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Xiphosurid (horseshoe crab) trace fossils are much more abundant in the fossil record than their body fossils. Xiphosurid trace fossils from the Pottsville Formation (Westphalian A, about 310 million years old) at the Union Chapel Mine site of northwestern Alabama, USA, provided sufficient trace fossil data to study information about xiphosurid and population dynamics of the horseshoe crabs. Many burrows show a sinusoidal pattern (resembling a sine wave) in a vertical plane. This behavior is contrary to the habits of the modern horseshoe crab Limulus polyphemus. Many hypotheses were put forth, including: predation of a specific species of worm; burrowing through sand to the contact between the sand and mud, with mud acting as a confining layer, possibly having interfered with breathing; and that the sinusoidal burrowing was simply normal behavior 310 million years ago. Previous researchers have concluded that the sinusoidal pattern is not uncommon in the fossil record. However, there is evidence in one specimen that this behavior was continuous through significant depths of mud, which would result in suffocation in modern horseshoe crabs. This fact suggests that the gills of xiphosurids have been modified in the 310 million years seperating these fossils from the present. A distinct difference in body size is shown in the tracks, with 26 trackways between a minimum of 1.5 cm and a maximum of 3.9 cm wide, and two specimens of 5.8 and 8.4 cm wide. One possible reason for this extreme deviation from mean size is either a rapid or seasonal deposition, preserving two or more distinct generations of xiphosurids in the 3 meters (or less) of shale. Another hypothesis is that the area was used as breeding ground, with adults only present for part of the time, and juveniles staying in the area untill they reach a certain size, then moving to deeper water. Also, based on observations of modern horseshoe crabs, the tracks show suggestions of sexual dimorphism in several fossil specimens (three males, three females based on tracks, one female based on size). Dimorphism is expressed in modern horseshoe crabs as a difference in the first pair of walking legs. The normal walking legs are pinschers, resembling the pinschers of crabs or scorpions, but the males have distinct graspers, adapted to hold on to the female during mating. The morphological difference can be expressed in tracks. However, determining horseshoe crab sex from the fossil record (body fossils or trace fossils) has never been accomplished in any paleontological study before. Therefore, this research will add a new dimension to studies in the future.
Modern day horseshoe crabs (Limulus polyphemus) are studied for many reasons. Their immune system is studied on the forefront of cancer research, their copper-based blood is studied because of the interesting properties of biota-metal interactions, and their eyes are studied for optical properties. In paleontology, they are studied because they have exhibited little change since their first known occurrence in the fossil record nearly 500 million years ago. These “living fossils” are interesting, but they are also rare in the fossil record. Their exoskeleton has always made of chitin, which is not nearly as resilient as the calcium carbonate shells of gastropods and coral (Babcock, 1998). When body fossils are found, they are generally in poor shape, with nothing left of the walking legs or gills. However, horseshoe-crab trace fossils are fairly common in the fossil record. Although trace fossils are not as aesthetic or popular as corresponding body fossils, the information that can be derived from trace fossils is far greater than information from body fossils alone.
The Union Chapel Mine Site: The trace fossils were found in northwestern Alabama, USA, in a formation called the Pottsville Sandstone. The dominant lithology is sandstone, but layers of shale, coal, and lenses of varied lithology are common. In one of these layers of shale, an incredible tracksite was discovered in the Union Chapel Mine, a surface coal mine in Walker County, Alabama. The Birmingham Paleontological Society, an amateur fossil-collecting group, organized collection and documentation of all fossils found. Many of the tracks were then loaned to the authors of this poster for formal study. The included measured section (Figure 2.2) represents the stratigraphy of the Union Chapel mine and related strata.
