Ontogeny and covariation in the Toarcian genus Osperleioceras (Ammonoidea)

Abstract

Abstract. -Starting from embryonic (protoconch-ammonitella) and early juvenile shells, which are indistinguishable at the species level, growth curves of Osperleioceras from the Reynesi Subzone (Upper Toarcian) of the Causses Basin (Aveyron, France) show a continuous radiating range of correlated variation in dimensional and ornamental characters, such as involution, whorl compression, rib strength and rib density. This covariation pattern can be observed among single-horizon assemblages, as well as during individual ontogenetic development.The existence of a continuous intergradational series of shells, ranging from stout coarsely ribbed to smooth suboxycone morphologies, rules out functional or ecological selectivity to explain this non-random variability pattern. The complex interdependence of shape and sculpture can be simulated by a model in which sculpture intensity depends on mantle curvature [Guex, 1999].The expression of covariation in subadult specimens since the base of Upper Toarcian reveals a rise in variability, concomitant with a size decrease, both contemporaneous with environmental instability. It developed in successive bursts from a fairly long low variability period spanning the whole Middle Toarcian.Ontogenèse et covariation chez le genre toarcien Osperleioceras (Ammonoidea)

Ammonoid Intraspecific Variability

“…In Mesozoic taxa showing this covariation, a remarkable range of intraspecific variation in ornamentation still remains in forms with the same shell morphology and size (Morard and Guex 2003). Wiese and Schulze (2005) reported a marked variation in the umbilical width of Neolobites vibrayeanus from a funnel-like deepening to a well-developed umbilicus reaching 18 % of the diameter, which did not show a covariation with either the ribbing strength or the degree of inflation.…”

“…• the Triassic (e.g., Silberling 1956;Silberling and Nichols 1982;Dagys and Weitschat 1993a, b;Checa et al 1996;Dagys et al 1999;Dagys 2001;Monnet and Bucher 2005;Weitschat 2008;Monnet et al 2010), • the Jurassic (e.g., Tintant 1963Tintant , 1980Westermann 1966;Sturani 1971;Howarth 1973;Dzik 1985Dzik , 1990aWestermann and Callomon 1988;Mitta 1990;Bhaumik et al 1993;Beznosov and Mitta 1995;Guex et al 2003;Courville and Crôni-er 2005;Morard and Guex 2003;Bert 2004Bert , 2009Morard 2004Morard , 2006Zatoń 2008;Chandler and Callomon 2009;Baudouin et al 2011Baudouin et al , 2012Bersac and Bert 2012a, b), • and the Cretaceous (e.g., Haas 1946;Reeside and Cobban 1960;Kennedy and Hancock 1970;Kennedy and Cobban 1976;Kennedy and Wright 1985;Meister 1989;Kassab and Hamama 1991;Kennedy 1991, 1998;Courville and Thierry 1993;…”

“…He suggested that the covariation between septal suture and shell plication could be explained by functional requirements ("the stiffening of the phragmocone against shear, a strongly incised suture line furnishing a better fixture of the septum against shear and more even distribution of stresses from the septum onto the outer shell vice versa"), particularly when this covariation could be demonstrated in multiple, not closely related genera. By contrast, Morard and Guex (2003) stated that the sculpture probably does not influence suture complexity directly, but that the sculptural and sutural elements both depend on a common third factor, the whorl shape (Guex 2001(Guex , 2003. Casey (1961) illustrated that the shape of the suture line at more or less the same diameter can also vary with ornamentation (tubercles) within the same specimen (Fig.…”

Geographic Information Systems technology as a morphometric tool for quantifying morphological variation in an ammonoid clade

“…Paleontologists recognize numerous examples of interspecific and intraspecific variation in ammonoids (Reeside and Cobban, 1960;Westermann, 1966;Kennedy and Cobban, 1976;Howarth, 1978;Landman et al, 1991;Hohenegger and Tatzreiter, 1992;Dagys and Weitschat, 1993;Landman and Geyssant, 1993;Checa et al, 1996;Dagys et al, 1999;Morard and Guex, 2003;Kakabadze, 2004;Harada and Tanabe, 2005;Monnet and Bucher, 2005;Gangopadhyay and Bardhan, 2007;Gerber et al, 2007;Weitschat, 2008;Monnet et al, 2010;Monnet et al, 2011;De Baets et al, 2013). Some of these examples, such as the ammonoids described by Weitschat (2008), demonstrate a large spectrum of morphological variation that is present among stratigraphically cooccurring specimens of the same species.…”

Latest Toarcian-earliest Bajocian (Jurassic) Grammoceratinae (Hildoceratidae, Ammonitina) of the western Tethys: Their palaeobiogeographic and phylogenetic significance

“…In western Tethys Osperleioceras from Meneghinii Zone has high variability in both size and ornamentation (Morard and Guex, 2003). Some of the more involute Osperleioceras (e.g., o. autheIini [Monestier], o. subbeticum [Rivas]) from the upper part of the Reynesi biozone (Rivas, 1975) have ribbed inner whorls and a striated body-chamber similar to primitive Vacekia (e.g., V. sourensis and V. striata) and may represent its origin (Fig.…”