Fossil of the month: Asterosoma
What is this strange shape? Some people find this fossil and think it may be a fossil starfish. Actually, it’s a type of trace fossil (also called ichnofossil), formed by a soft-bodied organism moving through the sediment. Trace fossils like Asterosoma are important for interpreting the original environmental conditions in rock strata where body fossils (fossils of actual organisms with hard parts like shells) are lacking.
Description. Asterosoma is a three-dimensional trace fossil. It has multiple, radiating lateral arms (tubes) around a central bulging axis. The arms and central axis are burrows of a soft-bodied invertebrate (lacking a backbone) organism, rather than the body parts of an organism. Individual lateral burrows can be several inches long and generally less than an inch wide. They tend to be widest in the middle, thin toward the central axis, and thin towards slightly rounded ends. Lateral burrows commonly are covered with longitudinal (parallel to long axis) wrinkles. Some Mesozoic-age forms have branching lateral arms. When Asterosoma is viewed from the side, each lateral burrow looks like a semi-circular tube filled with sandstone, siltstone, and sometimes minor mudstone or claystone. Lateral burrows may be concentrically or irregularly laminated (called spreite) or they may be massive and unlayered (Chamberlain, 1971; Häntzschel, 1975; Miller and Knox, 1985). When viewed from the side, Asterosoma’s lateral burrows can look very similar to another type of trace fossil, called Teichichnus. Teichichnus is sometimes described as looking like small, stacked gutters.
Species. Asterosoma is a trace fossil, rather than a specific animal or plant fossil. It is given a genera name as a distinctive pattern or trace. Different species names have been given to different variations of the Asterosoma pattern, although many researchers do not use a species name with trace fossils. The common form found in Kentucky with distinct, non-branching arms, has been called Asterosoma radiciforme (Chamberlain, 1971; Martino, 1989). Other names are given to branching forms, which are more common in Mesozoic strata (Seilacher, 2007).
Asterosoma vs. starfish and starfish traces. Some people think Asterosoma looks like a starfish, sea star, brittle star or related asterozoan fossil. When Asterosoma has five radiating burrows or tubes, it is easy to understand how at first glance, it could be confused with a starfish or starfish resting trace. Starfish usually have five arms (although some have more). More importantly, each arm is similar in appearance to each other, and each arm is symmetrically spaced (similar distance between arms) around a central axis. Asterosoma, however, commonly has more than five arms; arms are not equal in size; and arms are not equally spaced from each other.
Actual starfish and asterozoan trace fossils are preserved and are called Asteriacites. Asteriacites trace fossils are usually resting impressions of a starfish or brittle star (e.g., Seilacher, 2007). Asteriacites are relatively rare, but have been found in the Mississippian Borden Formation of eastern Kentucky (Chaplin, 1980) and the Pennsylvanian Fentress Formation of neighboring Tennessee (Miller and Knox, 1985).
Starfish and other asterozoan body fossils are also preserved as fossils, although they are rare. Body fossils preserve details of body structure, rather than just the outline of the body. The upper surface of a starfish or brittle star fossils exhibit arrangements of tiny plates on their arms. The underside of arms have a central groove (called the ambulacral groove), body plates, and the opening for the mouth.
Range. Asterosoma are long ranging. They occur in rock strata from the Precambrian to Recent (Crimes, 1975; Ekdale and others, 1984). In Kentucky, they are mostly found in interbedded sandstones and shales of Pennsylvanian age. Asterosoma is relatively common in the Tradewater Formation of western Kentucky, and the Grundy Formation of eastern Kentucky (Greb and Chesnut, 1992; Greb and others, 1992; Martino and Sanderson, 1993; Greb and Chesnut, 1994). These units are 310 to 320 million years old. Asterosoma has also been reported from Pennsylvanian strata in many surrounding states including West Virginia (Martino, 1989; 1996), Tennessee (Miller and Knox, 1985), Ohio (Chamberlain, 1978a), Indiana (Archer and others, 1994), and Illinois (Devera, 1989; Devera and others, 1990).
Trace makers and paleoecology. Asterosoma are the traces of a soft-bodied organism. Most researchers have inferred the trace makers for Paleozoic Asterosoma were marine worms (polychaetes) or worm-like organisms (e.g., Chamberlain, 1971; Martino, 1989; Miller and Knox, 1989); shrimp (Seilacher, 2007); and for branching Mesozoic forms, small, decapod arthropods (Neto de Carvalho and Rodrigues, 2007). The layering or “spreite” found in some many lateral Asterosoma burrows were formed by the soft-bodied organism moving through the sediment, pressing sediment against the walls and backfilling the burrows as it probed the sediment interface between beds (Seilacher, 2007). Different patterns (and species) of Asterosoma are formed by different movements of the trace makers.
Asterosoma are commonly found in beds that lack shelly fossils of organisms but are fairly well bioturbated. Bioturbation is the disturbance of sediment by organisms (burrows, tracks, trails, etc.). Other trace fossils commonly found with Asterosomain Kentucky include Arenicolites, Asterophycus, Conostichus, Monocraterion, Rosselia, and Teichichnus. This association of traces is usually placed in the Cruziana ichnofacies (e.g., Ekdale and others, 1984; MacEachern and others, 2007). Ichnofacies are a characteristic grouping of trace fossils, which form in a certain range of shallow marine environmental (salinity, energy, etc.) conditions. In fact, traces like Asterosoma help geologists interpret the original depositional conditions of the beds in which they occur, based on comparisons of ichnofacies to modern environments of deposition. Cruziana trace fossil assemblages are characteristic of modern shallow marine (typically shoreface) and coastal environments (Chamberlain, 1978a; Miller and Knox, 1985; Pollard, 1988; Greb and Chesnut, 1992, 1994; Martino, 1996; MacEachern and others, 2007).
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- Neto de Carvalho, C., and Rodrigues, N.P.C., 2001, Compound Asterosoma ludwigae Schlirf, 2000 from the Lusitanian Basin (Portugal): Conditional strategies in the behaviour of Crustacea: Journal of Iberian Geology, v. 33, no. 2, p. 295-310.
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Text and illustrations by Stephen Greb (KGS).