Kouphichnium: Ichnology is the science of identifying and interpreting traces of biological activity. Sometimes, a trace is identified, but the tracemaker is never found. Also, many different animals can leave similar traces. Another problem is that one animal can leave many different kinds of traces. For these reasons, the trace is given a species name that is entirely different than the animal. This also makes sure that the distinction is made between the body fossil and the trace fossil. The standard trace made by a horseshoe crab is called Kouphichnium. Kouphichnium is a trackway that can have varied appearance. The variables are: the sediment the size of the animal the speed of locomotion the depth of the sediment that the trace appears in and many other more subtle variables. Pictures 3.1 through 3.7 are of Kouphichnium, 3.8 through 3.10 are resting traces, and 3.11 through 3.12 is another trace that shows periodic outlines of the body. This trace has been interpreted to be a burrow that resembles a sine wave in a vertical plane. Sinusoidal Burrows These sinusoidal burrows show interesting behavior. The only burrowing observed in modern horseshoe crabs is just below the surface, plowing through the sand in shallow water (Eldridge, 1970). The sediment in the rocks is very fine (mud), and continuous burrowing in similar sediment would result in suffocation for modern horseshoe crabs. The clay-sized particles would interfere with normal gill functions, so modern horseshoe crabs could not survive if burrowing in the sediment preserved in the record. Thus, a preliminary conclusion was reached that the sinusoidal pattern was a response to the sediment. However, one specimen showed the pattern through several cm of mud, so this pattern seems to be normal behavior for at least one specimen, so this behavior may have been normal in the entire population. Also, this pattern is not unheard of elsewhere. The gills of the modern horseshoe crab have been slightly altered from their predecessors, becoming more specialized, requiring more specific salinity contents than in the past. Perhaps the gills have been further specialized, becoming intolerant of clay-sized particles. In any case, this study suggests that horseshoe crabs of 310 million years ago were able to thrive in environment that the modern genus Limulus cannot live in today.
Undertracks: When one thinks of tracks, the immediate mental picture is one of a simple surface expression. However, tracks extend down into the substrate far beyond the simple depth that the foot entered. Many times the footprints that are left in the fossil record are actually from several layers below where the animal stepped. This is because of the constant weathering of the surface in any environment. What is most often left in the fossil record is what is easiest to preserve. Surface tracks may last for hours to days, or even weeks, but the undertracks are left for considerable time after the surface tracks are weathered away. For more detailed information on this Kouphichnium as undertracks, see Goldring, 1971.
Population Dynamics: So, what data are represented by this collection of tracks? Width measurements are easy to collect, and show an interesting pattern. Of the 28 specimens measured, 26 ranged from 1.5 to 3.9 cm wide, but two were much larger, 5.8 and 8.4 cm wide. Two hypotheses are proposed for this interesting pattern. The first hypothesis is that the sediment that the tracks are found in was deposited seasonally or in a short time frame, possibly a month or two. This would preserve two or more generations as distinct sizes. Another possibility is that the area was near a breeding ground, with young horseshoe crabs living there, while adults were rare visitors, or just passing through. The accompanying graph (figure 6.1) shows the raw data. Mean trackway width is 2.9 cm, n=28
Sex lives: (Well, just sexual dimorphism) Sexual dimorphism in modern horseshoe crabs is expressed in the first pair of walking legs. The normal morphology of the walking legs is shown in pictures 4.1-4.2. The foot terminates with a pincher, resembling a crab’s or a scorpion’s claw. The first pair of walking legs in females are roughly identical to the normal walking legs, only slightly smaller. However, the male’s first pair of walking legs, shown in figure 4.3, are remarkably different. They are graspers, designed for holding onto the female’s carapace during breeding. This difference should be recognizable in the animals’ footprints. Indeed, the footprints of three of the specimens that were studied in detail showed different foot morphology in several tracks. The differences are subtle, but figures 7.5-7.6 show the most obvious example. This study is the first time that any study of fossil horseshoe crabs has been able to identify sex of an individual.
Special Thanks goes to: The SURE Program at Emory University The Birmingham Paleontological Society Nick Pyenson Dr. Andrew Rindsberg, Department of Geology, University of Alabama Dr. Ronald Buta, Department of Physics and Astronomy, University of Alabama Dr. Lyall Anderson, Curator of the Invertebrate Collection, National Museum of Scotland Dr. Loren Babcock, Department of Geology, Ohio State University Dr. Judith Morgan, Department of Biology, Emory University This research was funded by Grant No. 52003071 from the Howard Hughes Medical Institute.
Modern horseshoe crabs are virtually identical to fossil horseshoe crabs going back at least 380 million years. The only notable difference is size. However, preservation of certain details and soft-tissue in fossil horseshoe crabs is sketchy at best. This is unfortunate, considering that sex is impossible to tell except with soft tissue, specifically the first pair of legs. This study was the first to attempt to identify individual horseshoe crab’s sex through tracks. The tracks that were studied were from a coal mine in northwestern Alabama, and were 310 million years old. Population dynamics were studied, resulting in a reconstruction of a juvenile-biased population. The study also revealed undulating burrowing, a behavior that is not known to exist in the present. This behavior was through a fine-grained mud, which is not a suitable habitat for modern horseshoe crabs. This suggests that the gills have been modified to become more specialized in the modern forms, but were more versatile in the past. With that perspective, the fact that they look almost identical to modern forms is interesting. All aspects of the study will be interesting, but determining sex in horseshoe crab tracks will add a new dimension in future study.
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