Middle Ordovician – Late Ordovician (?Silurian)
Primaspis is an extinct fast moving, low level epifaunal carnivore.
Identification in Hand Sample:
- Glabella small, 3p lateral lobes.
- Occipital ring not greatly lengthened or inflated, small occipital lobes.
- Librigenal spines broad at base, adjoining short lateral cephalic spines.
- Thorax with 10 segments; posterior pleural bands inflated at fulcra, stout posterior pleural spines, small anterior pleural spines.
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- On the librigenae, 13 large tubercles or short spines ornament the ventral side of the border. Six pairs of marginal spines diverge initially at an angle of 30 degrees; a very short distance behind the pygidial margin they curve inward so that their tips converge at an angle less than 15 degrees without meeting. Three pairs of anterior spines decrease in size forward. Of the two pairs of posterior spines the median pair is the larger; the second smaller pair is rooted in the base of the major pair of spines.
- Cranidium trapezoidal in outline, moderately convex axially, strongly convex transversely; glabella widest across basal glabellar lobes, narrowing anteriorly; two pairs of prominent lateral glabellar lobes, basal largest, both pair sub-oval in outline, much smaller third pair of lateral lobes apparently developed in anterior portion of glabella; frontal lobe of glabella slightly expanded anterolaterally; narrow anterior border separated from glabella by moderately deep furrow; occipital ring with pair of small spines and median granule, separated by shallow occipital furrow from glabella; occipital furrow deeper laterally with pits behind basal lobes; fixigenae strongly convex; adaxial corners of genal regions merging with anterolateral parts of occipital ring; anterior branch of facial sutures runs forward-inward posterior branch runs outward-backward; cranidium tuberculate except in deeper parts of furrows.
Treatise on Invertebrate Paleontology, Part O (1959):
- Glabella with small 3p lateral lobes; occipital ring not greatly lengthened or inflated, median tubercle or small paired occipital spines, small occipital lobes, eye lobes opposite basal glabellar lobes and about halfway across genal regions; librigenal spines broad at base, adjoining short lateral cephalic spines. Thorax with 10 segments; posterior pleural bands inflated at fulcra, stout posterior pleural spines, small anterior pleural spines.