High Peak (Sidmouth), East Devon

[SY 092 838]–[SY 131 873]

Highlights

The Otter Sandstone Formation at Sidmouth is the richest active Mid Triassic reptile site in Britain. Ten or more species of amphibians and reptiles have been found here, most of them recently, and the site represents one of the most promising terrestrial reptile localities of its age anywhere in the world.

Introduction

The fossiliferous beds are developed in the series of high cliffs to the west of Sidmouth between Chiselbury Bay [SY 092 838] and Chit Rocks [SY 121 869], and at Port Royal, just east of Sidmouth [SY 1297 8730]. The whole locality (Figure 4.7) is important as one of the most productive sources of tetrapods of Mid Triassic age in Britain and fresh finds are made every year (1980–94) after cliff falls. However, it is difficult and dangerous to collect from the cliff face and most of the fossils have come from fallen blocks on the foreshore, or in situ from ledges at beach level (Figure 4.8).

Whitaker (1869) distinguished 'red sandstone' overlain by 'red marl' in the New Red Sandstone at High Peak [SY 144 858], which is in turn overlain by Cretaceous Upper Greensand, and he reported the first finds of vertebrates from the Otter Sandstone Formation. Lavis (1876) reviewed the Sidmouth coast in more detail, and Seeley (1876a) described a fine lower jaw and other bones of Mastodonsaurus lavisi and a possible Hyperodapedon (=Rhynchosaurus) tooth plate which Lavis had collected. Hutchinson (1879) further reported fossil plant remains that he identified as stems of an equisete or calamite. Ussher (1876), Metcalfe (1884), Carter (1888), Irving (1888, 1892, 1893), Hull (1892) and Woodward and Ussher (1911) discussed the stratigraphy and dating of the coastal section near Sidmouth, with particular attention to occurrences of fossil vertebrate material. Metcalfe (1884) figured remains of Rhynchosaurus, Mastodonsaurus jaws, and other bones collected from fallen blocks near High Peak, while Carter (1888) described further remains, including fish scales and coprolites.

A second phase of work on the Otter Sandstone Formation coast section began in the 1960s. Laming (1966, 1968) and Henson (1970) provided further information on the sedimentology and stratigraphy of the formation. Warrington et al. (1980), Laming (1982) and Warrington and Scrivener (1990) discussed the problems of correlating the Otter Sandstone with other Triassic sequences. Leonard et al. (1982), Selwood et al. (1984), Mader and Laming (1985), Lorsong et al. (1990), Mader (1990), Smith (1990), Purvis and Wright (1991), Smith and Edwards (1991) and Wright et al. (1991) carried out studies on the sedimentology of the Otter Sandstone Formation, focusing on the palaeosols and other climatic indicators. Spencer and Isaac (1983), Milner et al. (1990), Benton (1990c) and Benton et al. (1994) described collections of fishes and tetrapods made between 1982 and 1994 by P.S.S. that greatly enlarged the faunal list.

The Otter Sandstone Formation has been regarded as 'sparsely fossiliferous' (Spencer and Isaac, 1983). This mistaken impression may be the result of the steepness and height of the cliffs and the fact that most fossils so far collected have come from fallen blocks on the shore. The Sidmouth to Budleigh Salterton section has yielded the largest number of remains of fossil reptiles and amphibians from the New Red Sandstone of Devon, and one of the widest ranges of fossil amphibians and reptiles from the British Middle Triassic, and it continues to produce new finds. Type specimens of Mastodonsaurus lavisi and Rhynchosaurus spenceri come from High Peak, and other unusual finds include the ?ctenosauriscid neural spine, the tanystropheid tooth and the exquisite small procolophonids. These small fossils may be of biostratigraphic value.

