page_title: Adelophthalmus
section_title: Age-based segregation
hierarchical_section_title: Adelophthalmus / Paleoecology / Age-based segregation
caption_attribution_description: Limulus polyphemus&#160;(Linnaeus, 1758) English: Atlantic horseshoe crab at St. Lucie County Marine Center in Fort Pierce, St. Lucie County, Florida, U.S.A.
context_page_description: Adelophthalmus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Adelophthalmus have been discovered in deposits ranging in age from the Early Devonian to the Early Permian, which makes it the longest lived of all known eurypterid genera, with a total temporal range of over 120 million years. Adelopthtalmus was the final genus of the Eurypterina suborder of eurypterids and consisted the only known genus of swimming eurypterids from the Middle Devonian until its extinction during the Permian, after which the few surviving eurypterids were all walking forms of the suborder Stylonurina.
  Fossils of Adelophthalmus have been described from four continents; North America, Europe, Asia, Australia, which indicates that Adelophthalmus might have had a nearly cosmopolitan distribution, one of few eurypterid genera to achieve one besides potentially Pterygotus. The territorial expansion of Adelophthalmus had begun early, with representatives found in both Siberia and Australia during the Devonian, but it first gained its almost cosmopolitan distribution following the amalgamation of the supercontinent Pangaea during the Carboniferous and Permian.
context_section_description: In the Madera Formation, Adelophthalmus and associated organisms lived in bodies of brackish to fresh water in what is assumed to have been a deltaic plain. The lack of large coal beds suggests that the fossil localities which have yielded Adelophthalmus was a moderately elevated region with less dense vegetation and better drainage than the swamplands that occupied much of the rest of the United States. The discovery of a large assemblage of A. luceroensis, including several adults and juveniles, allowed researchers to determine different habitat preferences for different age groups. Larger individuals (adults) are found associated with large plant fragments, including branches of Walchia and leaves of Cordaites, but smaller individuals (juveniles) are found in fossil beds containing less organic material and mostly smaller plant fragments. The large plant fragments of the adult habitat were deposited in quiet conditions, likely through leaves dropping into enclosed lagoons or standing ponds.
  The juveniles appear to have developed and lived in somewhat different conditions than the adults. In beds were juveniles are more common, insect fossils are more common as well, indicating a lack of adults that were capable of devouring them, and the presence of smaller plant fossils suggest a less prolific vegetation cover, the juvenile environment possibly having been lower areas on the delta plain between the ponds. Periodically, storms would drive marine water into the ponds, where salinity would thus be variable, while juveniles could live in fresher and less variable environments further away from the shoreline. It is possible that the adults mated in the streams that fed the ponds, and then returned to live in the ponds because of a richer food supply being present.
  Age-based segregation of this kind between juveniles and adults of the same population is relatively normal in arthropods, for instance, juveniles of the related and modern Limulus live in different environments and regions than the adults. The advantage of this form of segregation is not only to allow younger individuals to live in conditions more stable from a salinity standpoint, but also to keep juveniles safe from situations in which substantial amounts of marine water decimated the populations in the ponds by altering the living conditions too much. In such a situation, younger populations could after some time recolonize the old habitats.
caption: The same type of age-based segregation of individuals seen in modern horseshoe crabs (genus Limulus, pictured) has been inferred from Adelophthalmus.
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