Durlston Bay

[SZ 035 772]–[SZ 039 786]

Highlights

The fish fauna from the Purbeck Limestone Group of Durlston Bay, Swanage is of special interest as it represents the latest occurrence in Britain of typical Jurassic fish assemblages. The fish remains are mostly fairly well preserved and 32 species have been described from this locality.

Introduction

The 2 km stretch of coastal sea cliffs between Peveril Point and Durlston Head near Swanage, Dorset, displays the finest sections of the Purbeck Limestone Group in Britain and comprise the type locality for the Formation (Figure 12.24) and (Figure 12.25). The strata of the Purbeck Limestone Formation here are famous for their exceptionally diverse fauna, which includes mammal remains unique to the early Cretaceous of Britain, and important reptilian material. The remarkable fish fauna has been known since 1816 (Webster, 1816). Museum collections made in the mid- and late 19th century formed part of A.S. Woodward's monograph upon the fossil fishes of the late Jurassic and early Cretaceous succession of the south coast of England (Woodward, 1915–1919). Both freshwater and marine forms occur, including abundant holostcan material and teleost fishes. Durlston Bay is the type and only locality for two species of early teleost, Thrissops molossus and Pachythrissops laevis. It is also the type location for the coelacanth Holophagus purbeckensis.

Most of the vertebrates from Durlston have been obtained from the natural cliff exposures, but some of the more complete fish remains came from underground workings for the Purbeck 'Building Stones'. Although the latter source for fish material is no longer available, the extensive cliff exposures continue to yield new specimens. Many finds were made in the early 19th century by S.H. Beckles, in the course of his excavation of the cliff in an area, just north of the 'Zigzag path', that is, well south of the outcrop of the mammal bed at beach level.

The Purbeck section at Durlston Bay (Figure 12.26) has been described with varying degrees of accuracy by many authors (Austen, 1852, pp. 9–16; Bristow and Fisher, 1857, pp. 245–54; Strahan, 1898, pp. 91–6; Arkell, 1933, pp. 521–9; Clements, in Torrens, 1969a, figs A35–37, 1993). Cope and Clements (in Torrens, 1969a, pp. A57–A64), and MacFadyen (1970, pp. 134–52) gave a history of research on the stratigraphy and palaeontology of the Durlston section.

Description

The section of the Purbeck Limestone Group at Durlston Bay is based on the annotated stratigraphical logs of Clements (in Torrens, 1969a; 1993), where bed numbers are prefixed DB (Figure 12.25) and (Figure 12.26), (Table 12.1). Clements' reappraisal of the stratigraphy provided a bed-by-bed account of the succession (1993, pp. 183–203). Other sections are given by Wimbledon and Hunt (1983, p. 270) and Wimbledon (in Benton and Spencer, 1995, p. 205, fig. 7.14). (Figure 12.25)A details the northern part of Durlston Bay (between Peveril Point and the 'Zig-zag' path), showing the occurrence of numbered Purbeck beds based upon those by Nunn (1992) and Clements (1989a, 1993). The succession in the southern part of Durlston Bay ('Zig-zag' path to Durlston Head) was sketched by Strahan (1898; (Figure 12.25)B).

The Purbeck beds of Durlston Bay were formerly united in the lithostratigraphical Purbeck Limestone Group by Townson (1975), Melville and Freshney (1982) and Clements (1993), although other authors (e.g. Ensom, 1985; Allen and Wimbledon, 1991) have treated them as a formation. Traditionally the beds are divided into three informal units, the 'Upper', 'Middle' and 'Lower' divisions, and the group as a whole has been split into two formal lithostratigraphical formations, the Durlston Formation above, and the lower, Lulworth Formation (Townson, 1975; Clements, 1993). Ensom (1985) formalized the minor lithostratigraphical divisions of Bristow (Bristow and Fisher, 1857, and in Damon, 1884) as members ((Figure 12.26) and (Table 12.1), modified after Clements, 1993, and Benton and Spencer, 1995). Individual beds within the members of the Purbeck Limestone Group are often informally named; this nomenclature is ingrained in the literature and was not dropped during the formal definition of the section by Clements (1993).

The Purbeck Limestone Group is generally taken to span the Jurassic–Cretaceous (Portlandian–Berriasian) boundary. However, ammonites have not been found in these beds, and the stratigraphy is based on palynomorphs, ostracods and gastropods. The position of the boundary between the Cretaceous and Jurassic has been in dispute (Birkelund et al., 1978), but an integrated approach to correlation has made it possible to determine the base of the Berriasian in the section. Recent opinion places the system boundary within the 'Cypris'Freestone Member (Beds DB10–DB33; (Figure 12.26); (Table 12.1)), quite low in the formation (Allen and Wimbledon, 1992).

