Introduction

A selection of ammonites and other fossils from Osmington Mills is shown, both above and below. It should be noted that these are only a few of the specimens illustrated and described in the literature. This selection can be used to give an idea of the type of fossils present in the Osmington Mills area, but the specialist literature needs to be consulted so that full fossil lists can be found and so that names can be corrected and updated if necessary. it would be wrong to give an impression that large numbers of ammonites are readily available here. They are not, and good specimens appear only occasionally. On a brief visit many bivalves will be seen, particularly "Trigonia" and "Pecten" (scallops such as Chlamys), the occasional nautiloid, but probably few signs of ammonites will be encountered. Corals can be found in the Ringstead Coral Bed, with some luck, and providing the correct part of the cliff has been eroded clean by a recent storm. Trace fossils will be seen in some abundance in the Corallian strata.

Ammonites from the Trigonia Clavellata Beds

Two images above show how this ammonite was found, just west of Bran Point, in a loose boulder of ankeritic limestone from the Red Beds of the Trigonia clavellata Formation, one of the higher units of the Corallian Group. A worn side of the ammonite can be seen with well-defined ribs. It is a large specimen, as shown by the ruler for scale (30cm). On the right of it is a turreted gastropod and, further right, remains of a Myophorella ("Trigonia"). Other "Trigonias" can be seen to the left. Above the ammonite in the photograph are some small, white, calcite-filled brachiopods, probably rhynchonellids.

The ammonite has subsequently been collected by Ian Troth, a keen fossil-collector and student at Southampton University, who has arranged its preparation. It is now shown in excellent prepared condition on the left. You would not, at first, think that this was the same specimen! This was the other side of it; in the top photographs this side was still enclosed in the rock. It has well-preserved ribs which reveal it to be a fine specimen of Perisphinctes cautisnigrae Arkell (compare with photograph from Arkell, below). The originally aragonitic shells of the ammonite and the other fossils have been replaced by sparry calcite. Some ankerite (brown iron and magnesium carbonate) seems to coat the ammonite. (Photograph of prepared Perisphinctes cautisnigrae by Barry Marsh. For more ammonite photographs see SOES Geology Collection, Southampton. )

Like many fossils in the Dorset Jurassic limestones, and unlike those in the shales, the ammonite is uncompressed. No compaction is visible, probably because the carbonate sand was cemented into at least a partially lithified state at an early stage, before significant burial. It is interesting to note that giant perisphinctid ammonites occur in carbonate facies in the Upper Jurassic Corallian, as well as in the Upper Jurassic Portland Stone, where they are very well-known (Titanites). At first sight the Corallian Group and the Portland Group do not appear to be very similar, but, in fact, there are many similarities. There are similarities too between the ammonite-bearing Red Beds of the Corallian and the ammonite-bearing Red Bed and associated strata in the Middle Jurassic, Inferior Oolite at Burton Bradstock .

Examples of Corallian ammonites. Ian Troth has identified the specimen shown in the photograph above as belonging to Perisphinctes cautisnigrae Arkell. The photograph at the top here published by Arkell (1935-48) is the holotype of Perisphinctes cautisnigrae Arkell from the Trigonia clavellata Formation, near Osmington and it has the shell preserved. It is from the Damon Collection, Natural History Museum, London, no. C 7843. It is a large specimen, comparable to the one found by the students near Bran Point and is about 3 times the size shown on the zoomed screen. Maximum diameter 356mm. The fragment in the bottom right corresponds to a portion of the outer whorl of the holotype where the ribbing is undergoing modification.

The lower specimen is Perisphinctes (Biplices ?) damoni Arkell. This is holotype from the Trigonia clavellata Formation "near Weymouth". The shell is completely preserved. Damon Collection, Natural History Museum, London, no. 23898. There is a paratype from "Redcliff" and both are probably from west of Osmington, according to Arkell (1935-1948). The maximum diameter of the holotype is 83mm; of paratype 95mm.

More examples of Corallian ammonites from near Osmington Mills. Perisphinctes (Perisphinctes) uptonensis Arkell. This is the holotype from the Trigonia clavellata Formation near Osmington. It is from the Damon collection, Natural History Museum, London, no. C 7367. It is a large specimen, round about 3 times the size seen on the zoomed computer screen - maximum size = 405mm. Perisphinctes (Biplices ?) pachachii Arkell. This is the holotype from the Red Beds of the Trigonia clavellata Formation. It is an internal mould from Arkell's collection. The size is near the screen size, maximum diameter 118mm. See Arkell (1935-48) for full description and details.

