21140 - Soft Bodied Aglaspid (Tremaglaspis) + Xiphosurid (Horseshoe Crab Ancestor) Lower Ordovician
21140 - Soft Bodied Aglaspid (Tremaglaspis) + Xiphosurid (Horseshoe Crab Ancestor) Lower Ordovician 21140 - Soft Bodied Aglaspid (Tremaglaspis) + Xiphosurid (Horseshoe Crab Ancestor) Lower Ordovician 21140 - Soft Bodied Aglaspid (Tremaglaspis) + Xiphosurid (Horseshoe Crab Ancestor) Lower Ordovician 21140 - Soft Bodied Aglaspid (Tremaglaspis) + Xiphosurid (Horseshoe Crab Ancestor) Lower Ordovician 21140 - Soft Bodied Aglaspid (Tremaglaspis) + Xiphosurid (Horseshoe Crab Ancestor) Lower Ordovician 21140 - Soft Bodied Aglaspid (Tremaglaspis) + Xiphosurid (Horseshoe Crab Ancestor) Lower Ordovician 21140 - Soft Bodied Aglaspid (Tremaglaspis) + Xiphosurid (Horseshoe Crab Ancestor) Lower Ordovician 21140 - Soft Bodied Aglaspid (Tremaglaspis) + Xiphosurid (Horseshoe Crab Ancestor) Lower Ordovician

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21140 - Soft Bodied Aglaspid (Tremaglaspis) + Xiphosurid (Horseshoe Crab Ancestor) Lower Ordovician

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Species
Tremaglaspis unite - Aglaspid + Xiphosurid (Horseshoe crab ancestor)
Age
Lower Ordovician, Floian stage (~477 Million Years)
Location
North of Zagora, South Morocco
Formation
Upper Fezouata Formation (Outer Feijas Group)
Size
24.4 mm   •    in
Weight
403 g   •    oz
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Description

Xiphosurans are an order of arthropods related to arachnids, or, according to one recent study, actually arachnids. They are sometimes called horseshoe crabs (a name applied more specifically to the only extant family, Limulidae). They first appeared in the Tremadocian (Lower Ordovician) from Fezouata Fm in South Morocco. Currently, there are only four living species. They are members of the order Xiphosura, which contains two suborders, Xiphosurida and Synziphosurina.

The group has hardly changed in millions of years; the modern horseshoe crabs look almost identical to prehistoric genera such as the Jurassic Mesolimulus, and are considered to be living fossils. The most notable difference between ancient and modern forms is that the abdominal segments in present species are fused into a single unit in adults.

Aglaspidida is an extinct order of aquatic arthropods that were once regarded as primitive chelicerates. However, anatomical comparisons demonstrate that the aglaspidids cannot be accommodated within the chelicerates, and that they lie instead within the Artiopoda, thus placing them closer to the trilobites. Aglaspidida contains the subgroups Aglaspididae and Tremaglaspididae, which are distinguished by the presence of acute/spinose genal angles and a long spiniform tailspine in the Aglaspididae.

Aglaspidida, Walcott (1912) represents a major and diverse –
yet historically problematic – group of early Palaeozoic euarthropods typified by a biomineralized phosphatic exoskeleton (Briggs & Fortey, 1982). Although
aglaspidids are relatively poorly understood, recent studies have produced significant insights into their morphology (e.g. Van Roy, 2006), biostratigraphic range (e.g. Fortey & Rushton, 2003, 2009; Lerosey-Aubril et al. 2013), palaeobiogeographic distribution and phylogenetic position
within the evolutionary context of Artiopoda, and even the entire phylum Euarthropoda.

The conservation of soft parts in the fossil record is somewhat exceptional and only occurs under certain and exclusive taphonomic conditions. For example, anoxic substrates without oxygen, where the organic matter has difficulty decomposing.

The Fezouata Formation occurs in the lower part of the Lower to lower Middle Ordovician Outer Feijas Group, in the western, central and eastern Anti-Atlas
Mountains, southern Morocco (Destombes et al.1985; Gutierrez-Marco & Martin 2016).

It comes from The Konservat-Lagerstätten of Lower Fezouata Fm that spans the entire Tremadoc stage of the Ordovician system and is only present in the Moroccan Anti-Atlas. The discovery of the Fezouata biota in the latest Tremadocian of southeastern Morocco has significantly changed our understanding of the early Phanerozoic radiation. The shelly fossil record shows a well-recognized pattern of macroevolutionary stasis between the Cambrian Explosion and the Great Ordovician Biodiversification Event, but the rich soft-bodied Fezouata biota paints a different evolutionary picture. The Fezouata assemblage includes a considerable component of Cambrian holdovers alongside a surprising number of crown group taxa previously unknown to have evolved by the Early Ordovician. Study of the Fezouata biota is in its early stages, and future discoveries will continue to enrich our view of the dynamics of the early Phanerozoic radiation and of the nature of the fossil record.

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