The habitat of the early vertebrates: trace and body fossil evidence from the Harding Formation (Middle Ordovician), Colorado

A unique assemblage of trace fossils, body fossils, and sedimentary structures from the lower Harding Formation near Florence, Colorado show the Middle Ordovician vertebrates lived in an estuarine environment dominated by merostome and arachnid arthropods consisting of Aglaspidida, Synziphosurina, Limulina, and Scorpionida. Bec1a'ithia gigantea n. sp., a large aglaspidid, Cruziana canonensis n. sp., probable aglaspidid resting sites, Homopodichnus polydactylus n. gen., n. sp., a Synziphosuran walking trace, Xouphichniwn cordiformis n. sp., a limuloid resting site, Corpusculichnus caudatus n. gen., n. sp., a limuloid burrow, Paleohelcura delicatula n. sp., a scorpion walking trace, Arachnomorphichnus thorsoni n. gen., n. sp., resting sites and locomotor traces of a transitional arachnomorph sharing both scorpionid and eurypterid affinities, and Caridoidichnus curtisae n. gen., n. sp., the probable trace of a syncarid crustacean, are all described and figured. The geologic ranges of the Synziphosurina, Limulina, Scorpionida, possibly the Syncarida, and the aglaspidid genus Beckwithia are adjusted to the Middle Ordovician. The fauna and ichnofauna is associated with an articulated posterior portion of the vertebrate Astraspis desiderata Walcott and with Agnathichnus fOssor n. gen., n. sp., an ostracoderm feeding trace. Members of the ecosystem inhabited fluvial point-bar deposits in an estuarine environment during a time of rising sea level. Predation by merostomes was probably an important factor in the evolution of heterostracan armor plate.