Exceptionally preserved fossils are a useful tool for understanding and reconstructing past environments. They provide information about ancient organisms, including their morphology, ecology, and biogeography; and can act as indicators of environmental conditions such as temperature, humidity, and water chemistry. Early Cretaceous laminated limestones in the Crato Formation, a site known for plentiful fossil deposits in Northeast Brazil, have an abundance of exceptionally preserved insects, arachnids, fish, other vertebrates, and plants. Thin laminations suggest a calm lake environment during the deposition of the Crato Formation 100 million years ago (Heimhofer et al., 2010). Though several studies have tackled reconstructing the depositional environment of the Crato Formation, the salinity of the ancient lake has been debated. Interpretations for the salinity range from fresh water (Maisey, 1990) to hypersaline (Martill et al., 2007).
The lack of bottom-dwelling organisms (Martill and Wilby, 1993) in the Crato Formation has made it difficult to determine the salinity. Most of the fossils preserved are insects and spiders. Spiders are relatively rare in the fossil record compared to the insects. This kind of fossil assemblage is seen in similar deposits around the world, including localities in Colorado (The Green River Formation and the Florissant Formation). The spider fossils from the Crato Formation are of particular interest because their legs are curled tightly under their bodies, unlike spider fossils from other, similar deposits. Most people are familiar with dead spiders in and around homes, deceased with their legs curled up towards the center of their bodies. Spiders have muscles in their legs for contraction, but not extension – so their legs naturally curl when they die. Spiders extend their legs by a hydraulic mechanism in which spider blood is pumped into the legs creating a fluid pressure. When a spider dies on land, the hydraulic mechanism stops, and the legs give way to the flexor muscles causing them to contract. When a spider dies by drowning in normal water, the spider becomes waterlogged. The water filling the spider creates a pressure that causes the legs to extend, which overpowers the flexor muscles for contraction.
It would seem that spiders preserved as fossils in rocks deposited by water, such as an ancient lake, should have become waterlogged and displayed extended legs in their fossilized state. But the spiders in the Crato Formation had curled legs – which came as an unusual surprise. Other formations deposited by ancient lakes like the Green River and Florissant Formations usually have spiders with extended legs. Perhaps a connection exists between the unique curled leg pattern of spider fossils and the unknown salinity of the Crato Formation.
To test this idea, spider fossils from three localities representing ancient lakeside environments were examined, and experiments on modern spiders drowned in different salinities were done to look for patterns in leg orientation. The Crato, Green River, and Florissant Formations all yield well preserved spiders where leg orientation can clearly be seen. The Green River Formation includes several ancient lakes that had variable salinity (Buccheim, 1994; Cole 1985). The Florissant Formation was deposited in a volcanically dammed lake and is represented by mostly shales and volcanic deposits. Freshwater conditions existed during the Florissant Formation’s deposition, and are suggested by the presence of freshwater fossils (Meyer, 2003). Leg angles of the femur-patella joints, where most of the curling occurs. , were measured for spider fossils in the three formations and the leg orientation of each spider was determined.
The experiments on modern spiders were a little more gruesome. Spiders were drowned in solutions of varying salinity. Fresh, marine, and hypersaline conditions were tested to see if leg orientation differed between the three salinities. Spiders were placed in a sealed vial and left to expire. After death, they were photographed and the leg orientation was determined. A pattern was evident in the fossil spiders and in the recently drowned ones.
Spiders that were drowned in freshwater conditions exhibited legs that were extended. Spider fossils from the Florissant Formation also had an overwhelming display of extended legs. Both the fossils and the modern spiders had leg angles near 180 degrees and exhibited fully outstretched legs. The Florissant Formation was deposited in fresh water, and the fossils matched the fresh water experiments on modern spiders.
In marine conditions, curled legs and extended legs were relatively equal. Spider fossils from the Green River Formation had spiders with extended legs, but had a higher proportion of spiders that had curled legs. Those legs that were curled were not curled as tightly under the body as what was seen in hypersaline conditions. This trend was in accordance with the previously recorded pattern, as the Green River varied from mostly saline to marine salinity.