Fossil fishes have been recorded from several levels in the sequence, between the Mammal Bed (Bed DB83 of the Marly Freshwater Member) at the base of the 'Middle Purbeck Beds' and the Upper 'Cypris'Clays and Shales Member (Beds DB224–DB245), found in the highest part of the 'Upper Purbeck Beds' succession:

Although fish remains are found throughout the scqucncc, the best specimens arc conccntratcd within the Intermarine Member or Upper Building Stones (DB112-DB145) of the 'Middle Purbeck Beds' (Woodward, 1915–1919). Many of the bony fishes in these limestone units are represented by whole or partial skeletons. However, many specimens have been crushed during lithification and details are lost (Woodward, 1915–1919).

Fauna

Thirty-two species of fish have been recorded here (Figure 12.27). Many specimens, however, are labelled merely 'Middle Purbeck Beds: Swanage' (e.g. Woodward, 1915–1919), and they could have come from some of the inland quarries. Fish collections in the following institutions were examined: NHM, BGS(GSM), CAMMZ, CAMSM, DORCM and OUM. References include Davies (1887) and A.S. Woodward (1890, 1895a, 1915–1919).

Chondrichthyes: Elasmobranchii: Euselachii: Hybodontoidea

Asteracanthus verrucosus Egerton, 1854

A. semiverrucosus Egerton, 1854

Hybodus ensis Woodward, 1915–1919

H. strictus Woodward, 1915–1919

Hybodont cephalic spines

Osteichthyes: Actinopterygii: Neopterygii: Halecostomi

Lepidotes minor Agassiz, 1833–1837

L. notopterus Agassiz, 1835–1837

Lepidotes sp.

Macromesodon daviesi Woodward, 1890

Macromesodon sp.

Eomesodon barnesi Woodward, 1906

E. depressus Woodward, 1915–1919

Eomesodon sp.

Proscinates (Microdon)radiatus Agassiz, 1839–1844

Ophiopsis dorsalis Agassiz, 1844

Histionotus angularis Egerton, 1855

Osteichthyes: Actinopterygii: Neopterygii: Halecomorphi

Caturus (Strobilodus)purbeckensis (A.S. Woodward, 1890)

C. tenuidens A.S. Woodward, 1895

Amiopsis (Megalurus)austeni (Egerton, 1858a)

A. damoni Egerton, 1858

Aspidorhynchus fisberi Egerton, 1854

Osteichthyes: Actinopterygii: Neopterygii: Teleostei

Pholidophoristion ornatus Agassiz, 1843–1844

Pholidophorus granulatus Egerton, 1855

Ichthyokentema purbeckensis Davies, 1887

Pleuropholis crassicauda (Egerton, 1858a)

P. longicauda (Egerton, 1858a)

Leptolepis sp.

Pachythrissops laevis Woodward, 1915–1919

Thrissops molossus Woodward, 1915–1919

Osteichthyes: Sarcopterygii: Actinistia

Holophagus (Undina)purbeckensis Woodward, 1915–1919

Interpretation

The shark fauna of the Purbeck Limestone Group at Durlston Bay is similar to other Jurassic selachian assemblages, being composed largely of the isolated teeth, fin and cephalic spines of hybodonts. Four hybodont species have been described from the 'Middle Purbeck Beds' of the Swanage region. Hybodus ensis Woodward, 1915–1919 is a tooth genus ((Figure 12.27)A), although Woodward referred fin spines from the same bed to the same species. H. strictus Agassiz, 1837 was named from a large dorsal fin spine from the Purbeck Limestone of Swanage. Spines have also been referred to Asteracanthus (Egerton, 1854; Woodward, 1915–1919). Both A. verrucosus Egerton ((Figure 12.27)K) and A. semiverrucosus Egerton were named from distinctive large ornamented dorsal fin spines recovered from the 'Middle Purbeck Beds' and housed in the DORCM.