Description

The Otter Sandstone Formation (Sherwood Sandstone Group) is exposed in a series of fine sea cliffs along the coast from west of Ladram Bay to just east of Sidmouth. The nature of the cliffs was described by all the Victorian authors mentioned above. Whitaker (1869) noted that most of the cliff at High Peak was formed by the 'red marl', which was heavily weathered above the harder 'red sandstone'. Irving (1888, pp. 152–3) stated that the latter was underlain by 'massive, strongly current-bedded (Bunter) sandstones' which continue to the mouth of the Otter River. The succession is summarized below, with measurements estimated from Lavis (1876, fig. 1), on the assumption that High Peak is 155 m high (contour on 6-inch topographic map).

The Otter Sandstone Formation (the 'red sandstone') comprises c.118 m of medium- to fine-grained red sandstones which dip gently eastwards in the coast section. The formation continues northwards to Somerset and eastwards as far as Hampshire and the Isle of Wight beneath younger Triassic sediments (Holloway et al., 1989). It rests unconformably on the Budleigh Salterton Pebble Beds, a 20–30 m thick unit of fluvial conglomerates (Henson, 1970; Smith, 1990; Smith and Edwards, 1991). The contact is marked by an extensive ventifact horizon (Leonard et al., 1982) that represents a non-sequence of unknown duration and is interpreted by Wright et al. (1991) as a desert pavement associated with a shift from a semi-arid to an arid climate.

Calcretes occur abundantly at Otterton Point, Budleigh Salterton (see below), but farther east they are rarer and the formation is dominated by sandstones in large and small channels, with occasional siltstone lenses. The sandstones occur in cycles, often with conglomeratic bases, and fine upwards through cross-bedded sandstones to ripple-marked sandstones. The Otter Sandstone Formation is capped by water-laid siltstones and mudstones of the Mercia Mudstone Group.

Henson (1970), Laming (1982, pp. 165, 167, 169) and Mader and Laming (1985) interpreted the Otter Sandstone Formation as comprising fluvial and aeolian deposits. Sandstones near the base are aeolian, and middle and upper parts of the formation are of fluvial origin; sandstones were deposited by ephemeral braided streams flowing from the south and south-west (Selwood et al., 1984). The comparatively thin mudstones are interpreted as the deposits of temporary lakes on the floodplain. The calcretes indicate subaerial soil and subsurface calcrete formation in semi-arid conditions (Mader and Laming, 1985; Lorsong et ea., 1990; Mader, 1990; Purvis and Wright, 1991). The climate was semi-arid, with long dry periods when river beds dried out, and seasonal or occasional rains leading to violent river action and flash floods.

Recent collections of amphibian and reptile bones have come from the top 40 m or so of the Otter Sandstone Formation and occur in all lithologies, but most commonly in intraformational conglomerates and breccias (Spencer and Isaac, 1983). Lower in the sequence, in breccias exposed west of Chiselbury Bay (Figure 4.7), the abundance of tetrapod finds declines significantly. The bones are generally in a fine- to medium-grained reddish sandstone that often contains clasts of pinkish, greenish or ochreous calcrete and mudflakes up to 20 mm in diameter. The more complete fish specimens are, however, preserved in dark red siltstone, sometimes in association with plants and conchostracan crustaceans. Plant remains are preserved in iron oxide in all the lower-energy deposits, and their occurrence appears to be controlled by the sedimentology.

The only specimens found in situ by Spencer and Isaac (1983, p. 268) came from 'the lowest of three intraformational conglomerates', but these were 'indeterminate bone fragments'. Since 1983, four rhynchosaur specimens (EXEMS 60/1985.284, 285, 292, and 7/1986.3) have been collected in situ from a single horizon at beach level, and a partial rhynchosaur skeleton was found at the top of the foreshore exposures in Ladram Bay in 1990 (EXEMS 79/1992). It is likely that fossils occur at numerous levels throughout the Otter Sandstone Formation, but most have been found in fallen blocks on the shore and locating the original horizons in the cliffs is difficult.

The Victorian authors believed that one or more discrete bone beds occurred at the eastern end of the outcrop. Lavis (1876) and Metcalfe (1884) placed it 'about 10 feet from the top of the sandstone'; Hutchinson (1906) and Woodward and Ussher (1911) placed it 'about 50 feet below the base of the Keuper Marls', some 40 ft (13 m) lower in the section.