Most of the spiders drowned in hypersaline conditions had legs that were curled tightly under the body. The femur-patella joint angles were acute, indicating tightly curled legs. Spiders from the Crato Formation were also overwhelmingly curled. Spiders in each of the salinities tested appear to match spiders from one of the formations studied.
Fossil spiders from the Crato Formation (A – C) showing tightly curled legs. Spiders from the Green River Formation (D – F) are typically extended, and those that have curled legs are not tightly as curled as spiders from the Crato Formation. Spiders from the Florissant Formation (G – I) almost always display legs fully extended. Photos by Matt Downen and James Lamsdell.
When trying to infer the salinity of an ancient lake, normally one would expect to examine some aquatic organism. One of the last things anyone would expect to use is a spider. However, the special mechanism for leg movement in spiders and osmosis helps one understand why this is so. In a freshwater lake, a spider becomes waterlogged. The water filling the spider creates a pressure that overpowers the contracting muscles to extend the legs. In a hypersaline environment, the surrounding water is saltier than the inside of the spider legs. Instead of becoming waterlogged, water moves along a concentration gradient – by osmosis – from the spider into the area of higher salt concentration. This results in the spider leg muscles to contract – the only way they are capable of moving.
The results from this study are suggestive of hypersaline conditions at the time of deposition of the Crato Formation, and show that the leg orientation of spiders is related to salinity. Spiders may be a useful tool for interpreting salinity in deposits like this when other forms of evidence are absent. Studies like this may also help in determining salinity of certain environments – which could be useful in climate studies dealing with rates of evaporation.
Buchheim, H. P., 1994. Eocene Fossil Lake, Green River Formation, Wyoming: A history of fluctuating salinity. In Renaut, R., Last, W. (ed.), Sedimentology and geochemistry of modern and ancient saline lakes. SEPM Special Publication 50, 239-247
Cole, R. D., 1985. Depositional Environments of Oil Shale in the Green River Formation, Douglas Creek Arch, Colorado and Utah. In Picard, M. D. (ed.) Geology and energy resources, Uinta Basin of Utah. Utah Geological Association. 211-225.
Heimhofer, U., Ariztegui, D., Lenniger, M., Hesselbo, S. P., Martill, D., Rios-Netto, A. M., 2010. Deciphering the depositional environment of the laminated Crato fossil beds (Early Cretaceous, Araripe Basin, North-eastern Brazil) Sedimentology, 57, 2, 677-694.
Maisey, J. G. 1990. Stratigraphy and depositional environment of the Crato Member
(Santana Formation, Lower Cretaceous) of northeast Brazil, pp. 15–19. In Grimaldi, D. A. (ed.), Insects from the Santana Formation, Lower Cretaceous, of Brazil. Bulletin of the American Museum of Natural History 195.
Martill, D. M., Wilby, P. R., 1993. Stratigraphy, 20–50. In Martill, D. M. (ed.), Fossils of the Santana and Crato Formations, Brazil. Field Guides to Fossils, 5. London: The Palaeontological Association.
Martill, D.M., Loverage, R., Heimhofer, U., 2007 Halite pseudomorphs in the Crato Formation (Early Cretaceous, Late AptianeEarly Albian), Araripe Basin, northeast Brazil: further evidence for hypersalinity. Cretaceous Research, 28, 4, 613–620.
Meyer, H. W., 2003. Fossils of Florissant. Washington, D.C., Smithsonian Books
Matt Downen was the recipient of PRI’s 2013 J. Thomas Dutro Jr. Student Award in Systematic Paleontology. He is working at the University of Kansas as a graduate student seeking a Master’s degree in geology, with an emphasis in paleontology. He graduated from Western Kentucky University in 2011 with a bachelor’s degree in geology. In undergrad, he focused on volcanoes and meteorites, but switched to paleontology for graduate school.
The Paleontological Research Institution, Ithaca, New York, is pleased to sponsor Paleontology content for This View of Life. Founded in 1932, PRI has outstanding programs in research, collections, and publications, and is a national leader in development of informal Earth science education resources for educators and the general public.