EARLIEST EVIDENCE of ANIMAL LIFE on LAND Samuel E. Miller and James W. Hagadorn Department of Geology, Amherst College, Amherst MA, 01002

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PALEOPIONEERS: EARLIEST EVIDENCE OF ANIMAL LIFE ON LAND Samuel E. Miller and James W. Hagadorn Department of Geology, Amherst College, Amherst MA, 01002

1.200 MILLION Data Set for All Sections GEOLOGIC TIME YEARS A B, cast of surface Imprint Diameter Size Distribution Lognormal Cumulative Distribution Function PERIOD AGO 25 1.000 TERTIARY Section 1 NEOGENE Section 2 0.800 Trilobate trace Section 3

e 20

PALEOGENE g

n All Sections 0.600 a R

Bilobate trace Normal n i Distribution Fit 0.400 s

t 15

n Mean = 3.47 i r

CRETACEOUS p St. Dev. = 1.15 Furrows 0.200 m I

o 10

r 0.000 e

b -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 m u

N -0.200 JURASSIC 5 1.200 Data Set for All Sections Normal Cumulative Distribution Function 1.000 0 TRIASSIC T 0.25 0.75 1.25 1.75 2.25 2.75 3.25 3.75 4.25 4.75 5.25 5.75 6.25 0.800 Protichnites Diameter (+/- 0.25 mm) PERMIAN How does one know that the imprints were made by 0.600 North Late Silurian: Earliest raindrops? Both raindrop imprints and gas escape assemblage of 0.400 CARBONIFEROUS terrestrial arthropod structures appear as craters with well-defined rims. body fossils.1 T 1. In cross-section no vertical escape shafts were observed. 0.200

Late Ordovician: Vertical vents are diagnostic of gas escape. Earliest terrestrial 2 0.000 DEVONIAN 2. In three randomly selected 25 cm sample areas, no -4 -3 -2 -1 0 1 2 3 metzoan body fossils.2 2 cm 1 cm imprints with diameters greater than 6.00 mm were found 423 SILURIAN Equator (above). Raindrops break up at ~5.5 mm in the atmosphere. Early Ordovician: FIGURE 2 This constraint does not exist for gas escape structures. With size sorting 461 Earliest terrestrial There are three types of trace fossils on the surface (A). Protichnites is produced by a large arthropod. It is defined by metazoan trace 3. Modern raindrop imprints exhibit a lognormal size ORDOVICIAN fossils (literature).3 parallel sets of footprints and medial tail drag grooves (T). Certain Protichnites are associated with splayed scratches distribution (bottom right). Full raindrop spectra are (outlined in A). These might indicate loss of traction as the trace maker turned a corner. Other Protichnites with indistinct loglinear, but this is not the case when small drops go 488 Late Cambrian: and lumpy footprints (trackway outlined in B) include enigmatic perpendicular furrows. Could the trace maker have been 501 Earliest terrestrial unrecorded. Application of the chi-square goodness-of-fit CAMBRIAN metazoan trace scraping a microbially-bound substrate for food? Without size sorting 543 fossils (this study). test to our data shows that at a significance level of 25%, one cannot reject that the imprint diameters are distributed Five hundred million years ago, animals emerged from the oceans onto tidal flats of the ancient A second type of trace fossil consists of an irregularly meandering trench bounded by a ridge of excavated sediment on

