Błażejowski et al.—A new Triassic limulid from Tunisia
communities during the end-Permian mass extinction and its Triassic aftermath (Gall and Grauvogel-Stamm, 2005). In the detailed investigations of Lamsdell (2016), L. bronnii is placed within the Limulidae, next to genera forming the xiphosurid crown group (the extinct Crenatolimulus [Feldmann et al., 2011] and the extant Carcinoscorpius, Limulus, and Tachypleus).
Preservation and habits of Limulitella tejraensis n. sp.—The state of preservation of the material found at Tejra is not ideal, owing to the nature of its compaction, as indicated by wrinkling along the anterior of the prosomae. The vast majority of the specimens are incomplete (i.e., preserved as isolated prosomal and opisthosomal fragments) and represent all four stages of disarticulation (Fig. 3), as observed by Babcock et al. (2000). The specimens described here are relatively small, and with respect to their presumed close relationship with extant limulines they mostly probably represent the remains of juveniles only. Jurassic horseshoe crabs known from the fossil record seem to represent juvenile forms (Błażejowski, 2015; Błażejowski et al., 2015, 2016). The giant limulid trackways, Kouphichnium lithographicum Oppel, 1862, reported from Germany and France (Schweigert, 1998; Gaillard, 2011) seem to support this interpretation. Within the strata containing Limulitella tejraensis n. sp.,
thin-shelled bivalves and clam shrimp (conchostracans) are present (Fig. 3.8), and these may have provided a diet for the horseshoe crabs. The diet of fossil and the modern Atlantic Limulus is highly diverse and consists of a variety of small marine organisms, including soft-shelled bivalves, gastropods, polychaetes, and crustaceans (Botton, 1984; Botton and Ropes, 1989; Kin and Błażejowski, 2014). Sedimentological and paleontological data from the section
that yielded the Limulitella tejraensis n. sp. specimens suggest brackish/freshwater conditions. Forms inhabiting such environ- ments are known from the fossil record since the late Paleozoic (Lamsdell, 2016). In his phylogenetic analysis, Lamsdell (2016) demonstrated that limulines colonized non-marine environ- ments many times throughout their evolutionary history. In addition to the suborder Limulina, the Belinurina are a common component of late Carboniferous to early Permian freshwater/ brackish coal swamps (Filipiak and Krawczyński, 1996; Anderson, 1997; Anderson and Selden, 1997; Lamsdell, 2016). The loss of marine habitat and colonizing of brackish/ freshwater environments are probably direct effects of the turbulent history of shallow-marine ecosystems at the end of Paleozoic (Foster and Twitchett, 2014). Rapidly alternating cool and warm periods during the ensuing Carboniferous Ice Age caused deep changes in the inhabitable environments (DiMichele et al., 2001; Scheffler et al., 2003; Peyser and Poulsen, 2008). Coastlines fluctuated widely owing to local basin subsidence and worldwide sea-level changes (Haq and Shutter, 2008). Continents aggregated, forming Pangea; coast- line length decreased; more common deltaic environments supported fewer corals, crinoids, and bryozoans (Stanley and Powell, 2003; Veron, 2008), and many groups of animals, including bivalves, gastropods, bony fish, and horseshoe crabs, were forced to adapt to freshwater/brackish environments
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(Wesselingh, 2007; Sallan and Galimberti, 2015; Lamsdell and Selden, 2016). Environmental conditions changed in the late Permian,
when, as a result of decreasing glaciations, the interiors of continents became drier and brackish/freshwater areas were probably reduced (Erwin, 1993; Twitchett et al., 2001; Clarkson et al., 2015). At that time, the largest mass extinction (end-Permian extinction) recorded in the history of life on Earth began (Shen et al., 2006). Although it affected many groups of organisms in many different ecosystems, shallow-marine communities suffered preferentially. The current fossil record indicates that the belinurines did not survive the late Paleozoic environmental changes. A few horseshoe crab taxa, including ancestors of the Triassic genus Limulitella, were presumably so tolerant of changing conditions throughout the late Paleozoic that they survived the end-Permian mass extinction. In the early Mesozoic, the ancestral survivors presumably evolved into the Limulidae horseshoe crab lineage, which survives into modern times. In the Early–Middle Triassic, some lineages with unusual or aberrant morphologies also evolved (e.g., Austrolimulus). The late Early to early Middle Triassic represents a time of recovery following the end-Permian mass extinction (Erwin, 2006; Knoll et al., 2007; Hu et al., 2011; Chen and Benton, 2012). Thus, it can be considered an important stage in the evolution history of Limulina. Despite the poor state of preservation, horseshoe crabs known from the Lower Triassic (Olenekian) from Ankitokazo Basin, Madagascar (Hauschke et al., 2004) and Vetluga Series of Russia (Ponomarenko, 1985) were identified as Limulitella. Worldwide distribution of this group in various localities and different ecosystems provides evidence for rapid adaptive radiation of the group shortly after the biotic crisis, developing adaptive capability for survival in divergent environmental niches. The assumed high degree of environmental tolerance of the genus Limulitella reflects their ability to exist in both marine habitats and freshwater/brackish environments when forced to do so. All three extant genera exhibit a high tolerance to changes of salinity in inhabited areas (Shuster, 1982; Ehlinger and Tankersley, 2009). The Indian species, Carcinoscorpius rotundicauda, is observed to migrate astonishing distances from the mouth up to 150km to the source area of the Hooghly (Ganges) River (Annandale, 1922). Thus, the evolutionary success of horseshoe crabs results in
its most important part from the specialization, which played a key role during the re-establishment of the biota after the end- Permian extinction event.
Conclusion
Because xiphosurid arthropods are extremely rare in the fossil record, the recent discovery of the Middle Triassic fossils are of considerable importance in bridging the existing gap in our knowledge of limulid distribution and diversification during the early Mesozoic. The state of preservation and associated fauna indicate that the depositional area may have been a feeding zone for Limulitella tejraensis n. sp. While this new discovery of Triassic horseshoe crabs has only a limited bearing on phylogenetical relationships within the group, it certainly sheds new light on several aspects of the origin of extant
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