Early Ordovician – Late Ordovician
Trinucleidae is an extinct family of fast-moving low-level epifaunal suspension feeders.
Description of the Family
- Cephalic fringe broad, sloping outward, with numerous pits on external surfaces.
- Occipital ring convex.
- Thorax with six segments,deep apodemal pits in articulating furrows.
- Pygidium triangular, length 0.25 to 0.5 of width; axis with many rings; pleural fields with shallow pleural furrows.
- Glabella and genae may have articulate pattern of raised edges.
Treatise on Invertebrate Paleontology, Part O (1959):
- “Cephalic fringe broad, sloping outward, bilaminar, with numerous opposed pits on external surfaces, extending posterolaterally somewhat behind rest of cephalon; convex occipital ring commonly with backward-directed spine, occipital furrow with deep apodemal pit; glabella expanding forward, reaching to inner margin of fringe, deep anterior pits at extremities of axial furrows; genae subtraingular in outline, with or without eye tubercles and faint eye ridges; lower lamellae of fringe bearing genal spines. Thorax with 6 segments; convex axial rings, with deep apodemal pits in articulating furrows; pleurae with broad diagonal pleural furrows, pleural tips bent down. Pygidium triangular, length 0.25 to 0.5 of width; axis with many rings; pleural fields with shallow pleural furrows. Surface of glabella and genae may bear erticulate pattern of raised ridges, margin of fringe and pygidial border with terraced lines.”