Naraoia compacta
The Burgess Shale arthropod exhibits characters in common with trilobites, most notably, limb morphology, a cephalon, and hypostome.
Tegopelte gigas, a large Burgess Shale species that Fortey & Theron (1994) considered a naraoiid, but other analyses place in another clade, the Tegopeltidae.
Please see related pages:
Paleozoic Arthropods
a discussion of the relationship of trilobites to other Paleozoic arthropods, and descriptions of other close trilobite relatives.
Trilobite Features
for a pictorial guide to the specific features that define trilobites.
Trilobite Ontogeny
to see how trilobites develop through three growth phases.
Evolutionary Trends
a discussion of the many directions that trilobite evolution has taken.
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| Naraoia spinosa, from the Early Cambrian Chengjiang deposit, Yunnan, Southern China. | | Tariccoia arrusensis, an Ordovician naraoiid from Sardinia, Italy. | Soomaspis splendida, another Ordovician naraoiid from the Soom Shale, South Africa. |
Similarity of limbs between Naraoiids and Trilobites
The similarity between the limbs of naraoiids and primitive trilobites is quite striking. Based on limb structure alone it would seem incontrovertible that trilobites and naraoiids belong to the same clade. The limb reconstructions of the naraoiid Misszhouia longicaudata and the trilobite Eoredlichia intermedia demonstrate this startling similarity quite clearly.
In the two reconstructions above, the podomeres (leg segments) of the endopod (walking limb) are numbered, while the gill- bearing exopod is shown attached above, unlabeled. In life, the exopod would have been held perpendicular to the endopod (i.e., turned so that the gill filaments are pointing away from the viewer), but when individual limbs are preserved, the orientation shown above is commonly seen.This suggests that where the exopod and endopod are joined along the top of the coxa there is a flexible hinge-like joint that allows the gills to be held relatively stationary as the limbs move.
Order Nektaspida of the Class Trilobita?
Based on the similarities to trilobites (particularly limb similarities), Whittington (1985) considered Naraoia and its relatives to be members of an order of trilobites (Nektaspida) that lacks a calcified exoskeleton (he called them "soft-bodied trilobites"). Some analyses place species such as Naraoia, Misszhouia, Liwia, Buenaspis, Tariccoia, and Soomaspis closer to trilobites than to any other known arthropod clade, although they lack some important trilobite features. In his discussion of the relationship between naraoiids and Trilobita, Fortey (1997) suggested that the handful of naraoiids did not require ordinal status, and they would be treated as trilobites of uncertain ordinal status, based on their many similarities to calcified species.
Naraoiid Controversy:
The proposal to include Naraoiidae in the Class Trilobita is by no means unanimously supported. The naraoiids (and liwiids) could conceptually be included in the class Trilobita (with uncertain ordinal status, as Fortey proposes, or as a full order, as Whittington and others have suggested). However, although some cladistic analyses (e.g., Wills et al 1998) bring the Naraoiidae into the same clade as Trilobites, others (e.g., Edgecombe & Ramskøld 1999, Cotton & Braddy 2000, Zhang et al 2007) separate them distinctly among early Paleozoic clades. Cotton & Braddy (2000) place the pygidium-bearing arachnomorph families such as Helmetiidae and Xandarellidae closer to trilobites than the Naraoiidae or the Liwiidae. Moreover, in those analyses, the Tegopeltidae are removed from the Naraoiidae, since the Chengjiang tegopeltid Saperion clearly bears ventral eyes on stalks. By 2007, clarified by the excellent preservation of the Chengjiang fauna, Zhang et al flatly excluded naraoiids from the Trilobita, while further clarifying the relationships of trilobite-like arachnomorphs.
Developmental origins of agnostids and naraoiids
On the surface, naraoiids and agnostids seem similar: both are marked by species that are eyeless, isopygous, with reduced thoracic segment number, but with limbs similar to other trilobites. Considering their developmental origins draws attention to some important contrasts. In the course of typical development of trilobites (see figure below), the protaspid stage lacks both thoracic segments and pygidium. The transistion to the meraspid stage is marked by the addition of the pygidium, and subsequently, thoracic segments are added in each molt until the adult number of thoracic segments is achieved (holaspid stage). Naraoia superficially resembles a gigantic 0 degree meraspid, bearing a pygidium, but no thoracic segments (see trilobite ontogeny). The evolutionary process wherein changes in the rate or timing of growth, development, and reproductive maturity lead to evolution of new clades, is called heterochrony. There are two kinds of heterochrony: progenesis and hypermorphosis. The comparison chart below shows how hypermorphosis in naraoiids differs from progenesis, the process that gave rise hypothetically to the Agnostida. Progenesis involves maturation at an earlier developmental stage, which tends to create miniaturized species (see evolutionary trends in trilobites)
NORMAL MODE: normal growth rate, normal development rate, normal onset of maturation
The normal progression of growth and development in a typical trilobite begins with a protaspis of 0 segments (far left), a series of meraspid stages with increasing numbers of segments, leading ultimately to a holaspid with the adult number of segments (far right).
AGNOSTID MODE: normal growth rate, normal development rate, early onset of maturation
Via progenesis, normal protaspid to meraspid metamorphoses occur, but sexual maturity is reached in the early meraspid larval stage of a normal trilobite, leading to mature, but miniscule individuals with few segments, the typical state of Agnostida.
NARAOIID MODE: higher growth rate, inhibited development, normal onset of maturation
In contrast, hypermorphosis in naraoiids results in a higher growth rate, but development is greatly suppressed, leading to large adults that resemble gigantic meraspid larvae with zero or few thoracic segments.
Fortey and Theron (1994) suggested that because of the distinctly different developmental routes to produce naraoiids vs agnostids, their superficial similarities (few thoracic segments) were not sufficient to argue for a close phylogenetic relationship. Their differences (e.g., small size in Agnostida, non-calcified exoskeleton and macropygy in Naraoiidae) argued they belonged to distinct clades. Agnostida arguably represents a secondary loss of eyes, reduction in size and segment number, and specialization of limb form and function within the trilobite clade, while the Naraoiidae never developed dorsal eyes, calcified exoskeletons, or the numerous segments of the typical trilobite, yet share the primitive limb form of most arachnomorphs, including trilobites, helmetiids, tegopeltids, xandarellids, and naraoiids.