An exceptional primitive neopterygian fish fauna (Figure 12.27) was recovered from the 'Middle Purbeck Beds' of Swanage in the 19th century. Primitive actinopterygians such as those at Lyme Regis or Whitby are not known from the Purbeck Limestone Group of Dorset, though they have been reported from the Lower Purbeck of Teffont, Wiltshire (Woodward, 1891a). The Durlston Bay fauna is dominated by the semionotids and pycnodontid neopterygians. Two species of the semionotid Lepidotes occur at Swanage, including the type of L. minor Agassiz. This large species (up to 0.4 m in length), represented by numerous near complete specimens (Figure 12.27)B, varies between a deep-bodied form with a rounded back (e.g. BGS(GSM) 27975) and a slim-bodied fish (Woodward, 1915–1919). Lepidotes notopterus Agassiz, a more delicate species known from the Lithographic Limestone of Solnhofen, Germany, also occurs. Jaws and common isolated teeth of pycnodonts in the Purbeck Limestone Formation of Durlston Bay are mostly specifically indeterminate. However, more complete and rare specifically diagnostic specimens have been found at this and other Purbeck localities in Dorset and Wilstshire. Macromesodon (Mesodon) daviesi (Woodward, 1890), named upon a nearly complete fish ((Figure 12.27)D) from the 'Middle Purbeck' of Swanage, is a small (up to 0.25 m in length) deep-bodied pycnodont, with saddle-shaped scales ornamented with a row of four or five denticles (Woodward, 1890). The similar pycnodont, Eomesodon, is also present in the Purbeck Limestone Formation of Swanage and the Portland Stone of the Isle of Portland (Woodward, 1915–1919). Eomesodon had a much deeper body than Mesodon and possessed a prominant dorsal 'hump' which overhung the pectoral girdle and head. The head of Eomesodon is also more triangular in profile (Woodward, 1915–1919). Two species from the 'Middle Purbeck Beds' of Swanage are E. barnesi and E. depressus Woodward. The latter species is based upon a poor specimen from the Purbeck Formation of Durlston (Woodward, 1915–1919). Proscinates radiatus Agassiz (Figure 12.27)E was also named from a partial skeleton recovered from the 'Middle Purbeck' of the Swanage area. The species is represented by several specimens better preserved of features than the type (Woodward, 1915–1919). Proscinates possessed a deep and almost disc-shaped body in which the body depth almost equalled the length from the head to the base of the tail. The semionotid and pycnodonts had a specialized dentition of strong, stud-like palatine teeth and chisel-shaped premaxillary teeth, and are thought to have eaten a durophagous diet of molluscs and crustaceans.

The halecostomids are also represented in the Purbeck Limestone Group of Durlston Bay by the macrosemiids, which usually possessed a large and elegant dorsal fin (Bartram, 1977; Frickhinger, 1991) such as Ophiopsis dorsalis Agassiz and Histionotus angularis Egerton (Figure 12.27)H. The type species of the macrosemiid genus Histionotus was based on a partial specimen from the 'Middle Purbeck Beds' of Swanage described by Egerton (1855) as Histionotus angularis. Histionotus was a medium-sized fish with a large head and pronounced dorsal hump, known from the Swanage species, two or three other species from the (Solnhofen) Lithographic Limestone of Germany and France (Woodward, 1915–1919) and H. angularis Egerton from Tisbury, Wiltshire (Woodward, 1915–1919).

The halecomorphs are represented in the Purbeckian fish fauna by several species of the order Amiiformes, with caturids, amiids and aspidorhynchids present in the Durlston Bay sections (Woodward, 1915–1919). The ubiquitous Jurassic genus Caturus is known from fragments in the Purbeck Limestone Group of Dorset, with two species, C. purbeckensis Woodward and C. tenuidens A.S. Woodward named from fragmentary specimens (mostly jaws) from the 'Middle Purbeck Beds' of Swanage ((Figure 12.27)J). Caturus is a medium-sized fish with an elongate, fusiform trunk, robust head and deeply incised forked tail. C. purbeckensis was a much stouter and larger form than C. tenuidens.

The amiid Amiopsis austeni (Egerton) was also first described from a specimen from the 'Middle Purbeck' of Swanage and was assigned the genus Megalurus by Egerton (1858a). It was tentatively distinguished from the contemporaneous species Amiopsis damoni Egerton, but should be considered to be a nomen dubium. Amiopsis is a typical amiid with a large head, small, stout, styliform teeth and powerful jaws. It had an elongate, laterally compressed body, covered in large oval scales, and a convex border to the caudal fin; it is known widely from the Lower Cretaceous of Europe ((Figure 12.27)G).

Ophiopsis dorsalis Agassiz is a robust (up to 0.2 m) form which possesses a fusiform body and deeply forked caudal fin ((Figure 12.27)F; Woodward, 1915–1919). The genus is represented by O. penicillata Agassiz in the 'Lower Purbeck' of Weymouth, Dorset, and the Purbeck Beds of the Isle of Portland and the Purbeck of Wockley, near Tisbury, Wiltshire (represented by O. breviceps Egerton). Species are diagnosed on their scale types, cranial anatomy and fin morphology (Woodward, 1915–1919).

The aspidorhynchids had a long, thin, fusiform body with robust, smooth rectangular scales and an elongated, slender rostrum, which projected out in front and overhung the mandible. They are also characterized by their symmetrically forked tail and relatively tiny paired and unpaired fins. The aspidorhynchids are represented in the Purbeck Limestone of Dorset by Aspidorhynchus fisheri Egerton. This is a species attaining a length of approximately 0.4 m, the head tapering to an acute rostrum, which projected a total of one-third of the length of the cranium (Woodward, 1915–1919). Aspidorhynchus fisheri is the last representative of this important Jurassic genus.

