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The giant ammonite of the Portland Stone is a perisphinctid of the genus Titanites. It is not clear whether this specimen is Titanites anguiformis Wimbledon and Cope, 1978 or Titanites giganteus (Sowerby). By its stratigraphical position it should be the former. The ammonite is not complete; the outer whorl is missing and an indentation marks its former position. The inner whorls are not properly preserved. This particular example, now a garden ornament, comes not from Portland where such giant ammonites are common, but from the Shrimp Bed of the Portland Freestone near Blackers Hole (just east of it on the cliff top) in the Isle of Purbeck; it was found by me in about 1953 (and proved a heavy load on my back cycling back to Bournemouth). Some septal sutures are present.

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The shells of the ammonites was originally aragonite, lustrous, that like of a modern nautilus shell, and probably coloured. This was lost in solution after burial, probably when the Portland Stone was uplifted above sea-level at the end of the Jurassic or in the early Cretaceous (i.e. Purbeck or Wealden times). A specimen of Titanites above is peculiar in two respects. It does not showing the cavity between the interior fill and the surrounding rock, after the dissolution phase. Of more interest is the fact that this was fractured on the sea-floor, prior to cementation. This feature is common in some other giant ammonites, the Coroniceras bucklandi ammonites in the Lower Jurassic, Blue Lias of Lyme Regis.

Titanites anguiformis is the characteristic giant ammonite of the Portland Freestone Formation. Titanites giganteus, at one time thought to be the common Portland Stone giant ammonite, actually occurs lower down, in the Basal Shell Bed of Portland in equivalent strata (Wimbledon and Cope, 1978).

T. anguiformis has been described by Wimbledon and Cope, (1978) gave a diagnosis as follows:

"Very large, up to 900mm diameter, evolute, round-whorled. Umbilical diameter 48-58 percent. whorl height 19-26 percent, and whorl thickness 21-30 percent of total diameter. At umbilical diameter of 120, 36 ribs; at 140, 34-44; at 170, 38-49; 200, 40-56; 230, 43-56; 260, 44-63; 290, 47-62; 320, 50-65; 350, 52-71; 380, 53-74. Outer whorl with 70 to over 90 ribs, frequent simple ribs with tendency to fading of ribs on body chamber. Growth lines frequently the only ornament towards aperture. Body chamber 340-360 degrees. Holotype - BMC 2956 - from the Roach, top of Portland Stone, Portland (probably from Kingsbarrow Quarry complex)."

Amongst other places, particularly on Portland, Titanites anguiformis has also been found at the base of the Shrimp Bed in Swanworth Quarry, Worth Matravers, Dorset. This is at approximately the same horizon as the specimen shown here.

Wimbledon and Cope, (1978) further commented on this species:

"Very large evolute form with dense strong ribbing. The holotype is 665mm in diameter, has a whorl height of 155mm, whorl thickness of 170mm and an umbilical diameter of 340mm. It has 85 primary ribs on the last whorl... The holotype shows predominantly bifurcatre normal ribs to the brown coloured plain peristome. Paratype C 2927, on the other hand, shows a marked fading of the ribs at maturity with well-developed grown lines... Since this species of Titanites is much more finely-ribbed than other species of this genus, it is readily recognisable in the field. Thus field identification tends to be more reliable than with some other species or genera. Many specimens of the species have been seen in situ, but have proved impossible to extract."

Most of the giant ammonites from the Portland Stone are probably Titanites anguiformis, but do not necessarily assume this. See the paper of Wimbledon and Cope, (1978) and compare any specimen with the coarser-ribbed Titanites giganteus in addition. Large ammonites from the Portland Sands may be Glaucolithites or other genera, and probably not Titanites. Many Portland Stone Titanites can be seen in the fossil garden at the Portland Heights Hotel, on the summit of the Isle of Portland.

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Bivalves and Some Other Fossils

For more information on Portland bivalves see Cox (1928), A synopsis of the Lamellibranchia and Gastropoda of the Portland Beds of England. Part 1 - Lamellibranchia.

For other fossils see: Liassic Fossils,
and
Kimmeridge - Fossils.