Graf & Cummings (2006):
Morphological Characters and States

Shell Characters

Descriptions of the morphological characters and their states used to construct the matrix of morphological characters used by Graf & Cummings (2006) are given below. It is our intention to eventually illustrate as many of these as possible. Comments, suggestions and potential images should be forwarded to Dan Graf.

Characters are divided into six categories for convenience: Shell, Ctenidia and Labial Palps, Mantle, Other Soft-Anatomical, Life-History, and Larval. These characters have been largely drawn from those employed in published studies. Those previously used characters are coded with the following abbreviations:

• G Graf, D.L. 2000. The Etherioidea revisited: a phylogenetic analysis of hyriid relationships (Mollusca: Bivalvia: Paleoheterodonta: Unionoida). Occasional Papers of the University of Michigan Museum of Zoology 729: 1-21.
• GO Graf, D.L & D. Ó Foighil. 2000. The evolution of brooding characters among the freshwater pearly mussels (Mollusca: Bivalvia: Unionoidea) of North America. Journal of Molluscan Studies 66: 157-170.
• GW Giribet. G & W. Wheeler. 2002. On bivalve phylogeny: a high-level analysis of the Bivalvia (Mollusca) based on combined morphology and DNA sequence data. Invertebrate Biology 121: 271-324.
• HBH Hoeh, W.R., A.E. Bogan & W.H. Heard. 2001. A phylogenetic perspective on the evolution of morphological and reproductive characteristics in the Unionoida. pp. 257-280. In: Bauer G, Wächtler K, eds. Ecology and Evolution of the Freshwater Mussels Unionoida. Ecological Studies 145. Springer-Verlag, Berlin.
• LMD Lydeard, C., M. Mulvey & G.M. Davis. 1996. Molecular systematics and evolution of reproductive traits in North American freshwater unionacean mussels (Mollusca: Bivalvia) as inferred from 16S rRNA gene sequences. Philosophical Transactions of the Royal Society of London B 351: 1593-1603.
• RH Roe, K.J. & W.R. Hoeh. 2003. Systematics of freshwater mussels (Bivalvia: Unionoida). pp. 91-122. In: Lydeard C, Lindberg DR, eds. Molecular Systematics and Phylogeography of Mollusks. Smithsonian Series in Comparative Evolutionary Biology. Smithsonian Books, Washington DC.

Such citations follow the character description in squared brackets. Character state “X” refers to a condition that does not occur among the sample of taxa in our study but that has been of treated as a synapomorphy elsewhere.

1. Shell microstructure. — 0 = Shell composed of two or three aragonitic layers. 1 = Shell composed of both aragonite and calcite. All palaeoheterodonts are coded as having an aragonitic shell (Taylor et al., 1969; Taylor, 1973). [GW/3]

2. Nacre. — 0 = Shell composed of three layers, an outer prismatic layer, a middle layer of lenticular nacre and an inner layer of sheet nacre. 1 = Middle nacreous layer absent. 2 = Both nacreous layers absent. All palaeoheterodonts coded as having a three-layered, prismato-nacreous shell except Etheria and Acostaea, which are both missing the middle layer lenticular nacre (Taylor et al., 1969; Taylor, 1973) [GW/4-5, 8-9, 12]

3. Hinge Dentition type. — 0 = Differentiated hinge teeth, of heterodont type with true cardinal teeth. 1 = Dentition schizodont, typically with both posterior lateral and anterior pseudocardinal teeth, although either may be secondarily reduced or modified. Mytilus is coded as “inapplicable” because it lacks differentiated hinge teeth. [G/1; GW/38-39]

4. Secondary modifications to schizodont dentition. — 0 = Present and unmodified. 1 = Dentition secondarily reduced or edentulous. 2 = Pseudotaxodont. Coded as inapplicable among outgroup taxa lacking schizodont dentition. [G/2-6; HBH/21-22; RH/40-44]

5. Hinge ligament. - 0 = External, opisthodetic and parivincular. 1 = Alivincular or irregularly multivincular. 2 = Amphidetic. Etheria elliptica has an amphidetic ligament (Yonge, 1962: fig. 8). [GW/30-32]

6. Lamellar-ligament fossette. — 0 = Fossette shallow, not V-shaped. 1 = Robust hinge nymphae, resulting in a deep V-shaped fossette at the posterior end of the hinge (Waller, 1990: fig. 1). A V-shaped ligamental fossette occurs in the families Etheriidae, Iridinidae and Mycetopodidae. [HBH/23; RH/45]

7. Number of adult adductor scars. — 0 = Dimyarian. 1 = Monomyarian, with only the posterior adductor remaining in the adult. All palaeoheterodonts have two adductors except Acostaea and Pseudomulleria (Yonge, 1978: fig. 7). [G/8; GW/21]

8. Anterior adductor scar shape. — 0 = Round. 1 = Elongate. Coded as “inapplicable” in Acostaea and Pseudomulleria where the anterior adductor is absent. [HBH/27; RH/54-55]

9. Pedal elevator scars. — 0 = Present but weakly developed, usually as a series of impressions in the umbo cavity or along the buttress of the anterior teeth, if present. 1 = Well-developed, a robust impression under the umbo. 2 = Inconspicuous, apparently hidden behind the hinge. Inapplicable in those taxa where the foot is highly reduced. Coded as “inapplicable” in Mytilus and Astarte (Cox, 1969: N38). This character has not been used in previous phylogenetic analyses of the Palaeoheterodonta.

