Fossil record

Xiphosuran fossils are comparatively rare in the fossil record due to taphonomic factors. The carapace (exoskeleton) is composed of unmineralised chitin hence preservation is dependant on the environmental setting. Fossil Xiphosura are usually only found at sites of extraordinary preservation (Lagerstatten) such as coal swamps as opposed to Trilobites, which are a common occurence in the fossil record. They have a mineralised exoskeleton thus are found in a wide range of lithological facies. Comparatively, xiphosuran body fossils are rare in comparison to their trace fossil occurence. Trace fossils have been used to study population data and to identify sexual dimorphism and maturity. A type specimen for xiphosuran trackways (undertracks) is known as Kouphichnium from the Union Chapel Mine, northwestern Alabama, USA, dating to 310 million years ago.

Xiphosurid evolution stems from the ancestral order Synziphosurida, which itself has a fossil record going back to the Silurian. The xiphosuran fossil record begins in the Late Devonian presumably with a descendant of Kasibelinurus, which is the last and most advanced synziphosurid.

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Order Synziphosurida

Cyamocephalus loganensis
Length about 5 cm, from the family Pseudoniscidae. Found in Scotland, Silurian age.

(Drawing by Lyall Anderson, used with permission)

The synziphosurines dating from the Silurian to the Early Devonian are a fairly diverse ancestral group of the earliest Xiphosura (see taxonomy). Phylogenetic analysis places the synziphosurines as a paraphyletic group to the Xiphosura. They were rather trilobite-like in appearance with a large cephalothorax covered by a carapace. The opisthosomal segments (abdominal) were unfused (free) and not chelate unlike in the modern xiphosurans. Additionally the telson (tail) was shorter than in modern xiphosurans. The most common palaeoenvironmental habitats in which fossil Synziphosurids are found are brackish or freshwater biomes, and occasionally in marginal marine environments. Synziphosurid remains are often associated with eurypterids and ostracoderms.

Picture	Taxonomic Classification	Time Range	Habitat	Representative Taxa	Description
	Family - Weinberginidae	Pragian to Emsian	Marine	Weinbergina opitzi named by Richter and Richter, 1929 Willwerathia laticeps named by Størmer, 1936 Legrandella lombardi named by Eldredge, 1974	Medium-sized forms with large semicircular smooth prosoma. Small trilobite-like abdomen. The most primitive of the Synziphosurida, representing a side branch of Xiphosuran evolution.
	Family - Bunodidae		Marginal Marine	Packard 1886	Small elongate forms, with an ornamented prosoma, reduced abdominal segments and distinct post-abdominal segmentation.
	Subfamily - Bunodinae	Llandovery to Ludlow	Marginal Marine	Bunodes lunula named by Eichwald, 1854	Small forms with a vaulted and radially-lobed prosoma, post-abdomen with distinct segmentation.
	Subfamily - Limuloidinae	Wenlock to Ludlow	Marginal Marine	Limuloides limuloides	Small forms with a ridged and spiny prosoma
	Family - Pseudoniscidae	Llandovery to Pridoli	Marginal Marine	Pasternakevia podolica Cyamocephalus loganensis Pseudoniscus aculeatus	Small forms with a large, smooth and flat prosoma. No post-abdominal segmentation. Marginal (pleural) spines.
	Family - Kasibelinuridae	Givetian to famennian	Marginal Marine	Kasibelinurus amicorum named by Pickett, 1993	The most advanced Synziphosurines, presumably the ancestors to the Xiphosura.

(Drawings by Lyall Anderson. Used with permission.)

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• Characters/ anatomy
• Major subgroups in the phylum
• Fossil record 2
• Modern forms
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Author: Andrew Przewieslik
Last updated: 21/11/05
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