How to recognize in situ fossil cephalopods: evidence from experiments with modern Nautilus

Abstract

Field and flume experiments with modern Nautilus pompilius establish two prerequisites to recognize in situ preservation of fossil cephalopod shells (soft parts were within body chamber in situ at the time of fossilization): occurrence of the upper jaw within the body chamber and the position of jaws within the body chamber. Morphology of shells and jaws in modern and fossil nautiloids is so similar that these prerequisites can be applied for fossil nautiloids and provide implications for ammonoids. The upper jaws of Nautilus start to move at a water velocity of > 0.2 m/s, when the shells are reoriented with the aperture downstream; jaws are therefore unlikely to be secondarily deposited near the shell aperture by bottom currents. The lower jaws, moved at the velocity of > 0.1 m/s, can be deposited around the shell aperture by weak current (0.1–0.2 m/s in velocity), but never enter the inside of body chamber. Neither jaw is likely to be separately and selectively displaced from the inside of the body chamber through scavenging of the soft parts by burrowing infaunal animals. An upper jaw preserved inside the body chamber, together with a lower jaw, is thus a reliable indicator of in situ preservation; a sole lower jaw preserved around the shell aperture is likely to be secondarily deposited. Sedimentary structures inferring rapid burial events and jaw size are useful as additional evidence. Smaller jaws were more likely to be displaced from the body chamber by scavenging by infaunal animals after in situ burial, so that smaller jaws preserved within the body chamber suggest less scavenging. These findings are crucial to interpreting the taphonomic history and palaeo‐ecology of fossil cephalopods.

Ammonoid Habitats and Life History

“…Additionally, in situ fossil buccal masses are commonly located near the centre of the body chamber, supporting a jaw-function rather than an operculum-function (Schindewolf 1958;Lehmann 1976Lehmann , 1980Lehmann , 1988Kennedy and Cobban 1976;Tanabe 1983;Morton and Nixon 1987;Tanabe and Fukuda 1987;Mapes 1987;Bandel 1988;Nixon 1988Nixon , 1996Seilacher 1993;Westermann 1996;Kennedy et al 2002;Wippich and Lehmann 2004;Doguzhaeva et al 2007;Keupp 2007;Landman et al 2007b. As shown by Wani (2007), in situ preservation of buccal masses in the body chamber of ammonoids coincides with the in situ deposition of the ammonoid shell itself (Chamberlain et al 1981;Wani et al 2005).…”

Scaphites of the “Nodosus Group” from the Upper Cretaceous (Campanian) of the Western Interior of North America

“…This association has generally been interpreted as indicating that the animals lived, died, and were preserved at approximately the same site (Tanabe, 1979;Landman et al, 2007a;Wani et al, 2005;Wani, 2007). If an ammonite floated after death, its soft body would have fallen out, and the jaw would have either been lost or preserved at a different site than the shell.…”

Ammonoid Taphonomy

“…20.10). In addition, the field experiments revealed that neither jaw is likely to be separately and selectively displaced from the inside of the body chamber through scavenging of the soft parts by burrowing infaunal animals (Wani 2007b). These experimental results suggest an upper jaw preserved inside the body chamber, together with a lower jaw, is a reliable indicator of an autochthonous preservation of the jaws; a sole lower jaw preserved around the shell aperture is likely to be secondarily deposited in allochthonous preservation.…”

“…However, there are two scenarios for such jaw occurrence within the body chamber: (1) true autochthonous preservation, or (2) jaws were secondarily deposited within the empty body chamber by water currents (i.e., allochthonous preservation). Based on the laboratory experiments with modern nautili, Wani (2007b) revealed (1) The upper jaws of modern nautili start to move at a water velocity of > 0.2 m/s, when the shells are reoriented with the aperture downstream; jaws are therefore unlikely to be secondarily deposited near the shell aperture by bottom currents; and (2) the lower jaws, moved at a velocity of > 0.1 m/s, can be deposited around the shell aperture by weak currents (0.1-0.2 m/s in velocity), but never enter the inside of body chamber (Fig. 20.10).…”