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Rafinesquina Constraining functional hypotheses: controls on the morphology of the concavo-convex brachiopod Rafinesquina LINDSEY R. LEIGHTON Leighton, L.R. 1998 12 15: Constraining functional hypotheses: controls on the morphology LETHAIA of the concavo-convex brachiopod Rafinesquina. Lethaia, Vol. 3 I, pp. 293-307. Oslo. ISSN 0024-1 164. The concavo-convex shape of strophomenoid brachiopods has been inferred to be adaptive to a free-lying state. Such functional hypotheses should be constrained by identifylng the factors controlling morphology. A typical strophomenoid, Rafinesquina alternata, is used here to study morphological influences. Specimens of R. alternata were collected from ten localities in Indi- ana, USA. Beds are Upper Ordovician (Richmondian) mudstones and limestones of the Dills- boro and Whitewater Formations. Length and hingeline width of the pedicle valve were mea- sured, and elongation (length divided by width) calculated for each specimen. Specimens were qualitatively identified as geniculate or arcuate. Regression analyses using stratigraphy (time), mudstone percentage (substrate), grainstone percentage (disturbance), ratio of Strophomena planumbona to R. alternata (competition), and length (ontogeny) as independent variables, were performed to determine the factors influencing morphology. Elongation was most strongly influenced by grainstone percentage and the S. planumbona ratio. Populations of R. alternata from grainstone-rich intervals are less elongate than other samples. The lateral mar- gins of transverse R. alternata may have functioned as sediment traps during periods of high turbidity. Alternatively, variation in elongation may be a character displacement due to inter- specific competition with the related S. planumbona, which is inferred to have had a similar life mode. R. alternata specimens found in beds dominated by S. planumbona are more elongate than R. alternata from beds in which S. planumbona is rare or absent. Geniculation was influ- enced by stratigraphic position, suggesting an evolutionary trend, and by limestone percentage. Geniculate individuals are most common in muddier intervals, supporting the hypothesis that geniculation enabled R. alternata to employ an ‘iceberg’ strategy, ‘floating’convex down on the soft muds. The habitat distribution of R. alternata is inconsistent with the hypothesis that con- cavo-convex brachiopods lived convex-valve-up, as suggested by Lescinsky (1995). Both elon- gation and geniculation may be examples of phenotypic plasticity. OFunctional morphology, concavo-convex brachiopods, RAFINESQUINA,Ordovician, Indiana. Lindsey R. Leighton [ [email protected]],Museum ofpaleontology, University ofMichigan, Ann Arbor, MI 48109, USA; 7th October, 1997; revised 1st October, 1998. Functional morphology provides insight into the aute- Many brachiopods of the superfamily Stro- cology of organisms. However, research that demon- phomenoidea, including R. alternata, lacked a functional strates plausibility of a functional interpretation does not pedicle as adults and so lived unattached to the substrate. prove definitively that a given interpretation is correct, Free-lying brachiopods may have been subject to greater merely that the interpretation is feasible. To constrain risk than attached brachiopods from transport by cur- functional hypotheses, the factors influencing the mor- rents or from burial in soft sediments. Many workers phology of an organism should be identified. Moreover, (Lamont 1934; Rudwick 1970; Richards 1972; Alexander as suggested by Savarese ( 1995a, b), the interpretation 1972, 1975, 1989; Alexander & Daley 1994; Bretsky & should be consistent with the organism’s environmental Bretsky 1975; Savarese 1994) have interpreted the exter- occurrence. The intent of this study is to identify the fac- nal morphology of these brachiopods as an adaptive tors influencing morphological change in a common response to the free-lying condition. Rather than possess- Upper Ordovician strophomenoid brachiopod, Rafines- ing the biconvex shape common to most other brachio- quina alternata (Conrad, 1838), in order to clarify further pods, strophomenoids have one convex and one concave the autecology of the species and possibly that of higher valve. In the case of R. alternata, the pedicle valve is con- taxonomic ranks. vex and the brachial valve concave. In addition to stro- 294 Lindsey R. Leightori LETHAIA 3 1 ( 1998) Functional inferences regarding geniculation and plan- view morphology are significant in reconstructing the ecology of R. alternata. Lamont (1934) and, subsequently, Richards ( 1972) inferred that concavo-convex brachio- pods were semi-infaunal organisms, living convex-valve- down. Richards believed that geniculation was a direct response to