Continental trace fossils and museum exhibits - Geological Curators ...

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entombed within the trace itself (e.g., Hasiotis andMitchell 1993, Hembree et al. 2004).In the following sections we illustrate examples ofways in which traces can be used in educationalexhibits on continental trace fossils, incorporatingmodern and ancient examples of organism behaviourfrozen in time.Crayfish burrowCrayfish (Decapoda: Astacoidea and Parastacoidea)burrows are simple to elaborate in morphology andtheir depth of penetration is based on the depth of thesaturated zone (e.g., Hobbs 1981, Hasiotis andMitchell 1993). Burrows can be horizontal and justbelow the sediment-water interface or reach depthsof up to 9 m below the sediment-air interface (Hasiotisand Mitchell 1993, Hasiotis and Honey 2000, Hasiotis2004). Simple crayfish burrows are composed of asingle shaft with one or more openings that terminatein a single chamber or as tunnels that branch laterallyat the shaft terminus. More elaborate crayfish burrowshave one or more openings, chambers and tunnelsthat originate or occur within a central shaft (Figures5A, B). The burrow entrance is circular and oftenmarked by a chimney composed of pellets of soilexcavated from the burrow (Figures 5C, D). Thesurficial morphology of the burrow walls containmm-scale clusters of scratch marks, cm-scaletransverse scrape marks, mm-scale rounded striations,knobby and hummocky texture, and discontinuoussediment linings composed of excavated soil matrix(Hasiotis and Mitchell 1993).The architectural and surficial morphologies ofcrayfish burrows appear to have changed little overtheir 280-million-year history (Hasiotis and Mitchell1993, Hasiotis et al. 1993, Hasiotis 1999, 2002).Examples of crayfish burrows from the Upper TriassicChinle Formation (Colorado Plateau, USA) andPaleocene Fort Union Formation (Wyoming, USA)illustrate how similar their morphologies are to eachother, as well as to the burrows of modern crayfish(Figures 5E, F). The burrow morphologiesdemonstrate that the chelae were used to constructand maintain the burrows, and that the burrows wereconstantly modified during the life of the crayfish aswell as by other members of the same species thatinhabited the burrow after it was abandoned or afterthe original owner died.The exhibit of crayfish burrows could contain actualcasts of the modern and ancient burrows, as well asphotographs of the outcrop and the fossil specimensassociated with the burrows. Live specimensburrowing in an aquarium set-up would allow theFigure 5. Crayfish burrows. (A-B) Cast of a moderncrayfish burrow from the University of KansasEcological Research Station; burrow is 720 mm tall.(C-D) An open (C) and closed (D) chimney of acrayfish burrow, composed of soil pellets; (C) is 120mm wide, (D) is 80 mm wide. (E-F) Triassic ChinleFormation (E) and Paleocene Fort Union Formation(F) crayfish burrows in outcrop; lens cap in left sideof photograph in (E) = 50 mm, Jacob staff in (F) is 1.5m tall.-219-
public to see how crayfish construct and maintaintheir burrows.Scorpion BurrowScorpion (Arachnida: Scorpiones) burrows are uniquein morphology because they produce a spiral, flattenedtunnel with several whorls that are well constrainedin construction with respect to the tunnel angle andradius of coiling (Figures 6A, B). The burrow iscomposed of a single entrance, spiral tunnel andterminal chamber. The terminal chamber is slightlywider in diameter than the spiraled tunnel. Theposition of the chamber is directly under or awayfrom the direction of the burrow opening. The burrowentrance is distinctively crescent shaped with theterminations of the crescent pointing upwards (Figures6C, D). The morphology of the opening indicates theupward, ready position of the scorpion’s chelae as itexits the burrow.Although scorpion burrows have not been describedfrom the geologic record, their terrestrial body-fossilrecord occurs at least as early as the Carboniferous(Petrunkevitch 1953), and possibly the Silurian(Kjellesvig-Waering 1986). Scorpion burrowmorphology is so distinct that it should be easy toidentify the presence of scorpion burrows in the rockrecord.Figure 6. Scorpion burrows. (A-B) Cast of a modernscorpion burrow from the Simpson Desert about 80km south of Alice Springs, Northern Territory,Australia; cast is 550 mm tall. (C-D) Crescentshapedentrance of a scorpion burrow (C) prior topouring the fiberglass into the burrow and excavationof a cast (D); lens cap is 50 mm wide.Scorpion burrow casts can be displayed alongside thetrace fossil Gyrolithes (e.g., Pemberton et al. 1992)or Xenohelix (Mansfield 1927), described fromvarious marine deposits. The purpose of this exhibitwould be to illustrate the subtle, but significant,difference in morphology between these types ofspiral burrows that would indicate the difference inanatomy of the constructors that made them, as wellas the environments they occur in.Skink burrowSkink (Squamata: Scincidae) burrows are composedof a main tunnel that slopes downward gently between15 o and 30 o from horizontal (Figures 7A, B). Themain tunnel may have one or more branches whichform nearly identical, parallel tunnels. An inverted,flattened U-shaped cross section to these tunnels isproduced by a weak to robust, longitudinal, medialgroove on the floor. Several upward branching tunnelsare used to evade predators and begin from the maintunnels. These escape tunnels are composed of shorttunnel segments connected to each other andswitchback on them in an upward direction, forminga crude pseudospiral. The burrow entrance isdelineated by a dome shape with a flat floor andarched ceiling. In some cases the flattened, invertedU-shape of the tunnel can be seen.Small and large diameter burrows of the LowerTriassic Fremouw Formation, Antarctica (Figures7C, D), have similar burrow morphologies to those ofmodern skinks, which were used to help interpret theburrows as constructed by tetrapods (Hasiotis et al.2004). The larger diameter burrows were interpretedas tetrapod in origin; however, the smaller diameterburrows were interpreted as being excavated bycrayfish (Miller et al. 2001). The reinterpretation ofthese small diameter Triassic burrows was based onseveral key morphological features found in skinkburrows. The overall architecture is not much differentfrom the Triassic burrows, including the inverted U-shape of the cross-section. The Antarctic burrowsare also similar to therapsid burrows described fromPermian and Triassic continental rocks of thesouthwestern part of the Karoo basin in South Africa(Smith 1987, Groenewald et al. 2001). Thelongitudinal median groove in the modern skink andancient burrows was produced by the sprawling stanceof the lizard, and the locomotion of the front and rearlimbs on either side of the body that formed thegroove. The longitudinal scratches on the outside ofthe burrow and along the median groove wereproduced by the predominantly lateral digging motionused by these lizards.-220-
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Page 44 and 45: Figure 2. Life size reconstruction
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