Scaphopoda

From Palaeos

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

Introduction

Scaphopods are a small group of mostly small, infaunal or semi-infaunal, marine mollusks. There are an estimated 500 extant species known. They are fairly common in deeper water environments (being found at depths from 6 to 1,830 meters), and only a few species live nearshore. Because of this they are little known to the average person and, unlike gastropods and bivalves, their shells are rarely washed up on beaches.

The scaphopod shell is quite unlike the usual molluscan form. It is a slender tapering and usually gently curved tube, not unlike a miniature elephant tusk, hence the common name "tusk shells". It is open at both ends, the large end of the shell being the anterior. The exterior has finely spaced growth lines and usually some type of longitudinal ornamentation or ribs. In color it is usually white or yellowish, although one species is a brilliant jade green. Barnes (1980: 434).

Most scaphopods are small; the largest living forms being usually no longer than 8 cm, with the maximum diameter of about 6 mm; and most species being much smaller than this (often only about 4 or 5 mm.). The largest living species, Dentalium vernedei, found off the coast of Japan, has a maximum length of 15 cm. A fossil species of the genus is known to have attained twice that length, with a maximum diameter of well over 3 cm,

A protrusible burrowing conical foot extends from the anterior end; the mantle cavity is large and extends along the ventral side of the animal to the smaller, posterior opening, through which respiratory currents pass. There are no gills, so respiration is accomplished by direct exchange with mantle tissue. The head is poorly-developed. The animal feeds on microscopic organisms, mostly foraminifers and organic detritus. This food is passed to the mouth via tentacles (called captacula) and tentacular cilia, and is processed by the radula.

Scaphopoda first appear in the Ordovician (see also Reynolds for references for the Ordovician date) or the Devonian (Engeser & Riedel) or even the Mississippian, depending on one's interpretation of the earliest forms. Yochelson (1978). They are fairly rare in the fossil record, except for occasional concentrations in Cenozoic deposits.

At one time scaphopods were believed to be closely related to bivalves, as their early development very closely parallels that of marine bivalves (in the course of development the shell is originally bivalve). It was suggested that both groups descended from extinct bivalved mollusks called rostroconchs. Runnegar & Pojeta (1974). There is now strong evidence (Steiner & Dreyer, 2002) that the bivalves diverged from the main evolutionary sequence of the mollusks much earlier than the scaphopods, which are actually much more closely related to cephalopods. According to one hypothesis, both cephalopods and scaphopods evolved from a tiny, semi-infaunal helcionellid ancestor, probably some time in the Late Cambrian. However, Engeser and Riedel (1997) argue for an origin from Devonian conocardioid rostroconchs, as the early scaphopod shell resembles the adult conocardid shell. If this hypothesis is correct, then the scaphopods are by far the last molluscan class to emerge.

The Class Scaphopoda is divided into two orders, the Dentaliida and the Gadilida or Siphonodentalioida). The latter is distinguished by the constriction of the anterior aperture and the shape of the central tooth of the radula (Reynolds). The Dentaliida may or may not be paraphyletic, as they appear much earlier, so it is possible the Gadilida (which first appeared in the fossil record in the Permian) evolved from them. However, the systematics of the group are still poorly worked out.

Credits

MAK021228