Crinoid

Reteocrinus alveolatus (atypical)

• Ordovician
• Bobcaygeon Formation
• Brechin, Ontario, Canada

Size: 4 cm crown

According to Forest Gahn of BYU-I, this is likely a bizzarre mutant of Reteocrinus alveolatus. Quite unlike almost any other known crinoid and conflicting with its genus/species definition, this crinoid's radials bear 2 arms each on the A, C, D rays (effectively acting as axillary primibrachials) and no arms at all on the B, E rays. This sort of pattern is reminiscent of Cromyocrinid arm branching groupings, though that is a family of Pennsylvanian-aged cladids and thus far-removed from this specimen (an Ordovician-aged diplobathrid camerate) in time and phylogeny. In addition, according to Forest the strong association of the primanal with the C radial and its complete separation from the D radial makes it disparid-like in some respects. Perhaps this mutant hints at something fundamental to crinoid developmental programming. Forest has generously performed additional preparation and photography to produce the images here.