Primitive teleosts are fairly abundant in the rocks of the Purbeck Limestone Group of Dorset and include representatives of the orders Pholidophoriformes, Leptolepiformes and Ichthyodectiformes. Both pectinate-scaled and smooth-scaled forms of Pholidophorous occur in the Purbeck Limestone, and P. ornatus Agassiz and P. granulatus Egerton have been described from the 'Middle Purbeck' of Swanage (Woodward, 1915–1919). Pholidophorus is also known from the Purbeck Group of Weymouth, Isle of Portland, and Teffont, Wiltshire. The pholidophorids of the Purbeck Group are the last occurrence of this primitive teleost group. The Pleuropholidae are only known from one genus, Pleuropholis, which was originally described by Egerton (1854) for a small (<0.1 m) sprat-like form with a tiny mouth, from the 'Middle Purbeck' of Sutton Mandeville, Wiltshire. Other species were later named from the Purbeck Limestone Group of Wiltshire and Dorset. The small and rare Ichthyokentema has been described by Griffith and Patterson (1963). The leptolepids are only represented at Durlston Bay by undiagnostic fragments of the form genus Leptolepis, but there are two species of ichthyodectids and Durlston Bay is the type and only locality for both (Figure 12.26). The first, Pachytbrissops laevis Woodward, is a large (0.5 m in length) species with a fusiform body, deeply forked tail and small head. The second, Thrissops molosus Woodward, is similarly a large form, attaining lengths up to 0.35 m, with a short, deep head and stout body. It also had a strongly incised forked caudal fin, with very delicate lobes. Both genera were covered in very thin and delicate cycloidal scales that are not often preserved as fossils, and had large bony gill-rakers (Woodward, 1915–1919). Thrissops curtus Woodward is distinguished from T. molosus on the basis of smaller size. Nybelin (1966, 1974a) and Forey (1973a) have suggested that Pachythrissops may be an early megalopid elop-iform, as it shares osteological characters with Cretaceous forms and the modern-day tarpon Megalops.

The coelacanth Holophagus purbeckensis Woodward is only known from a single incomplete specimen from the 'Middle Purbeck Beds' of Swanage. The specimen lacks a head, but sufficient of the posterior portion of the body and the dorsal and caudal fins was preserved ((Figure 12.27)C) for Woodward (1915–1919) to erect the species. Holophagus purbeckensis is a stout fish which may have reached lengths of up to 0.4 m, and it possessed scales ornamented with coarse elongated tubercles (Woodward, 1915–1919).

Fossil amphibians have been recovered from bulk sampling two microvertebrate-rich clay horizons in the Cherry Freshwater Member (?Lower Cretaceous) at several localities on the Isle of Purbeck, Dorset (Ensom, 1987, 1988; Ensom et al., 1991). The material includes abundant albanerpetontid salamander remains, and some discoglossid frog material which represent families known from the Middle and Upper Jurassic of Europe. However, more significant are the (as yet) undescribed jaw fragments representing the earliest record of a bactrachosauroidid salamander (Ensom et al., 1991). The initial discoveries were made at Sunnydown Farm, near Langton Matravers [SY 982 788], in two horizons correlated with DB102 and DB101 in Durlston Bay (Clements, 1993). DB102 has also been sampled at three other localties along a 6 km outcrop, including Durlston Bay, and at all these sites microvertebrates were recovered (Ensom et al., 1991).

However, so far, no amphibian material has been recovered from the Durlston Bay section, although P.C. Ensom (pers. comm., 1995) considers this to be partly due to collection failure.

Comparison with other localities

Similar Purbeck Limestone Formation faunas have been recovered from many localities in Dorset and Wiltshire, and these have been documented above. Specifically, these include the microvertebrate-rich beds at Sunnydown Farm [SY 982 788], Suttle's Quarry [SZ 020 777] and Lovell's Quarry [SY 980 790], Dorset (Ensom et al., 1991), and an unconfirmed report of a fossil frog from the Purbeck of Swindon, Wiltshire (Hudleston, 1876). Purbeck fish faunas have been recovered from the 'Lower' and 'Middle Purbeck Beds' in the Vale of Wardour, Wiltshire, the Isle of Portland and Weymouth, Dorset, and from localities in Buckinghamshire (Woodward, 1915–1919).

Farther afield, elements of the Purbeck fish faunas are known in the Late Jurassic of Germany and France (Woodward, 1915–1919). The English Wealden (Early Cretaceous) fish assemblages and those from other parts of western and central Europe also contain remnants of the late Jurassic fauna.

Conclusion

The Purbeck beds of Durlston Bay have yielded one of the most important Late Jurassic fossil fish faunas in the world from which the site derives its conservation value. The Durlston fauna occurs in marine and non-marine rocks that occupy a unique position at the Jurassic–Cretaceous boundary; it includes amphibians and a diverse fish assemblage, including exceptionally preserved early teleosts.

References