals possess a further distinguishing character, in the development of a lateral keel and an articular face which has either a single shallow concavity or an open V-shape (Brown, 1981). The cryptoclidids, by contrast, have medium-length necks (28–32 cervical vertebrae), and the anterior cervical centra have a deep concavity with a convex rim.

The other plesiosaur remains from Encombe probably belong to Colymbosaurus (DORCM G172, G184, G5093) and Pliosaurus (DORCM G186, G639) respectively.

The ichthyosaur remains (BMNH R8693; DORCM G.8) could belong to one of several genera that occur elsewhere in the British Kimmeridgian (e.g. Macropterygius, Grendelius, Nannopterygius, Ophthalmosaurus). Identification is based on the shape of the skull (e.g. snout length, shape, size and position of openings) or on features of the paddles. A new specimen of Grendelius in the BRSMG will be described shortly (McGowan, in prep.). The taxonomy of Late Jurassic ichthyosaurs is controversial (McGowan, 1976; A. Kirton, pers. comm., 1981), and fragmentary remains are hard to identify.

Comparison with other localities

Late Kimmeridgian reptile sites are rare, and none has been highly productive. Some ichthyosaurs, plesiosaurs and crocodilians have come from units equivalent to those described here at Ringstead Bay, Dorset [SY 75 81], and rare remains from the Hartwell Clay of Buckinghamshire (pallasioides Zone). A referred specimen of Kimmerosaurus (BMNH R1798) came from Weymouth, probably from a cliff exposure between Sandsfoot Castle and the old Portland Ferry Bridge and therefore Early Kimmeridgian in age (Damon, 1884; see Smallmouth Sands report). The disused Kimmeridgian pits on Shotover Hill, Oxfordshire ([SP 558 065], [SP 560 066], [SP 562 066], [SP 564 066], etc.) have yielded some reptiles from the pectinatus Zone (Shotover Grit Sands, Shotover Fine Sands), as well as more abundantly from the Early Kimmeridgian.

Conclusions

The whole coast section from Swyre Head to Chapman's Pool (Encombe Bay) represents the best British Late Kimmeridgian (=Early Tithonian) reptile site. It has produced a selection of marine reptiles that have not yet been described in full. One undescribed turtle may be the first specimen of the poorly known genus Pelobatochelys with limb remains. The two partial skulls and some postcranial remains of Kimmerosaurus langhami Brown, 1981 show that this was a plesiosaur with several unique features that may form part of a lineage separate from the commoner plesiosaurid–elasmosaurid and pliosaur groups. Marine faunas of this age are rare elsewhere in the world, with similarly isolated remains known from France and Germany. At the same time in North America and Africa, the only known faunas are of terrestrial organisms. This potential for future discoveries gives the site its conservation value.

References