Archeomalacology in Texas by Ken Brown

Ken Brown is a visiting researcher at TARL. This article is part of the June 2018 TARL newsletter.

Much of my research over the past couple of decades has focused on reconstructing Texas paleoenvironments by studying snails and sphaeriid clams. Snails are very moisture-sensitive, and most of the state (except east Texas) doesn’t have acid soils, so the shells usually preserve well, except where groundwater or shrink-swell processes have been active. If proper sampling is done, enough specimens can often be recovered to allow some rudimentary quantitative methods to be applied. Although snails are not especially temperature-sensitive, they make good paleo-moisture proxies, and because vegetation and moisture are correlated, snails can also hint at past vegetative cover. Aquatic snails are especially diagnostic of past hydrologic conditions. Some species can tolerate hot, stagnant, poorly oxygenated water with high solute levels, and others require cool, well-oxygenated, fresh flowing water, so an aquatic assemblage can tell us a lot about past stream conditions. Snails are sessile animals with short lifespans, which means that individual field samples capture paleoenvironmental profiles that are restricted in time and space.

Proper sampling means fine-mesh wet-sieving of fairly large samples and picking of whatever shows up, including shell fragments and juveniles. I use a standard nested set of oversize brass geologic sieves (18 inch diameter; Fig. 1). Mesh sizes are #10 (2 mm), #18 (1 mm), and #35 (0.5 mm). The British were the earliest proponents of this kind of research, and they often use one-liter samples, perhaps modeling their sampling methods after those of pollen analysts. But England is a wet place replete with snails, and while one-liter samples may be adequate there, Texas is a lot drier, so my target sample size is usually about 12-15 liters. There are perhaps 185 or so species of terrestrial snails native to Texas, and recent research suggests there might be another 60 or so aquatic species. Many of these (perhaps as many as 60% for terrestrial species?) are too small to be captured on quarter-inch archeological field screens. Most or nearly all the juveniles will fall through standard field screens, even for the larger species, so fine mesh sampling is essential if the complete spectrum of species is to be recovered.

Figure 1. Nested geologic sieves used for sampling snails.

For paleoenvironmental studies, I’ll try to collect a few pilot samples first to find out if snails are preserved at all, what condition they’re in, and how abundant they are. If the site looks promising, I’ll try to collect a continuous column of samples from a representative profile, perhaps in 10 cm vertical increments. To get the required 15 liters from a 5 cm thick sample requires a column about 60-80 cm wide, which is much more extensive than the usual pollen sample column. I try to avoid sampling features, since by definition features are places with extensive human disturbance, but for some reason many of the snail samples reported in the literature (for example, Wilson-Leonard) have come from cultural features.

In the field, I measure out 15 liters of sediment, plus a little extra allowing for shrinkage, bag it in sandbags, and return it to TARL, where it is weighed and the volume re-measured after drying. Then the 15-liter sample is soaked in tapwater overnight (with dispersant if clay-rich) and wet-sieved through nested sieves. Small specimens are identified with a binocular microscope at low magnification (usually no more than 10X). Shell fragments are also saved and weighed, because recovering fragments is the only way to estimate how many fragments have been lost to breakage. Juveniles are tabulated separately and identified if possible. Snails are much like vertebrates, in that individuals often don’t develop species-diagnostic characters until adulthood is reached, so sometimes juveniles can only be identified to the family or genus level. On the other hand, if there is only one species in a genus present at a site, it is sometimes possible to assume that all the juveniles are representatives of the same species. For example if the only adult Gastrocopta found at a site are Gastrocopta pellucida, it is probably safe to assume that any juvenile Gastrocopta belong to that same species. If juveniles can be identified, adult/juvenile ratios can be computed. This will tell us something about juvenile mortality, which is linked to environmental conditions. High juvenile mortality probably means severe water stress, although fecundity can also be an evolutionary adaptation to living in stressful habitats.

