Baculites grandis, Hall & Meek, 1856

Westermann, Gerd, 2013, Hydrostatics, propulsion and life-habits of the Cretaceous ammonoid Baculites, Revue de Paléobiologie 32 (1), pp. 249-265 : 261-262

BACULITES GRANDIS

I am here describing the possible events surrounding the life, death and biostratinomy (pre-burial taphonomy) of B. grandis . This story is told in finite terms, but naturally remains largely hypothetical even if based on the experiments and discussion given on previous pages. Life history

Growing up, the baculite followed in its ancestors’ habitat spending about three years among the plankton in the water-column. Life began in a free-floating egg mass in a neutrally buoyant, gelatinous matrix followed by a minute, self-efficient hatchling ( Fig. 8-1a) in the plankton (Westermann, 1990, fig. 8; 1996, figs. 15, 16). The juvenile baculite grew into a megaplankter (> 20 mm, Rhode, 1974; Fig. 8-1b) trapping micro-organisms with a wide-stretched ‘plankton net’ ( Fig. 8-3) while sinking and rising ( Fig. 8-1b), and sometimes diving to the seafloor at maximally 100 m depth or seeking out a seamount produced by a methane vent, to skim for small demersal fauna flushed from the surface, a behavior here called epidemersal ( Figs. 8-1c, d) – still all in sub-vertical posture. During the roughly year-long adolescence stage, the internal shell architecture changed greatly, the phragmocone growing disproportionately while its adapical chambers re-flooded, all the time keeping the animal at neutral buoyancy. During epidemersal feeding episodes, it could recline its long shell by the forward jet forces of its twin-nozzles, aided by its ‘streamlined’ cross-section. Neutral equilibrium occurred only when over half of phragmocone was flooded. To achieve some stability against rolling and minimally against pitching and yawing, its shell had grown slightly dorsally curved and the most adapical chambers were re-emptied to produce a secondary buoyancy unit ( Fig. 6d). Now it could swim and feed anywhere in the upper 100 m of the water-column, including epidemersally ( Figs. 8-2a, b).

The entire growth to maturity took perhaps five years, relatively short for such a large shell but due to the thin septa and rapid emptying rate in a shallow habitat. In comparison, estimates for large deep-water ammonoids, e. g. mesopelagic lytoceratids, range to at least 50 years ( Westermann, 1996).

Soon, a female and somewhat smaller male would mate at mid-water. After its eggs were fertilized internally, the female would produce a large number of them in a neutrally buoyant gelatinous mass, before releasing them into the well oxygenated water.

Death and aftermath

The pre-burial taphonomy or biostratinomy of the shell included a variety of probable events that differed from those of a bipartite orthocone as conventionally reconstructed and are therefore added here. Baculites are commonly found in calcareous concretions within bituminous shale (Tsujita & Westermann, 1998; Landman et al., 2010). Adults may occur alone, commonly incomplete, or are mixed with juveniles, or juveniles form the majority of the baculite fauna – all indicating strong biostratinomic bias. I envision the processes of post-mortem shell transport and deposition to have proceeded the following way:

1. The vertically oriented juveniles, after death naturally or by predator and removal of tissues, became positively buoyant and rose apex-up to the surface of their shallow sea ( Figs. 9-1a.b). Since orientation would remain vertical, decomposition gases were trapped inside the body-chamber, keeping the shell floating with part of its ‘empty’ phragmocone protruding into the air, allowing for extensive surface drifting. Current transport increased still more if part of the aperture was also broken away ( Fig. 9-1d). The chambers flooded and the shell sank to the seafloor, its orientation depending on the location of the flooded chambers – but probably mostly still apex-up ( Figs. 9-1c). When the sea was deep enough (> 100 m?), any ‘empty’ chambers that had remained untouched by post-mortem re-filling, would have imploded ( Fig. 9-1e), just like orthocerids (Westermann, 1985, fig. 7). Presumably this occurred in the pelagic waters along the Pacific coast.

2. Horizontally oriented adults, after natural or violent death and losing at least their tissues ( Fig. 9-2a), also became positively buoyant, but in addition lost their finely calibrated neutral equilibrium and turned apexdown while rising to the surface and beginning to drift ( Fig. 9-2b). Their relatives in oceanic habitats probably lived deeper, and after death their empty chambers would have flooded more rapidly under the higher ambient pressure, so that most of them sank promptly, apex-down to the bottom. This was confirmed by Olivero (2007, fig. 12 F-G), who described vertically embedded Baculites in apexdown orientation from a Santonian deep-water delta system of Antarctica. This seemingly wrong orientation can be explained only by assuming that this baculite had been in neutral equilibrium, i. e. a horizontal swimmer – consistent with Olivero’s illustrations of short body-chambers. Returning to the adult shells continuing to drift apex-down at or near the surface, they would finally encounter and collide with the seafloor, causing a good-sized, adapical part of the fragile, thin-shelled phragmocone to break off ( Fig. 9-2b, c). After rotating to an apex-upward orientation, it became a ‘race’ between water flooding through the siphuncle open at both ends and the uplift produced by removing water-filled chambers, if the remaining shell should resurface or sink down immediately. The shell would finally land, probably under an angle but perhaps with the broken end of the phragmocone up more often than down ( Figs 9-d, e).