10. Position of posterior pedal retractor scar. — 0 = Retractor scar not distinct from and generally dorsal to the posterior adductor. 1 = Retractor scar distinct from posterior adductor and generally anterior. Distinctly separate posterior pedal retractors occur widely among palaeoheterodont families. Coded as “missing” in Mytilus, Astarte, V. angasi and P. exilis. [GW/23]

11. Lateral muscle scars. — 0 = Absent. 1 = Present. Coding follows Smith (1983); lateral muscle scars occur only in Neotrigonia and margaritiferids among the ingroup. [G/11; HBH/25; RH/50]

12. Beak/post-larval shell sculpture. — 0 = Simple concentric or absent. 1 = Radial. 2 = Double-looped or zigzag (Modell, 1942: pl. 6). See Ó Foighil & Graf (2000) for electron micrographs of Neotrigonia post-larval sculpture. [G/10; HBH/24; RH/46-49].

13. Adult shell sculpture. — 0 = Shell more or less smooth. 1 = Radial. 2 = Plications. 3 = Zigzag. 4 = Pustules. Palaeoheterodont external sculpture was discussed by Watters (1994). Coded as “missing” among the asymmetrical etheriids. [LMD/14; G/9; HBH/20; RH/39]

14. Valve symmetry. — 0 = Equivalved. 1 = Asymmetrical, usually due to cementation. Etheria, Acostaea and Pseudomulleria attach to hard substrates by cementing (Yonge, 1962, 1978). [G/7; GW/53]

15. Asymmetrical “talon” (Anthony, 1907). — 0 = Absent. 1 = A “claw” or “spur” formed by the remnant of the pre-cemented, post-larval shell (Yonge, 1978: fig. 1). Known only from Acostaea and Pseudomulleria (Yonge, 1962, 1978). Coded as “inapplicable” among non-cementing taxa. This character has not been used in previous phylogenetic analyses of the Palaeoheterodonta.

16. Shell sexually dimorphic. — 0 = Shells of males and females alike. 1 = Shells of males and females externally distinct. Male shells usually less inflated, with a pointed posterior; females generally with a more squared posterior and much inflated. This character has not been used in previous phylogenetic analyses of the Palaeoheterodonta.

Other References to Shell Characters:

• Cox, L.R. 1969. General features of the Bivalvia. pp. N2-N129. In: Moore RC, ed. Treatise on Invertebrate Paleontology, Part N, Mollusca, 6 1: Bivalvia. Geological Society of America and University of Kansas Press, Lawrence, Kansas.
• Modell, H. 1942. Das natürliche System der Najaden. Archiv für Molluskenkunde 74: 161-191.
• Ó Foighil, D. & D.L. Graf. 2000. Prodissoconch morphology of the relict marine paleoheterodont Neotrigonia margaritacea (Mollusca: Bivalvia) indicates a non-planktotrophic prejuvenile ontogeny. Journal of the Marine Biological Association of the United Kingdom 80: 175-176.
• Smith, D.C. 1983. On the so-called mantle muscle scars on the shells of the Margaritiferidae (Mollusca, Pelecypoda), with observations on the mantle-shell attachment in the Unionoida and Trigonioida. Zoologica Scripta 12: 67-71.
• Taylor, J.D. 1973. The structural evolution of the bivalve shell. Palaeontology 16: 519-534.
• Taylor, J.D., W.J. Kennedy & A. Hall. 1969. The shell structure and mineralogy of the Bivalvia. Introduction. Nuculacea - Trigonacea. Bulletin of the British Museum of Natural History (Zoology), Supplement 3: 1-125.
• Waller, T.R. 1990. The evolution of ligament systems in the Bivalvia. pp. 49-71. In: Morton B, ed. The Bivalvia - Proceedings of a Memorial Symposium in Honour of Sir Charles Maurice Yonge, Edinburgh, 1986. Hong Kong University Press, Hong Kong.
• Watters, G,T. 1994. Form and function of unionoidean shell sculpture and shape (Bivalvia). American Malacological Bulletin 11: 1-20.
• Yonge, C.M. 1962. On Etheria elliptica Lam. and the course of evolution, including the assumption of monomyarianism, in the family Etheriidae (Bivalvia: Unionacea). Philosophical Transactions of the Royal Society of London B 244: 423-458.
• Yonge, C.M. 1978. On the monomyarian, Acostaea rivoli and evolution in the family Etheriidae. Journal of Zoology 184: 429-448.

Page last updated 28 December 2007.