Rostroconchia

From Palaeos.org

o Eutrochozoa
`--o MOLLUSCA
   `-+--Solenogastres
     `--+--Caudofoveata
        `--o Testaria 
           `--+--o Polyplacophora 
              `--o Conchifera 
                 |--Tryblidiida
                 |?-- Stenothecoida
                 `--+?--Bivalvia (if molecular phylogeny)
                    `==Helcionelloida (paraphyletic)
                       |  |--+-- †Yochelcionellidae 
                       |  |  `?-- Cephalopoda
                       |  |? -- †Criconarida  
                       |  `-+?--Bivalvia (if transitional fossils)
                       |    `-- †Rostroconchia
                       |         `-- Scaphopoda 
                       `--+--†Paragastropoda
                          `-- Gastropoda

The Rostroconchia are a group of Palaeozoic bivalved molluscs that were originally considered bivalves, but were then regarded as belonging to a distinct class of their own. Pojeta et al. (1972); Pojeta & Runnegar (1976). They are characterized by a single, pseudobivalved shell which enclosed the mantle and muscular foot. The anterior, probably downwards-orientated, part of shell has a gape from which the foot could probably emerged. They probably lived a sedentary semi-infaunal lifestyle.

To date, about three dozen genera and a greater number of species have been described. Most rostroconches are small, often less than two centimeters in length, but large forms, about 15 cm long, can be found in Devonian limestones of the midcontinental United States.

[edit] Physiology and Lifestyle

The following sketch © xxxx from McRoberts (1998) shows the main parts of the rostroconch shell.

Rostroconchs look superficially like bivalves and like them probably had an extensible foot, as indicated by a prominant anterior gape in the shell. However, the shape of the foot, and perhaps the internal anatomy, were probably much closer to the scaphopods, which, according to one theory, they gave rise to.

Rostroconchs began life as a small limpet-like, bilaterally symmetrical protoconch larva, probably (like the scaphopod larva) planktonic. From this the adult, likewise bilaterally symmetrical shell (dissoconch) grew down as a pair of valves.

The adult rostroconch resembles a bivalve superficially, but differs in that there is no functional hinge. Instead of a flexible ligament or articulating system that charcterises the bivalve hinge, and some or all of the shell layers are continuous across the dorsal margin; layers of stiff calcite extend across this region. The effect is that the rostroconch shell appears much like a taco shell. The two shell valves would have been rigidly fixed in place, and the dorsal margin could not have been more than a poorly elastic structure, if that. The shell apparently broke periodically at the margin to permit shell growth.

The posterior of the shell is usually elongated into a flattened tube called the rostrum through which inhalent and exhalent siphons may have emerged. It can be assumed that the animal positioned itself so that the rostrum was above the layers of sediment in which the creature had burrowed; perhaps also the rostrum which may have aided in water filtration.

[edit] Evolutionary History

The earliest known rostroconch genus, Heraultipegma, dates from the earliest Cambrian (Tommotian), but is much more primitive than the typical or true rostroconchs. These appeared and flourished during the Early Ordovician, almost rivalling true bivalves, with which they may or may not have shared a similar lifestyle, in numbers and variety. But with the end-early Ordovician turn-over they declined, and only one order, the Conocardioida, continued until the Permian. Clarkson (1993).

The earlier and more primitive rostroconchs such as Ribeiroia (order Ribeirioida) had a hinge in which all the shell layers traverses the dorsal margin, resulting in a rigid shell. In more advanced forms, like Conocardium, the outer layer does not cross the margin, suggesting an independent step towards the condition already achieved by bivalves. Conocardium has a gape at one end and very pronounced rostrum at the other.

According to Engeser & Riedel (1997), during the Devonian, the conocardoid rostroconchs gave rise to the Scaphopoda, which therefore are technically Rostroconchia (in the same way that birds are dinosaurs).

[edit] Evolutionary Relationships

Rostroconchs almost certainly evolved from helcionelloid molluscs (Pojeta & Runnegar, 1976; Peel, 1991; Wagner, no date) during the early Cambrian. It was originally suggested (Runnegar & Pojeta, 1974; Pojeta & Runnegar, 1976) that they gave rise to the bivalves on the one hand (by separation of the valves and development of a proper hinge), and the Scaphopoda on the other (with the valves forming into a long tube). However, some cladistic and molecular evidence (Waller, 1998; Steiner & Dreyer, 2002) indicates that bivalves are only distantly related to scaphopods, but that cephalopods are closely related to scaphopods.

Embryological evidence (scaphopod protoconchs) supports the thesis that scaphopods evolved from rostroconchs. Engeser & Riedel (1997). Moreover, cephalopods (Pojeta, 1980) or both scaphopods and cephalopods (Runnegar, 2002; Steiner & Dreyer, 2002), are closely related to, and descended from, laterally compressed helcionelloids. Now, these early primitive helcionelloids are precisely the same forms that can be seen as the ancestors of the Rostroconchia [Wagner no date]. So, the implication is that helcionelloids gave rise to both rostroconchs and cephalopods, but not to bivalves, and that rostroconchs in turn were the ancestors of scaphopods.

[Editor's Note: obviously, this is not quite the same as the backbone phylogeny we have adopted for organizational purposes. We warned you that molluscan phylogeny was a mess. ATW060303].

