Morphostructural constraints and phylogenetic overprint on sutural frilling in Late Jurassic ammonites

Abstract

The functional signi cance of frilled septa and complex sutures in ammonoids has generated ongoing debate. The 'classic' hypothesis envisages ammonoid shells and septa as designed for resisting ambient hydrostatic pressure, complex sutures being the evidence of strength in shells for colonization of deep habitats. Here we address the 'suture problem', focusing on the analysis and interpretation of variables in our database of Late Jurassic ammonites not included in previous studies, such as whorl height (W h ), whorl shape (S), shell coiling (WD), taxonomic grouping and basic planispiral shell shape. The results indicate that sutural complexity, as measured by the fractal dimension (D f ) value of the suture line, is positively correlated with W h , and that the sutures of oceanic shells tend to provide, for a given W h value, lower D f estimates than do those of neritic shells. No general trend of increase in sutural complexity was noted for specimens recovered from swell areas belonging to oceanic fringes with respect to those that inhabited epicontinental shelves. In fact, Perisphinctoidea, the clade best represented in the database analysed, shows a higher D f mean value in neritic species than in epioceanic ones. Signi cant differences in sutural complexity were detected for groups of ammonites classi ed according to shell shapes (WD, S). Oxycones and discocones, streamlined potential swimmers, show the highest D f mean values, while spherocones and cadicones, which were presumably vertical vagrants, present the lowest ones. This indicates that sutural complexity was more related to shell geometry than to bathymetry. & Teatinos, 29071 Ma´laga, Spain; Juan A. Pérez Claros [Juan. Perez@urz.uni-heidelberg.de], Geologisch Paläontologisches Institut, Universitaet Heidelberg, Im Neuenheimer Feld 234, D-69120 Heidelberg, Germany; 29th June 2000, revised 4th March 2002 The functional signi cance of uted septa and complex sutures in ammonoids has stirred debate during recent decades (for recent interpretations and controversy, see review and references in

<i>Globacrochordiceras</i> gen. nov. (Acrochordiceratidae, late Early Triassic) and its significance for stress-induced evolutionary jumps in ammonoid lineages (cephalopods)

“…The classical morphofunctional interpretation (often called the Buckland hypothesis) is that increasing suture indentation leads to forms adapted to deeper water depth, because it is supposed to provide buttressing against hydrostatic pressure on the phragmocone (Buckland 1836;Pfaff 1911;Saunders 1995;De Blasio 2008; and references inside). However, this function of the septal folding remains controversial (Saunders 1995;OlÕriz et al 2002;Lewy 2002) and constructional constraints complicate its interpretation (see discussion in Monnet et al 2011) leading to the idea that the evolution of the suture indentation of ammonoids exceeds their functional needs or limits.…”

Sutural complexity in Late Jurassic ammonites and its relationship with phragmocone size and shape: a multidimensional approach using fractal analysis

“…After more than 160 years of debate, the interpretation of extreme septal folding in ammonoids is still controversial, as sutural complexity has been tentatively related to a wide spectrum of functions, including: (1) structural reinforcement of phragmocones against the hydrostatic load or the pressure generated by predator teeth; (2) area increase for mantle‐to‐septum joining; (3) a role in buoyancy control; and (4) a metabolic function linked to respiration or cameral liquid transport (for reviews and references, Raup & Stanley 1971; Ward 1981; Saunders & Work 1996, 1997; Olóriz et al . 1997, 1999, 2002; Pérez‐Claros 1999, 2005; Saunders et al. 1999; Lewy 2002; Pérez‐Claros et al.…”

“…1990; Boyajian & Lutz 1992; Lutz & Boyajian 1995; Olóriz & Palmqvist 1995; Olóriz et al . 1997, 1999, 2002; Pérez‐Claros et al. 2002), and is the methodology we have chosen for our case study.…”

The Evolution and Development of Cephalopod Chambers and Their Shape

“…If septal complexity is biologically constrained by the morphology of the shell [ 76 , 77 ], then the evolutionary drive to enhance early physiological function in the shell that we propose can, at least partially, explain the origin of septal complexity as a consequence of high SA C :V C /SA S :V C . A correlation between conch shape and septal complexity has been observed and can be interpreted in favor of this idea [ 38 , 78 – 80 ].…”