Lavis (1876) made his finds in fallen blocks from a 'fossiferous zone' consisting of up to four beds and 'characterized by lithological differences, in as much as the matrix is composed of much coarser sandstone, containing here and there masses of marl varying in size from that of a pea to that of a hen's egg. In these beds ripple-marks are very plentiful. The fragments of bone which are found in this zone seem to be very slightly water-worn'. Metcalfe (1884) gave further details of this locality at High Peak, stating that bones were found in fallen blocks of sandstone from a light-coloured band in the cliff close below the base of the 'Upper Marls' (Mercia Mudstone Group). Carter (1888) recovered bone material and coprolites from this locality.

Hutchinson (1879, p. 384) gave the most detailed account of the fossiliferous horizons. He found equisetalean plant stems in a bed at the top of the sandstone and 'about eight or ten feet above' two or three 'white bands' which appear as clear horizons in the cliff face. Then, 'one or two steps below' the White bands 'is what I venture to call the Saurian or Batrachian band, in which Mr Lavis found his Labyrinthodon; but I cannot exactly say how many feet this band is below the white bands, because the fall down of the under cliff has concealed the stratification at this place; but it may be fifty feet below and amongst the beds of red rock. Be that as it may, the Saurian band rises out of the beach somewhere under Windgate, as the hollow between the two hills is called, and ascends westwards into High Peak Hill, and having proceeded for about half-a-mile, and having attained a height of sixty or seventy feet above the sea, a fall of the cliff enabled Mr Lavis to find his specimens on the beach, and I was so fortunate as to see them soon afterwards.'

Woodward and Ussher (1911, pp. 12–13) summarized an unpublished section drawn up by Hutchinson in 1878 in which he located the bone bed '100 feet above the talus on the beach, and about 50 feet below the base of the Keuper Marls'. No trace of any tetrapod-bearing horizon in the form of a bone bed can be seen today, and there is no evidence that one existed. The Victorian geologists evidently expected to find bones at discrete levels, and had no concept of restricted lenticular deposits, such as channel lags.

The tetrapod fossils (Figure 4.9) and (Figure 4.10) are generally preserved in a fine- to medium-grained, orange to reddish sandstone that often contains clasts, including reworked rhizolith concretions, up to 20 mm in diameter, and claystone intraclasts which may have a pinkish, greenish or ochreous colour. The bones occur as generally isolated elements: jaws, teeth, partial skulls or single postcranial bones, but some occur in articulation. Exceptions are the partial articulated skull and lower jaws of Rhynchosaurus spenceri (EXEMS 60/1985.292), the associated humerus, radius and ulna of that species (EXEMS 60/1985.282), two sets of vertebrae (EXEMS 60/1985.15, 57), and the recently collected partial rhynchosaur skeleton (EXEMS 79/1992), which comprises much of the trunk, the pelvis and the hindlimbs, with the bones in close association, but mostly slightly disarticulated (Benton et al., 1993). The tetrapod bones generally show little obvious sign of abrasion and some tiny procolophonid jaws are exquisitely well preserved. More details of taphonomy are given by Benton et al. (1994).

About half of the identifiable tetrapod bones found are rhynchosaur remains, and most of these are parts of the skull, especially the jaw elements, which have a high preservation potential. The amphibians are represented mainly by skull and pectoral girdle elements, all relatively dense and with characteristic sculpture. The small reptiles are represented by limited postcranial elements, a partial skull (with lower jaws articulated), teeth and small segments of jaw, and the larger archosaur(s) by teeth and vertebrae. Specimens of the fish Dipteronotus and fossil invertebrates (Figure 4.9)J, (Figure 4.10)H J obtained from a claystone lens, east of Windgate, are extremely well articulated and occur in association with a 'still water' fauna of branchiopod crustaceans.

