Trace fossils record the behavior of extinct organisms, rather than actual body parts. While it would seem that trace fossils could provide relatively little information, given their relatively simple structure, this is categorically not true. Trace fossils can be surprisingly complex. Further, this complexity frequently increases through time, reflecting the evolution of increasingly efficient behavioral patterns. But while trace fossils allow us to reconstruct the evolution of behavior, we are frequently unable to correlate a given trace fossil with a specific taxon of animal. Because of this difficulty, trace fossils are typically grouped by ecological similarity; similar trace fossils were produced by unrelated animals that had similar life styles. Each of these ecological groupings becomes an ichnotaxon. Ichnotaxa are further aligned in a hierarchical scheme like that of organisms. Consequently, there are ichnofamilies, ichnogenera, etc. As with organismal classification, ichnogenera and ichnospecies are italicized.

        Trace fossils occur in a variety of forms. Trails and tracks chronicle the passage of animals along a sedimentary surface. Burrows record the penetration of soft sediments by organisms, while boreholes reflect similar activities in hard substrates. Finally, bite marks reflect attempts of carnivores to subdue their prey.

        This simplistic classification masks much of the complexity and richness of the trace fossil record. For example, boreholes, which we will be examining in today's exercise, can arise from two very different behaviors. Many boreholes are defensive in nature, and reflect attempts by organisms to avoid predators by hiding within hard substrates. Others are offensive, in that they record the attempts of carnivores to penetrate the armor of their intended prey.

Boreholes in Anadara

        In this exercise, you will be evaluating the boreholes present in a collection of shells for the bivalve Anadara idonea from a single horizon in the St. Mary's Formation (late middle Miocene) of southern Maryland. This species is a common component of shallow water, soft sediment communities in the middle Atlantic states near the end of the middle Miocene.

        Five different types of boreholes are known to regularly occur in A. idonea. Three of these borehole types are defensive, serving as protection from predators.

        Two offensive boreholes are known from Anadara, both produced by carnivorous gastropods (ichnogenus Oichnus). These boreholes are oriented perpendicular to the shell surface, but are generally much larger than defensive boreholes. Typically these boreholes completely perforate the shell from outer to inner surface. The attacking gastropod everts its proboscis through the completed borehole and uses its radula to remove the prey's tissues. In some cases, the borehole is abandoned before they are completed; these holes do not penetrate the inner surface of the shell

Figure 1.  A = typical boreholes present in bivalves.  Enlarged longitudinal sections of complete and incomplete parallel Oichnus (B) and tapered Oichnus (C) are also shown.

        There are six sets of Anadara shells available in the laboratory for analysis. You will collect data from all six samples. For each specimen, collect the following data:
        1. shell length;
        2. type and frequency of boreholes;
        3. position of each borehole.
Record shell length, along with the type and frequency of boreholes. Record the position of each borehole on enlarged outlines of Anadara. Be certain to record the position of the external opening of the borehole, not the exposed interior. In some cases (particularly with Polydora) there may be huge numbers of juvenile boreholes over one area of the shell. In such cases, indicate the boundaries of the area covered by this cluster of boreholes, rather than attempting to mark each borehole.

After examining your data, be prepared to answer the following questions:

  • Which borehole type was most commonly observed in Anadara?

  • Examine the spatial distribution of this borehole type on Anadara valves. What does this distribution suggest about the behavior of borehole-producers?

  • What does the spatial distribution of this borehole type on Anadara valves indicate about the behavior of Anadara?

  • Is there a relationship between the size of individual Anadara and the number of these boreholes present?

  • How would you explain any such relationship?

  • When combined with life history and seasonal mortality data, what additional insights are gained into the life history of benthic invertebrates?

    Boreholes in Vertebrate Fossils

            Boreholes are not restricted to invertebrate fossils. Although less common, vertebrate fossils occasionally have distinctive boreholes, as well. For example, flask-shaped Gastrochaenolites boreholes are known in a rostrum of the extinct Miocene cetacean, Squalodon cf. tiedemani (Figure 2; redrawn from J. R. Boreske, et al. 1972. A reworked cetacean with [Gastrochaenolites] borings: Miocene of North Carolina. Journal of Paleontology 46: 130-139.). While these boreholes superficially resemble those you have already examined in this exercise there are some subtle differences. Compare these boreholes to those you have already studied in Anadara, noting both similarities and differences. Which of the five borehole types you have already studied most closely resemble those in Squalodon?

            Are there any aspects of the Squalodon boreholes that are incompatible with the known origins of similar boreholes in Anadara?

            Gastrochaenolites boreholes similar to those on Squalodon are also known from other vertebrates and from some invertebrates. Examples of these boreholes are available in three different matrices:
            1. bivalve hinges (Melina)
            2. shark teeth (Carcharocles)
            3. limestone fragments
    How do these boreholes compare with those in Squalodon?

    Can you suggest a plausible explanation for these boreholes?

    How would you explain boreholes in both animal remains and limestone?

    The apparent similarity of Gastrochaenolites boreholes in fossil bivalves, sharks and a whale rostrum suggest that they may have similar origins. However, this conclusion is based on qualitative observations, rather than on quantitative data. Table 1 lists published aperture length and width data for Gastrochaenolites boreholes in bivalves and a whale rostrum (data from J. R. Boreske, et al. 1972). Reexamine the Gastrochaenolites boreholes available in the laboratory and collect length and width data for each borehole. Record this data, along with the data from Table 1 in a spreadsheet. Examine two different relationships; (1) the relationship across all matrix types, and (2) between published and class data. Plot log-transformed data with trendlines and determine regression parameters for these two relationships.

    What do these graphs suggest about the origin of Gastrochaenolithes boreholes in different matrices?