Perisphinctes (Dichotomosphinctes ?) osmingtonensis Arkell, examples of an apparently smaller perisphinctids from Osmington Mills and elsewhere, as labelled in Arkell (1935-48) in plate 9. This is in a relatively early part of his monograph. Later in Section 4: The Perisphinctids of the Ampthill Clay and Upper Calcareous Grit, he modified his views and labelled this as Perisphinctes (Arisphinctes) osmingtonensis. Arkell commented that the later development of septa and ribs had been seen on a specimen discovered in fallen Red Bed material at Ringstead in 1946. He concluded that the specimen was, in fact, an Arisphinctes.

Small ammonites (Cardioceras-like in appearance) from the Red Beds include Amoeboceras (Prionodoceras) spp. (see Arkell (1935-48) , text-fig. 136).

Ammonites from the Osmington Oolite Formation

Cardioceras (Maltoniceras) maltonense (Young and Bird) has been found in a block from Osmington with an Osmington Oolite matrix (Arkell, 1935-48 ).

Ammonite from Black Head, Osmington Mills

This is a specimen of Amoeboceras (Priodonoceras) glosensis (Bigot and Brasil) found by Ian Troth in a fallen block at Black Head. The strata at the foot of the cliff is part of the Osmington Oolite, so this specimen cannot have come from below this level. Ooids are attached to the ammonite. Gastropods, probably Pseudomelania , were found in association. This ammonite is known from the Red Beds of Black Head and the Red Beds of Perry Ledge (east of Bran Point). See Arkell (1935-48, plate XII and p. 48 et seq.) for more information. The species was figured by Damon (1888, Supplement). (Photograph by Barry Marsh. For more ammonite photographs see SOES Geology Collection, Southampton. )

Ammonites from the Preston Grit

Cardioceras (Cardioceras) cautisrufae Arkell and Cardioceras (Scoticardioceras) excavatum has been found in the Preston Grit at Redcliff Point. A larger coarse-ribbed ammonite from the same bed is Aspidoceras (Euaspidoceras) paucituberculatum. This has been described as a giant spinous species, septate to at least 316 mm and probably reaching a diameter of 450mm when fully grown (Arkell, 1935-1948). Also present at Redcliff Point is Aspidoceras (Euaspidoceras) akantheen S. Buckman, Aspidoceras paucituberculatum Arkell, Cardioceras (Cardioceras) wrighti Arkell. See Arkell (1935-48) for more details.

Ammonites from the Nothe Grit

Aspidoceras (Euaspidoceras) catena (J.de C. Sowerby) occurs in the Nothe Grit at Redcliff Point (Arkell, 1935-1948).

More on Ammonites

Discussion of ammonites here is limited and largely based on old publications. For more up-to-date information on Upper Oxfordian ammonites, please see Wright (1986; 1996) and references in these papers. The Spaunton Sandstone of Yorkshire has ammonites species present in the Trigonia Clavellata Formation, which has a non-sequence at its base (Wright, 1996).

Nautiloids

This nautiloid was found in one of the ledges formed by limestones at the base of the Osmington Oolite Formation near Bran Point. Note the simple form of the septa which separate the chambers. These are quite different from the complex septa and septal sutures of the ammonites. The animal was presumably similar to the pearly nautilus of the present day. Notice that some of the chambers contain sediment, now lithified into limestone, whereas others were presumably empty because they have become filled with sparry calcite.

Brachiopods (including a basal Kimmeridge Clay species)

One of the most well-known of the Dorset brachiopods is the asymmetric rhynchonellid - Torquirhynchia inconstans (J. Sowerby). In older literature this was referred to by the synonyms Rhychonella inconstans and Rhactorhynchia inconstans. In theory it should be found in the basal Kimmeridge Clay, Pictonia baylei Zone, at Ringstead, Osmington Mills, Black Head and at Portland Harbour. The exposures were much better in the past and it is not easy now to find it at all these localities. Fortunately, the inconstans Bed is, at present (2008), well-exposed about 200m. northwest of the Slipway at Osmington Mills. The specimen shown above was found there. They are not numerous, but another was discovered nearby within a few minutes. There seems to have been some recent erosion of the cliff by storm waves. The equivalent of the Ringstead Coral Bed is underneath but does not usually contain corals at this particular locality.

See Brookfield (1978) for discussion of the possibly intertidal environment in which this brachiopod lived. Note that this fossil is uncrushed and therefore unlike most fossil remains in the Kimmeridge Clay.

BIVALVES

Ctenostreon proboscideum

Shells of Myophorella clavellata (J. Sowerby)

A loose block on the shore near Osmington Mills with Myophorella clavellata, shown at two magnifications. Note the thick shell, presumably originally of aragonite, but now replaced by calcite. This bivalve had a robust shell, probably well able to withstand storm wave conditions.