Fauna

The faunal list of invertebrates, fishes, ternnospondyl amphibians and reptiles is compiled from Benton (1990c), Milner et al. (1990) and Benton et al. (1994).

Arthropoda: Crustacea: Branchiopoda

Lioestheria Carapaces of adults and juveniles (BRSUG)

Euestheria Carapaces of adults and juveniles (BRSUG)

Arthropoda: Crustacea: Ostracoda

Two carapaces, apparently representing separate taxa (BRSUG)

Arthropoda: Insecta

Insect wing (BRSUG)

Mollusca: Bivalvia

Taxon unidentified; single valve (BRSUG)

Osteichthyes: Actinopterygii: Neopterygii: Cleithrolepididae

Dipteronotus typhus Egerton, 1854 Complete specimens, pieces of flank, individual scales and spines (EXEMS)

Osteichthyes: Actinopterygii: 'Palaeonisciformes'

Gyrolepis(?) and others

Scales

Sarcopterygii: Dipnoi: Lepisosteidae

Lepisosteus sp. Scales in coprolites

'Temnospondyli' : Mastodonsauridae

Mastodonsaurus lavisi (Seeley, 1876), nomen dubium

Skull fragments and part of a lower jaw (BMNH, EXEMS)

'Temnospondyli': Benthosuchidae

Eocyclotosaurus sp Remains of a skull and other fragments (EXEMS)

'Temnospondyli': Capitosauridae

Capitosauridae incertae sedi Posterior part of mandible (EXEMS)

Anapsida: Procolophonidae

Procolophonid incertae sedis Three small dentaries, a maxilla and an interclavicle (EXEMS, BRSUG)

Archosauromorpha: Rhynchosauridae

Rhynchosaurus spenceri Benton, 1990 Skull and mandible fragments, isolated maxillae and postcranial elements from about 29 individuals (BMNH, BGS(GSM), EXEMS, BRSUG)

Archosauromorpha: Prolacertiformes

Tanystropheus sp. A small tricuspid tooth (EXEMS)

?Prolacertiform jaw fragments and teeth (EXEMS)

Archosauria: Crurotarsi: indet.

Rauisuchids and others(?) Numerous teeth, a jaw, cranial and postcranial elements (BMNH, EXEMS)

Atnniota incertae sedis

?Ctenosauriscid archosaur

?Neural spine (EXEMS)

Interpretation

Attempts to recover palynomorphs from the Otter Sandstone Formation have so far not been successful (Warrington, 1971, and pers. comm. to P.S.S., 1983). Its age is poorly constrained by occurrences of Late Permian miospores in the lower part of the Permo-Triassic succession near Exeter (Warrington and Scrivener, 1988, 1990) and Carnian taxa in the Mercia Mudstone Group, 135 m above the Otter Sandstone Formation. The only other biostratigraphic indicator, the vertebrate fauna itself, is all that is available for consideration. Walker (1969, 1970a), Paton (1974a) and Benton (1990c) favoured a Ladinian age for the fauna, but Milner et al. (1990) argued that an Anisian age was most likely. The association of the perleidid fish Dipteronotus cyphus (Anisian–earliest Ladinian), Eocyclotosaurus (Late Scythian–Anisian), procolophonids (Scythian–Anisian), ?ctenosauriscid (Anisian–Carnian) and ?tanystropheid (Anisian–Ladinian) identifies the Anisian as the only shared date (Figure 4.2).

