(click on images for pictorial guide)
last revised 09 JULY 2020 by S. M. Gon III
Introduction: Split from the
Ptychopariida (as Harpetida, see Ebach &
McNamara 2002), but originally described by Whittington
1959 as Order Harpida, its most advanced members (family
Harpetidae) easily distinguished by marginal sutures and
lack of rostral plate, as well as the presence of the
|ADDITIONAL CLASSIFICATION NOTES FOR
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 suborder Harpina to ordinal status within the trilobite subclass Librostoma.
Adrain (2013) argued that Ebach & McNamara's use of the name Harpetida was not necessary:
" Ebach & McNamara (2002, pp. 237–238) changed the name of the taxon to Harpetida Whittington, 1959. The basis for this was ICZN Opinion 1436 (1987), which emended the family name Harpida Hawle & Corda, 1847 to Harpetidae owing to homonymy of the former. Names above family-group rank, however, are not governed by the Code. McNamara et al. (2009, p. 14) cited the name as Order Harpetida Ebach & McNamara, 2002, despite authorship actually having been attributed to Whittington in that work. As the concept of the taxon remains attributable to Whittington, and there is no requirement to alter its name in light of the ICZN, I see no reason not to maintain Whittington’s original spelling and authorship."
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 Harpida 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.
Adrain, J.M. 2013. A synopsis of Ordovician trilobite
distribution and diversity. in: Harper, D. A. T.
& Servais, T. (eds) 2013. Early Palaeozoic
Biogeography and Palaeogeography. Geological Society,
London, Memoirs, 38, 297–336.
Beu, A.G. 1971. Cassidae and Harpidae: Two family group
homynyms in Molluska and Arthropoda. Z.N.(S.) 1938. Bulletin
of Zoological Nomenclature 28:564-86.
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