sedimentation, enabling the brachiopod to keep its commissure above the sediment, and that these brachiopods were well adapted to soft, muddy substrates. More recently, Lescinsky (1995) suggested that concavo- convex strophomenoid brachiopods lived with their con- vex valve up. All of these studies used epibiont distribu- tion on brachiopods to infer orientation and life mode. These results may be strongly influenced by the tapho- nomic interpretation, because post-mortem encrustation of the host obviously is not a reliable indication of life ori- entation. These epibiont distribution studies may demon- strate functional plausibility but are not conclusive. Biomechanical studies (Leighton & Savarese 1995, Fig. I. hlorphological variation in R~~$tiqiiimciitrrtintn i A-D, G-HI, with Strophonieiin ~~i~~llI~tl~JOtll1(E-F \ for comparison. 3-€3. Trans- 1996) suggest that the convex-up position probably is det- verse morphotype, plan vieb,. 3-D. Equidimensional morphompe. rimental to the brachiopod on both soft and hard sub- plan view. LjE-F. 5. plo~~~~t?~~m~c~,plan vie!\. 7G-H. hrcuate iG) and strates, particularly if the individual is geniculate. On liq- Both geniculate (HI morphotypes, profile views, anterior to the left. uid-rich muds, the commissure of a convex-up specimens are oriented convex-valve (pedicie valve) up. A, C, E, G, and H are oriented convex-valve up i pedicle valve for R~~jitim~iciii~~,brachial brachiopod will sink below the substrate, rendering feed- valve for Strophonirnci. ing impossible (Leighton & Savarese 1995, 1996). On hard substrates, the convex profile of a geniculate, con- phomenoids, other strophomenides, such as plectambon- vex-up brachiopod acts as a bluff body to currents, gener- itoids, chonetidines, and productides, as well as some ating sediment scour upstream of the brachiopod, and pterioniorph pelecypods, commonly have a concavo- burying the downstream margin. Unless the individual convex shape. always was oriented with the commissure facing exactly upstream (assuming the highly unlikely possibility that The goal of many of the aforementioned studies has the current never changed direction), the brachiopod been to determine the functional benefits of the concavo- would have to filter large quantities of sediment when convex shape to a free-lying brachiopod. Rajiriesquim feeding (Leighton & Savarese 1995, 1996). Although alteriiata is ideal tor functional studies of the concavo- problematic, this life strategy is plausible; a convex-up convex morphology because the species displays a broad brachiopod living on soft substrates is not. If Lescinsky range of morphological variation. This range of variation (1995) is correct, then geniculate brachiopods would encompasses the morphologies of many other stro- favor a hard substrate. phomenoids. Variation of the external geometry of R. alterntlta is manifested primarily in three ways: Alexander (1972, 1975) was the first to quantify the shape of R. altertzata, and the first to test strophomenoid Plan-view morphology of R. altermta may be strongly life-mode hypotheses biomechanically. On the basis of transverse (wider than long), equidimensional, or flume experiments using models, and correlations rarely elongate (longer than wide; Fig. 1A). between paleoenvironment and morphology, Alexander Plan-view morphology may be alate (sensu Alexander suggested that geniculation was an ecophenotypic 1975, hinge line wider than width at midpoint) or response to high rates of sedimentation, possibly related ovate (hinge line approximately equal in width to mid- to storm events. Alexander’s hypothesis was further tested width). The two aspects of plan-view morphology may (Alexander & Daley 1994) in Upper Ordovician beds of not be independent. Ohio. Assuming that storm events and higher sedimenta- Curvature of the sagittal profile may change abruptly, tion rates occur closer to shore, the percentage of genicu- producing a sharp bend, termed a geniculation, in the late specimens was predicted to increase in nearshore profile. If the brachiopod lives long enough, this environments. This prediction was confirmed; genicula- change in the direction of shell accretion results in tion tended to increase from offshore to nearshore envi- greater relief of the sagittal profile. In contrast, other ronments through four separate shoaling-upwards cycles. individuals within the species may have a flatter, more Subsequent flume work by Alexander (1984) used fos- smoothly arcuate profile (Fig. 1B). sils on a fine sandy substrate. Concavo-convex, strophic LETHAIA 3 1 ( 1998) Controls on the morphology Of RAFINESQC‘INA 295 experiments. Geniculate, fossil specimens of R. alternata ‘floated’ in carbonate muds that were 50% water by vol- ume (Fig. 2). In contrast to geniculate
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