The remains of three forms of temnospondyl amphibian (M. lavisi, Eocylotosaurus, sp., capitosaur incertae sedis) are abundant in the Otter Sandstone Formation (Figure 4.9). These were all aquatic, superficially crocodile-like forms, and were probably carnivores or piscivores which fed at the waterside. The new eocyclotosaur material represents the first find of a benthosuchid from the Middle Triassic in Britain. It is similar to Eocyclotosaurus species from two European formations: E. lehmani from the Voltzia Sandstone of the Vosges in France and E. woschmidti from the Lower Rot of the Schwarzwald in Germany. There is also undescribed eocyclotosaur material from the Moenkopi Formation of Arizona (Welles and Estes, 1969; Morales, 1987). The remains of Mastodonsaurus lavisi show some resemblance in interorbital proportions and dermal sculpture to material from Coten End and Bromsgrove (Paton, 1974a, pp. 265–82) and these show closest resemblance to M. cappelensis from the Upper Buntsandstein (Anisian) of Baden-Württemburg, Germany (Milner et al., 1990). M. lavisi is the largest temnospondyl in the Otter Sandstone her-petofauna with an estimated skull length of 500–600 mm, and a body length of 2 m or more.

Rhynchosaurus spenceri (Figure 4.9) is the largest species of the genus, with an average skull length of 140 mm, and an estimated body length of 0.9–1 m (Benton, 1990c). The maxilla had two grooves, a major and minor one, which received two matching ridges on the dentary when the lower jaw was in full occlusion. The genus Rhynchosaurus is also recorded from the Bromsgrove Sandstone Formation of the Midlands (R. brodiei at Coten End Quarry, Leamington, and Bromsgrove) and from Grinshill Quarry, Shropshire (R. articeps) (see above). R. spenceri is distinguished from these forms in having a larger skull length (140 mm), a skull that is broader than it is long (otherwise a character of Late Triassic rhynchosaurs) and a tendency for the tooth rows on the maxilla to 'meander'.

Some recently collected procolophonid remains (Figure 4.10)A–C appear to belong to a primitive form, and Fraser (in Milner et al., 1990) suggested that they most closely resembled the Mid Triassic (Anisian) form Anisodontosaurus greeri from the Holbrook Member of the Moenkopi Formation of Northern Arizona, USA. Re-examination of the material by P.S.S. indicates that there may be up to three taxa, and the most closely related forms appear to be Kapes, Tichvinskia and Phaanthosaurus from the Lower and lower Middle Triassic Vetluga series of the Russian Platform.

A tricuspid tooth (Figure 4.10)E, the sole specimen ascribed to the Tanystropheidae, is reminiscent of the teeth of Tanystropheus from the Anisian and Ladinian of Central Europe (Wild, 1980a).

An elongate element from the Otter Sandstone (Figure 4.9)A may provisionally be assigned to a ctenosauriscid archosaur. Comparable occurrences of ctenosauriscids with such elongate spines are Ctenosaurus from the Anisian Upper Buntsandstein of Germany (Krebs, 1969), Hypselorhachis from the Anisian Manda Formation of Tanzania, and Lotosaurus from the Middle Triassic of China.

Dipteronotus cyphus, a deep-bodied perleidid fish, is represented at Sidmouth by many well-preserved partial and complete remains (Figure 4.9)J. Specimens of D. cyphus, including the holotype, have been obtained elsewhere only at Bromsgrove, from the upper member of the Bromsgrove Sandstone Formation. The Otter Sandstone specimens are better preserved than those from Bromsgrove (Gardiner, in Milner et al., 1990). Dipteronotus is known also from the Scythian of Europe and the Carnian/Norian of Morocco.

The only plants so far found in the Otter Sandstone Formation are stems and leaves of large horsetails (Hutchinson, 1879), and recent finds of fossils identified as Schizoneura, a form also known from the Bromsgrove Sandstone Formation (P.S.S., personal observation).

The Otter Sandstone fauna and flora (Figure 4.11) is comparable to that of Bromsgrove. It is also reminiscent of the Scythian/Anisian Upper Buntsandstein and Voltzia Sandstone faunas of Germany and France (Milner et al., 1990), although in these assemblages, Rhynchosaurus is absent. The closest comparable locality is Otterton Point.

Conclusions

The coast at Sidmouth offers vast potential for study of Mid Triassic reptiles. New finds are made all the time in situ and in fallen blocks, with erosion constantly supplying new specimens. This potential and the importance of past finds give the site its conservation value.

References