This would change the position of the centre of mass, and consequently the orientation of the shell ( Anim. Kakabadzé and Sharikadzé (1993) also proposed that these ammonites might have been able to adjust their bouyancy by shifting cameral fluid within the shell. This alternative orientation would make it possible for heteromorph ammonites to feed off the sea floor. More recently, Kakabadzé and Sharikadzé (1993) proposed that the heteromorph shell had two stable orientations, one with the aperture pointing upwards (as in previous interpretations) and a second with the aperture tilted towards the sea floor. Many workers have cited these upturned apertures as evidence that heteromorphs were pelagic plankton feeders (e.g., Klinger 1980, Westermann 1996). For heteromorph ammonites this model typically predicts upturned apertures ( Anim. Ammonite functional interpretations based on this model are characterised by a single stable orientation, with the centre of mass vertically below the centre of buoyancy. Trueman assumed that the entire living chamber was filled,and used this todeduce the centres of mass and buoyancy of the ammonite. The discussion by Trueman (1941) on this topic is widely quoted (e.g., in the Treatise on Invertebrate Paleontology, Arkell et al. Consequently, the assumption that the soft parts of the ammonite filled the entire living chamber of the shell is not neccessarily correct, and it is possible that without a way of sealing the aperture, ammonites may have been capable of withdrawing further into the shell than has heretofore been acknowledged.īody position in heteromorph ammonites is important because the association of the soft body (e.g., head, arms, viscera) with the shell is a key part of understanding how these animals lived and functioned. Moreover, ammonites lacked a hood or operculum. In contrast, the living chambers of ammonites are very long and often narrow. However, the Nautilus body chamber is short, approximately one third of a whorl in length, and the animal cannot withdraw far. The aperture is also sealed by a tough hood, functionally similar to the gastropod operculum (Wells et al. Nautilus has paired retractor muscles that can pull the head and arms into the shell when the animal is threatened. Moreover, it is unclear how far Nautilus is a valid analogue for ammonites. Fossil cephalopods that do not have such clear analogues, such as the Lower Cretaceous heteromorph ammonites, are more problematical. 1996 regularly coiled ammonites to Nautilus, Chamberlain 1976, 1980). Interpretations of the functional morphology of fossil cephalopods have tended to concentrate on those that resemble extant cephalopods (e.g., belemnites compared to squid, Monks et al. PE NOTE: some links in this article no longer exist 6/2013 INTRODUCTION Body position and the functional morphology of Cretaceous Heteromorph ammonites, Palaeontologia Electronica Vol. Key Words: ammonite, functional morphology, body chamber, anatomy, aperture, hydrostatics, aspinoconic, ancycloconic, hamiticonic, heteromorph, Ancycloceratida, Lower Cretaceousįinal citation: Monks, Neale and Young, Jeremy R. Young, Department of Palaeontology, Natural History Museum, Cromwell Road, South Kensington, London, SW7 5BD, UK. Neale Monks, Department of Palaeontology, Natural History Museum, Cromwell Road, South Kensington, London, SW7 5BD, UK. This analysis suggests that movement of the body would significantly alter the distribution of mass and hence orientation. The effect of this anatomy on the hydrostatics of aspinoconic, ancycloconic, and hamiticonic heteromorph ammonites is analysed. An alternative anatomy is proposed here: the ammonite animal was small, mobile, and capable of moving away from the aperture when threatened. Previous analyses of ammonite functional morphology have assumed that the animal filled the entire body chamber, and that movement of the animal (such as withdrawal of the head and arms into the shell) had little effect on orientation. Plain-language and multi-lingual abstracts Body position and the functional morphology of Cretaceous Heteromorph ammonitesĬopyright Palaeontological Assocation, 28 January 1998
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