Most of the formal archeomalacological studies in Texas (those involving extensive fine sieving of sediments) have been done east of the 98º longitude line, or in other words east of the 32-inch annual precipitation isohyet, in the better watered part of the state. I’ve done studies at the Berger Bluff, Vara Daniel, and Fish Creek Slough sites, and other researchers have done studies at the Aubrey, Wilson-Leonard, Richard Beene, and various other sites. There have been a few studies in the Texas Panhandle and High Plains, at Lubbock Lake, Lake Theo, and at Mackenzie Reservoir (species lists with no specimen counts or sample volume given are provided for the Rex Rodgers and Snail Bed sites). Species lists are available for the Plainview site and several other sites on the High Plains (Neck 1995), but again, specimen counts and sample volumes are not given. At least one site (41CC112; Treece 1992:Table 15) in west-central Texas has been reported.

The northeast and southwest quadrants of Texas, on the other hand, are a blank slate. The archeomalacology of East Texas remains completely unknown. The acidic, sandy soils and constant saturation with phreatic water are hostile to shell preservation. Indeed, I would speculate that there may be some calcium-poor areas in East Texas where even living populations are absent today.

The southwest quadrant of Texas is mostly unexplored. One of the earliest archeomalacological studies in the state was done by Cheatum (1966) as part of the Amistad paleoecological project at Eagle Cave, Bonfire Shelter, Devil’s Mouth, and Devil’s Rockshelter, but the number of samples, volumetric size of samples, and number of specimens are not disclosed; intrasite provenience is reported only by stratum. Thus, we are left only with a laundry list of species. More recent studies at Bonfire and Skyline Shelter provide quantitative data, but both of these sites are rockshelters, and the Lower Pecos remains understudied.

In the entire vast area of the Big Bend (more than 12,300 square miles, not counting parts of Culberson, Hudspeth, and Reeves counties), no formal archeomalacological studies have been done until recently, as far as I know. The Big Bend is truly a blank slate. The potential of the driest two-thirds of Texas for these studies is largely unknown. Pierce (1987) recovered large numbers of specimens at Lubbock Lake, but most of these came from bedload gravels or cross-bedded sand in Stratum 1. The research potential of alluvial terrace deposits in west Texas has been largely unknown. Cheatum’s faunal lists from Devil’s Mouth, and Devil’s Rockshelter offer promise, but without specimen counts, their productivity is unclear. We know that snails are moisture-seeking organisms, and we know that they are most abundant and diverse in the wettest parts of Texas, but how well do they function as paleoenvironmental proxies in the driest parts of the state?

Figure 2. Site locations in Brewster and Val Verde Co.

In July, 2013, I visited the Genevieve Lykes Duncan site (41BS2615), a stratified Paleoindian site on the O2 Ranch, near the confluence of Terlingua Creek and Davenport Draw in Brewster County, and collected a continuous column spanning the entire Holocene. In May, 2016, I visited the Sayles Adobe site (41VV2239), a high terrace site in Eagle Nest Canyon near Langtry. Tori Pagano collected a discontinuous series of eight pilot samples for me, from sandy sediments representing about a 2500-year span of the Late Holocene. I have now processed and analyzed the samples from both of these sites (Fig. 2), and in future issues of this newsletter, I hope to summarize what I’ve learned.

References Cited

Cheatum, E. P.
1966 Report on Mollusk Shells Recovered From Four Archeological Sites in the Amistad Reservoir. Pages 227-243 in Dee Ann Story and Vaughn M. Bryant, Jr. (assemblers), A Preliminary Study of the Paleoecology of the Amistad Reservoir Area. Final Report of Research Under the Auspices of the National Science Foundation (GS-667).

Neck, Raymond W.
1995 Molluscan Remains. Pages 59-67 in Vance T. Holliday, Stratigraphy and Paleoenvironments of Late Quaternary Valley Fills on the Southern High Plains. Geological Society of America, Memoir 186.

Pierce, Harold G.
1987 The Gastropods, with Notes on Other Invertebrates. Chapter 6 (pages 41-48) in Eileen Johnson (ed.), Lubbock Lake. Late Quaternary Studies on the Southern High Plains. Texas A&M University Press.

Treece, Abby C.
1992 A Study of Five Annular Burned Rock Middens from the O.H. Ivie Reservoir, West Central Texas. Unpublished MA thesis, University of Texas at Austin.

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