Ammonite aptychi from the Lower Jurassic of Port Mulgrave near Whitby, U.K., are reported for the first time in association with ammonites of the Family Hildoceratidae, Subfamily Harpoceratinae. The aptychi are preserved in shale, varying in their completeness and exhibiting a range of sizes, but are identified as Cornaptychus sp. and cf. Lamellaptychus sp. Some Cornaptychus specimens are preserved with ammonites identified as Tiltoniceras antiquum. The aptychi, especially Cornaptychus, are comparable with aptychi associated with ammonites of the Harpoceratinae elsewhere, notably in the Posidonia Shale of Toarcian age in Germany. The ammonite aptychi were further analysed using an Environmental Scanning Electron Microscope (ESEM) in order to determine their composition and assess whether this might have implications for their occurrence and preservation potential compared with that of the ammonite shells.
The term aptychus (plural: aptychi) is a general name given to calcareous or corneous structures (Moore & Sylvester-Bradley 1957) considered to be isolated hard parts of ammonoid shells. Aptychi have been documented in the fossil record since the 19th century (Meyer 1831; Voltz 1837; Quenstedt 1856–58) and superficially resemble the shells of bivalves, for which they have been mistaken in the past (Parkinson 1811). Their function has been discussed extensively in the literature. They have been regarded as opercula (Arkell 1957), i.e. hard plate-like structures used to close the aperture of the shell when the animal was retracted, as jaws of ammonites (Lehmann 1979), or as a combination of the two (Lehmann & Kulicki 1990; Seilacher 1993). They are now generally interpreted as plates that covered the outer surface of ammonite lower jaws (Tanabe & Landman 2002).
Aptychi have been recorded from Palaeozoic and Mesozoic successions. Mesozoic occurrences were reviewed by Engeser & Keupp (2002) and aptychi from the Upper Cretaceous were discussed by Tanabe & Landman (2002). Mesozoic records, which show their widespread geographical distribution, include those from the upper Toarcian of Spain (Martínez 2007), the Middle–Upper Jurassic of Russia (Rogov 2004a, b) and the Lower Cretaceous of Antarctica (Thomson 1972). Ammonite specimens from the Upper Jurassic (Tithonian) Solnhofen Limestone of Germany have yielded complete jaw apparatuses (Keupp 2007), including the first Jurassic ammonite jaw apparatus preserved in situ and in three dimensions (Schweigert 2009).
Aptychi have been found associated with ammonites in, for example, the Upper Cretaceous of Slovenia (Summesberger et al. 1996) and the Middle Cretaceous of Skye, Scotland (Morton 1973). Such discoveries have helped to develop an understanding of the relationships between aptychi and ammonites. They provide information on the function of aptychi relative to their shape, size and morphology, and in recent years have focussed attention on the feeding habits of ammonites and their feeding apparatus (Andrew et al. 2010; Kruta et al. 2011; Tanabe 2011).
Aptychi recorded from the Lower Jurassic of England include isolated specimens from Whitby, which were referred to Trigonellites whitbyensis (Simpson 1884), Ilminster (Moore 1851) and Northamptonshire (Morris 1852). Here, we describe aptychi associated with Harpoceratid ammonites from the Lower Jurassic Lias Group of the Yorkshire Coast at Port Mulgrave near Whitby, and investigate their preservation potential.