[edit] Systematics and Classification

The Class Rostroconchia (Pojeta, Runnegar, Morris & Newell,1972) is usually divided into two orders, as follows:

Order Ribeirioida (Early Cambrian to Early Silurian) Order Conocardioida (Late Cambrian to Late Permian)

The Ribeirioida are actually a paraphyletic grade, as they give rise to the Conocardioida. The following cladogram by Wagner shows the relationships between the earliest Rostroconchia and their Helcionellid ancestors:

graphic © Peter Wagner

Note that this cladogram has the Bivalvia (pelecypods - in purple) emerging from proto-rostroconchs. This is the most widely accepted hypothesis today, but if it is correct that means that scaphopods cannot have also evolved from Rostroconchs, since cladistic and molecular evidence implies that scaphopods are most closely related to cephalopods, and only distantly related to bivalves. Thus either scaphopods or bivalves, but not both, evolved from rostroconch ancestors. If bivalves evolved separately, that means that they independently developed similar morphologies as a result of exploiting similar infaunal lifestyles, rather than actual ancestor-descent relationships.

[edit] Taxonomy

<==Rostroconchia
   |  i. s.: Hippocardia Brown 1843
   |           |--H. alborza
   |           |--H. bohemica (Barrande 1881)
   |           |--H. cunea (Conrad 1840)
   |           `--H. kermani Hoare & Aghababalu 2001
   |         Riberia apusoides Schubert & Waagen 1903
   |         Heraultipegma
   `--Conocardioida [Conocardioidea]
        |--Pseudobigaleaidae
        |    |--Pseudobigalea Hoare et al. 1982
        |    |    `--*P. crista Hoare, Mapes & Brown 1982 (see below for synonymy)
        |    |--Exalloschema Hoare, Mapes & Yancey 2002
        |    |    `--*E. batilia (Hoare, Mapes & Brown 1982) [=Hippocardia batilia]
        |    |--Minycardita Hoare & Plas 2003
        |    |    `--*M. sectilis Hoare & Plas 2003
        |    |--Baiosoma Hoare, Mapes & Yancey 2002
        |    |    |--*B. pala (Hoare, Steinker & Mapes 1988) [=Hippocardia pala]
        |    |    `--B. cucullata (Hoare et al. 1988) [=Hippocardia cucullata]
        |    `--Hadropipetta Hoare, Mapes & Yancey 2002
        |         |--*H. ancora (Hoare, Steinker & Mapes 1988) [=Hippocardia ancora]
        |         `--H. nevadaensis Hoare & Plas 2003
        |--Bransoniidae
        |    |--Pseudoconocardium Zavodowsky 1960
        |    |    `--*P. licharewi
        |    |--Bransonia Pojeta & Runnegar 1976
        |    |    |--*B. wilsoni Pojeta & Runnegar 1976
        |    |    `--B. robustum (Fletcher 1943) [=Conocardium robustum]
        |    `--Apotocardium Hoare, Mapes & Yancey 2002
        |         |--*A. lanterna (Branson 1965) (see below for synonymy)
        |         |--A. cordatum (Hoare et al. 1982) [=Pseudoconocardium cordatum]
        |         |--A. obliquum (Meek & Worthen 1875) [=Conocardium obliquum]
        |         |--A. plautum Hoare, Mapes & Yancey 2002
        |         |--A. polymitarium (Hoare et al. 1982) [=Pseudoconocardium polymitarium]
        |         `--A. snideri (Morgan 1924) [=Conocardium snideri, Pseudoconocardium snideri]
        `--Conocardiidae
             |--Conocardium Bronn 1835
             |    |--C. acadianum Dawson 1891
             |    `--C. nicholasensis Price 1921 (n. d.)
             |--Oxyprora Hoare, Mapes & Yancey 2002
             |    |--*O. parrishi (Worthen 1890) [=Conocardium parrishi]
             |    |--O. missouriensis (Girty 1915) (see below for synonymy)
             |    `--O. oklahomaense (Beede 1902) [=Conocardium oklahomaense]
             `--Arceodomus Pojeta & Runnegar 1976
                  |--*A. glabrata (Easton 1962) [=Conocardium glabratum]
                  |--A. angusta Hoare, Mapes & Yancey 2002
                  |--A. langenheimi (Wilson 1970)
                  |--A. prolata Hoare & Mapes 1990
                  `--A. sphairikos Hanger et al. 2002

*Apotocardium lanterna (Branson 1965) [=Conocardium lanterna, Pseudoconocardium lanterna]

Oxyprora missouriensis (Girty 1915) [=Conocardium missouriensis, Pseudoconocardium missouriensis]

Pseudobigalea crista Hoare, Mapes & Brown 1982 [=Pseudoconocardium parrishi (Worthen) in Hoare, Sturgeon & Kindt 1978 non Conocardium parrishi Worthen 1890]

* Type species of genus indicated

[edit] References

Hoare, R. D., R. H. Mapes & T. E. Yancey. 2002. Structure, taxonomy, and epifauna of Pennsylvanian rostroconchs (Mollusca). Journal of Paleontology 76 (Suppl.): 1-30.

Hoare, R. D., & L. P. Plas Jr. 2003. Permian rostroconchs (Mollusca) from Nevada. Journal of Paleontology 77 (5): 873-875.

Prothero, D. R. 1998. Bringing Fossils to Life: An introduction to paleobiology. WCB McGraw-Hill: Boston.

Credits

MAK021208; taxonomy CKT071117