Here are two views of an isolated specimen of Myophorella clavellata (J. Sowerby). Unfortunately, it is somewhat worn but the tubercules show clearly. It belongs to the superfamily Trigoniacea and used to be called "Trigonia". (In technical descriptions: the Myophorella shell is trigonally ovoid with anterior erect inward pointing beaks and obtuse posterior carinae; posterior slope on each valve is bipartite with little ornament.) Myophorella is a genus which ranges from the Lower Jurassic to the Lower Cretaceous and is cosmopolitan. It was a shallow inshore burrower like the modern cockles (Cardium) and was a suspension feeder, filtering the water for microplankton.

The particular specimen here is paired, with both valves present and closed. It is easy to imagine these in large numbers on the Jurassic shore. In Dorset Myophorella is common not only in the Corallian but also in the Portland Stone, where they can be clearly seen in the Roach Bed at the top (and in other parts). Like the Corallian the Portland Stone is of shallow-water origin, and the Roach Bed, in particular, was probably deposited in seawater only a metre or two in depth.

"Trigonia" - ancient and modern. Here a Jurassic "Trigonia" or Myophorella, actually from the Kimmeridge Clay, is shown alongside its modern relative Neotrigonia margaritacea, the Australian Broach Clam, a "living fossil". This is a solid triangular shell with granulated ribs. The inside is pearly with large hinge teeth. The shell is pinkish white but with the brown periostracum shown here when fresh. The inside is gold tinged. It lives in southeastern Australia, including Tasmania (Dance, 1992). It is not suggested that the modern distant relative is living in the same conditions. It occurs in offshore mud. The general appearance of the shell is likely to have been similar though. The modern form has been used for making various kinds of ornaments, hence the name (Dance, 1992) (This guide by Dance to 500 species of seashells is very useful for finding modern analogues of ancient molluscs and is recommended).

More about the "Trigonias"
(Notes from a poster, undated, by Bryan Wasson a former student of Southampton University)

During the Mesozoic Era, the Trigoniidae became the dominant family of shallow-burrowing bivalves of near-shore habitat. Several bizarre morphological features make this the most unusual large family of bivalves ever to have existed. Forming a coadapted complex, these features enabled Mesozoic trigoniids to burrow with great efficiency, which largely accounts for their evolutionary success. Their adaptive zone includes numerous niches in areas characterised by coarse, shifting substrata. Enormous complex hinge teeth evolved to keep trigoniid valves aligned at wide angles of gape required by a large muscular cockle-like foot. The dentition and by inference, the enormous foot, evolved during the Triassic and ushered in a remarkable adaptive radiation. In many species, unusual arrays of knobs and ridges on the shell surface gripped the sediment to facilitate burrowing.

General Habitat

The adaptive zone of the Mesozoic trigoniids included relatively unstable, dynamic, coarse grain areas of the seafloor. Most populations lived in substrata of grain-supported, arenitic (sandy) lithologies and nearly all lived nearshore, in waters of 10-15m deep. The trigoniids of the Mesozoic were shallow burrowers that had life positions similar to that of Neotrigonia, the only living genus of the family. Trigoniids can be considered the cockles of the Mesozoic, but had less mobility, in part because of the friction of the complex hinge teeth. None the less, Mesozoic trigoniids were advanced bivalves, and had they not suffered almost complete extinction towards the end of the Cretaceous, they would still remain diverse today.

Bizarre Morphological Features

The Foot: It is thought that "Trigonia" had the ability to jump by the use of its unusually muscular T-shaped foot. This has been established by the observation of modern day relations (Neotrigonia - found in Australian waters) and by fossil record analysis. The foot is used for burrowing by means of the shell rocking back and forward. Although not itself fossilised the muscular foot can be traced back through Trigoniid phylogeny by indirect means because of its coadaptive relationship to certain skeletal features.

Hinge Teeth: Trigoniid hinge teeth are large and distinctive, radiating from the beaks far into the interior of the shell. Fossil evidence documents the origin of this type of dentition early in the Mesozoic. Once established, this specialised dentition became conservative, changing little to the present day.

External Ornamentation of the Shell: Unequalled in other families of burrowing bivalves is the variety of ornamentation on the shells. These consist of arrays of knobs, tubercules, short spines and ridges.

Ichnofossils - Trace Fossils

Plant Debris

This specimen is from almost the top of the Osmington Oolite at Bran Point beneath the Nodular Rubble. It consists of argillaceous micrite (marl) with some ooids but also contains an interesting piece of carbonised foliage. Land was probably not far away because plant material is fairly common in the Corallian section at Osmington Mills. The nature of this plant debris is not clear. It might be a fern fragment although it gives an impression of being fairly thick and not just a thin film. Any comments regarding its identity would be helpful.

The specimen was found by Kristen Munro, a student at the SOES, Southampton University, in November, 1999, and I am grateful to him for permission to place the image on the Internet. The specimen is in his possession.