normally or lognormally (above right). Our small sample Number of raindrops diameter continent Laurentia (above). These pioneers provide the earliest record of animal life on land. either side (A). Specimens average less than 1 cm in width. Some exhibit tight looping whereas others are nearly straight. population made both distributions fit the data equally well. Sandstones from Wisconsin, New York, and Quebec contain some of the best evidence to support These traits can converge. Three potential trace makers are suggested. i) Small arthropods, including certain millipedes Diameter this hypothesis. The evidence is scarce and includes raindrop-imprinted bed surfaces that and crustaceans, are capable of producing analogous bilobate grooves on a liquefied substrate.4 ii) Gastropod may also The presence of raindrop imprints on the survived the typical array of erosional processes. One exceptional surface from WI bears produce simple furrows.5 iii) Surface-moving worms can plow similar troughs.6 cross-cutting relationships between raindrop imprints and trackways. Study of these relationships studied surface indicates subaerial exposure. demonstrates that animals were living in subaerial conditions. C Some imprints are halved by trace fossils (Figs. 1B, 5A, 6A). Other imprints lie atop The third type of trace fossil on the surface is a trilobate traces (Figs. 1C, 5B, 6B and C, 7A). These trail. Two troughs are flanked by two outside levees and cross-cutting relationships imply that traces a low middle ridge (Fig. 2A). Similar traces are found in Bizarre offset of trilobate trace were created during or between rain events. Area around C Area around B NY, but those display parallel rows of stipple marks, in direction of arrow A interpreted as footprints (Fig. 3). The WI and NY traces B probably share a common producer. Deep puncture Furrow marks in the furrows of the WI traces evoke the NY footprints. Both are ~1 cm wide and show low sinuosity. One WI trace has a distinct offset (Fig. 4), which is a movement difficult to attribute to a wormlike maker. Certain arthropods create trilobate ribbons on soft A A surface films atop hard substrates.4 This sedimentary condition might be mirrored in microbially-bound Ridge surfaces. Evidence of microbial binding is found on the WI surface (Fig. 5). The NY traces are akin to slightly younger trackways thought to have been made by millipede-like organisms.7 1 cm FIGURE 4 Cast of surface containing an offset trilobate trackway and raindrop imprints. B 1 cm 1 cm

Sand stromatolite FIGURE 6 FIGURE 7 (domal microbial accretion) Cast of surface containing trilobate trackways Surface containing a trilobate cross-cut by and raindrop imprints. trackway and raindrop imprints. trilobate trace SUGGESTIONS FOR FURTHER RESEARCH One could expose sand to natural rainfall to analyze modern vs. fossil imprint distributions. One could also B determine whether the rain system recorded on the imprinted surface was continental or maritime, because Well-defined parallel there is a difference in large drop size frequency between the two types of systems. Experimentation with track rows modern animals could help constrain possible trace makers. Work on the preservational influence of A microbial mats could help reveal their taphonomic role in raindrop preservation. C ACKNOWLEDGEMENTS 1 cm Thanks to the Edward Hitchcock Fund for Student Research in Environmental Science for funding, the Krukowski family for quarry access, and D. Damrow and P. Groulx for their assistance and hospitality.

FIGURE 1 REFERENCES: 1 cm 1. Jeram, A. J., Selden, P. A., and Edwards, D., 1990, Land Animals in the Silurian: Arachnids and myriapods from Shropshire, England. Science, v. 250, no. 4981, p. 658-661. Cast of surface containing a trilobate trackway and raindrop imprints. 2. McNamara, K. J., and Trewin N. H., 1993, A euthycarcinoid arthropod from the Silurian of Western Australia. Paleontology, v. 36, p. 319-335. 1 cm 3. MacNaughton, R. B., Cole, J. M., Dalrymple, R. W., Braddy, S. J., Briggs, D. E. G., Lukie, T. D., First steps on land: Arthropod trackways in Cambrian-Ordovician eolian sandstone, southeastern Ontario, Canada. Geology, v. 30, no. 5, p. 391-394. Cross-cutting relationships between features on this surface provide information about the ages of those FIGURE 3 4. Uchman, A., and Pervesler, P., 2006, Surface lebensspuren produced by amphipods and isopods (crustaceans) from the Isonzo delta tidal flat, Italy. Palaios, v. 21, p. 384-390. features relative to each other. For example, the looping trace fossil above crosses itself at A, which allows Track-laden surface FIGURE 5 5. Miller, M. F., and Knox, L. W., 1985, Environmental control of trace fossil morphology, in Curran H. A., ed., Biogenic structures: Their use in interpreting depositional environments: Society of Economic Mineralogists and Paleontologists Special Publication 35, p. 167-176. reconstruction of the trace maker’s direction of locomotion. The trackway enters from the bottom corner, from the Potsdam Formation of NY. Surface containing a trilobate trackway, sand 6. Seilacher, A., 2007, Trace Fossil Analysis: Springer, p. 96. 7.Johnson, E. W., Briggs D. E. G., Suthren, R. J., Wright, J. L., and Tunnicliff, S. P., 1994, Non-marine arthropod traces from the subaerial Ordovician Borrowdale Volcanic Group, English crosses itself near C, and exits in the top center. stromatolite, and raindrop imprints Lake District. Geology Magazine, v. 131, no. 3, p. 395-406.