(click on images for pictorial guide)
last revised 22 April 2008 by S. M. Gon III
Split from the Ptychopariida (see Ebach &
McNamara 2002), its most advanced members (family Harpetidae)
distinguished by marginal sutures and lack of rostral
plate, as well as the presence of the "harpetid brim."
CLASSIFICATION NOTES FOR HARPETIDA:
Fortey (1990) indicated that two diagnostic characters of Order Ptychopariida are the presence of a rostral plate, and opisthoparian sutures. Ebach & McNamara (2002) pointed out that all members of the family Harpetidae lack a rostral plate and bear a marginal facial suture, therefore can not be defined as Ptychopariida. Consequently, they raised Harpetida to ordinal status within the trilobite subclass Librostoma.
Fortey erected the Librostoma (1990) to act as a high-level monophyletic group containing all of the "ptychopariid" (sensu Treatise 1959) orders and suborders. As Proetida, Asaphida, and Harpetida were distinguished from the old Ptychopariida clade, the subclass Librostoma serves to highlight their shared Ptychopariida ancestry.
Peng et al (2004) tentatively assigned the Upper Cambrian Baikadamaspis (see drawing, lower left) to the Harpididae, based on its "general similarity to Entomaspis." This is inconsistent with recognition of the family Entomaspididae, separate from the Harpididae, and I assume Peng et al do not recognize family Entomaspididae, lumping its members into Harpididae. I include Baikadamaspis in Entomaspididae in the genera listings above. Baikadamaspis bears a rostral plate and lacks marginal sutures, violating the defining features for advanced members of Harpetida sensu Ebach & McNamara (2002). The convergence of the anterior and posterior sutures, and the glabellar, pygidial, and other similarities to Entomaspis and Loganopeltoides do strongly suggest an affinity, and if the basal clade members of Harpetida lack synapomorphies of the more advanced members, this is analogous to basal members of Asaphida (e.g., Anomocaroidea) that lack the median ventral suture. More research and discussion on the evolution of the marginal sutures of Harpetida from its presumedly opisthoparian ptychoparioid ancestors is needed, in which the suture patterns of Baikadamaspis and Entomaspis may represent intermediate stages toward the marginal state. Basal Harpetida (especially the Entomaspididae) would be paraphyletic with Ptychopariida.
A note on the name Harpidae vs Harpetidae: Harpidae was once used as the name for the trilobite family containing the type genus Harpes. However, this is in conflict with the use of the same name for a family of extant mollusks. Beu (1971) pointed out the conflict and the precedence of Harpidae for the molluscan family. In 1987 Harpetidae Hawle & Corda 1847 and Harpididae Whittington 1950 were added to the official list "Names in Zoology" per Opinion 1436 of the ICZN.
Beu, A.G. 1971. Cassidae
and Harpidae: Two family group homynyms in
Molluska and Arthropoda. Z.N.(S.) 1938. Bulletin
of Zoological Nomenclature
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