I find the members of the Scaphopoda Class of mollusks to be beautiful and quite distinctive. This post is partly about how that distinctiveness may work against them in paleontological terms.
My interest in these animals was sparked by several fossil shells (shown below) from Dentalium attenuatum Say 1824, an extinct member of this class. These were a gift from a friend who collected them at Windmill Point on the St. Marys River. The formation shedding fossil shells at this location is considered Upper Miocene, deposited about 11.6-5.6 million years ago.
Marine biologist Helen Scales, when describing the various classes of mollusks, had this to say of the members of the Scaphopoda Class:
Scaphopods or tusk shells are fairly self-explanatory: they look like little tusks. Often they live buried in the seabed, head down, with the tips of their shells poking out. (Spirals in Time: The Secret Life and Curious Afterlife of Seashells, 2015, p. 27.)
In a wonderful aside in her discussion of the changing thinking about the relationships among different taxa of the mollusk family, she wrote of Scaphopoda:
The scaphopods continue to be a pain in the neck to identify; we simply don't know enough about them to be sure where exactly they fit in. (Scales, p. 35.)
Though there is uncertainty about the kinship of scaphopods with all of the other mollusk taxa, there is no doubt that they belong in the mollusca phylum. (Scales, p. 28.) Further, we also know that they are the newest of mollusk classes to show up in the fossil record, appearing perhaps as early as the Mississippian in the Carboniferous Period, about 362.5 million years ago. (Patrick D. Reynolds, The Scaphopoda, Advances in Marine Biology, Volume 42, 2002, p. 139.)
To elaborate on Helen Scales' brief description, each of the extinct and extant scaphopods generally positions itself in sea bottom sediment with its larger opening (the aperture) buried and its narrower opening (the apex) exposed. The aperture houses the animal's "creeping" foot and its mouth. Water is drawn in through the exposed apex and waste is excreted from the same opening. (See descriptions of tusk shells in: National Audubon Society Field Guide to North American Seashells by Harald A. Rehder, 1981, p. 655; and Invertebrate Fossils by Raymond C. Moore, et al., 1952, p. 273-275.)
I think it undeniable that the Scaphopoda Class, as a whole, is distinctive: shells of all members share the same morphology, one which readily distinguishes them from other mollusks. At the same time, it may be challenging to differentiate among species within the class. That morphological sameness has long been noted as a potential issue. For example, when the D. attenuatum shells were first described in 1824 by conchologist and entomologist Thomas Say (1787-1834), he noted of the Dentalium genera as a whole:
The species of this genus are very closely allied to each other, and at the same time they exhibit so few characters, that it is with difficulty that some of them are determined. (An Account of Some of the Fossil Shells of Maryland, Journal of the Academy of Natural Sciences of Philadelphia, Volume IV, Part 1, 1824, p. 155.)
They exhibit so few characters: from a taxonomic perspective, that's a fairly damning statement. Later, paleontologist Raymond Moore et al., in Invertebrate Fossils (cited above), characterized the Scaphopoda as being "simple tapered tubular shells [exhibiting] few sorts of difference other than size and surface markings." (Moore, p. 275) The authors, prompted by the general sameness of the various species in this class, concluded that these shells "are not very useful as index fossils." (Moore, p. 275)
Hmmm. Though their dismissal of the shells of scaphopoda as potential index fossils seems to have been sparked solely by the relative uniformity of the morphology of the individual species in the class, Moore et al. may not have been wrong about the possible utility of these shells as index fossils. I turned to Hervey Woodburn Shimer and Robert Rakes Shrock, who, in their Index Fossils of North America (1944), defined an index fossil as:
one which identifies and dates the strata or succession of strata in which it lies. The ideal index species has narrow or restricted stratigraphic range and broad, even worldwide geographic distribution. . . . A genus which has narrow stratigraphic range and rather broad geographic distribution is now considered an index fossil. (Shimer and Shrock, p. 1.)
There's nothing in that definition about whether the difficulty of distinguishing one species from another might affect what could be considered an index fossil, though that could be the case. The actual focus of Shimer and Shrock's definition is on the temporal and geographic distribution of a species:
Is it found in a short and well defined interval in the stratigraphic column and, so, could it be used to identify and date the interval?
Is it distributed widely enough that it could be useful for correlating formations across a broad geographic range?
I wondered what we do know of the distribution of the scaphopods in the Maryland Miocene formations. Of this class of animal here, paleontologist Harold E. Vokes et al., in their useful Miocene Fossils of Maryland (Maryland Geological Survey, second edition 1999), wrote that the scaphopods "occur throughout the [Maryland] Miocene deposits." (Vokes, p. 18.) I became hung up over that "throughout." Undoubtedly, this does not mean that each species in this class is found everywhere in the stratigraphic column. So what does it mean?
The seminal work on the Maryland Miocene published by the Maryland Geological Survey in 1904 is somewhat helpful in teasing this out. (Miocene, Text, Maryland Geological Survey, 1904, p. eviii-evix.) Five scaphopod species are identified in that publication. (I think it interesting that there is no statement in this volume regarding any particular difficulty in distinguishing among these species.) Data presented on the distribution of these five species among the Maryland Miocene formations show they may be widely, though unevenly, distributed in these formations. The distribution of these species across the three formations covered in the report is this: three of the species had been collected in localities in all three formations, two of the species were collected in only one formation (a different one for each). Does that pattern offer any support to using scaphopod species as index fossils or militate against such an application? Frankly, I don't think the data from this particular publication are sufficient to inform such a decision.
A better sense of the adequacy, or, rather, inadequacy, of the scaphopods as potential index fossils may be gained from their complete absence in the stratigraphic work on the mid-Atlantic coastal Miocene formations by paleontologist Lauck W. Ward. In Molluscan Biostratigraphy of the Miocene, Middle Atlantic Coastal Plain of North America (Virginia Museum of Natural History, Memoir Number 2, 1992), he proposed stratigraphic zones for these formations based on the distribution of various mollusk taxa, defining the boundaries of zones in his schema by the first appearance of a mollusk taxon in the stratigraphic column. For instance, the zone from which the D. attenuatum shells in my collection came is the Chesapecten/Glossus fraterna Interval-zone, which begins where fossil shells of the species Chesapecten santamaria first appear, and ends where fossil shells of Glossus fraterna enter the fossil record here. None of Ward's zones in this work involve scaphopods. (There is also no mention of scaphopods in Ward's later work with George W. Andrews titled Stratigraphy of the Calvert, Choptank, and St. Marys Formations (Miocene) in the Chesapeake Bay Area, Maryland and Virginia (Virginia Museum of Natural History, Memoir Number 9, 2008, p. 48).)
Ward characterized the mollusk species that he used in setting the boundaries for the zones as "well represented." Further, confidence in appropriateness of these taxa to mark zone boundaries was heightened
because of the relatively complete phyletic record of the aforementioned taxa and because this record is found over a wide geographic area. (Ward, Molluscan Biostratigraphy, p. 9.)
The scaphopoda species are missing because they, in general, did not meet these index standards. (Ward indicated as much in a private communication.) I would note that, for each molluscan zone, he also listed mollusk species restricted to that zone, as well as, various mollusk species common in, but not restricted to, that zone. Scaphopods are excluded from those lists as well.
In sum, it would appear that our knowledge of scaphophod fossils is limited, here in Maryland and elsewhere. Further, biologist Patrick D. Reynolds has argued that the morphological sameness of the scaphopods, coupled with the taxon's uncertain phylogeny in the mollusk family tree, "renders the scaphopods an enigmatic group." (Reynolds, p. 139, emphasis added.) Indeed.
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