THE ONTOGENY OF TRILOBITES:
THE  ONTOGENY OF PROETOIDS
GENERALITIES:
   The early ontogeny of proetoid trilobites is typically composed of two distinct larval stages (protaspid stages). The first stage (anaprotaspis) is represented by a bulbous larva, which probably had a planktonic mode of life. In comparison, the second kind of proetoid larvae (metaprotaspis) is flatter and devoid of lateral spines, and it greatly resembles adults of the same species. Also, it is generally accepted that proetoid metaprotaspides probably represented benthic larval stages.
    The significant differences (both morphological and ecological) between ana- and metaprotaspides suggest that a metamorphosis might have occurred in the early ontogeny of proetoid trilobites.          
Figure 1: Typical larval stages of proetoid trilobites. a. anaprotaspis. b. metaprotaspis. Scale 0.5 mm.
MORPHOLOGY & SYSTEMATICS:
   The anaprotaspis of proetoids remained highly conservative from the Ordovician to the Early Carboniferous. It is characterized by a bulbous shape, three pairs of spines that project both laterally and ventrally, and a lens-shaped hypostome bearing a postero-medial spine. All these characteristics are unique among the Proetida and suggest that this order may be composed of two different clades: the Proetoidea and the Aulacopleuroidea-Bathyuroidea group. Though more variable in terms of morphology, metaprotaspides also indicate the particularity of the Proetoidea within the order. Indeed, they are devoid of the complex pattern of tubercles visible in aulacopleuroid and bathyuroid metaprotaspides.   
Figure 2: Proetoid anaprotaspides from the Ordovician to the Lower Carboniferous. Note that proetoid anaprotaspis is very conservative through time concerning its shape but its size significantly varies. Same scale for all the drawings.
Proetoid protaspides have been described or figured in:

BEECHER, C.E. 1893. A larval form of Triarthrus. American Journal of Science, 46:361-362.
CHATTERTON, B.D.E. 1971. Taxonomy and ontogeny of Siluro-Devonian trilobites from near Yass, New South Wales. Palaeontographica (A), 137: 1-108.
CHATTERTON, B.D.E., & S.E. SPEYER. 1997. Ontogeny, p. 173-247. In R. L. Kaessler (ed.), Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita (revised). Geological Society of America and University of Kansas Press, Lawrence, 530 p.
CHATTERTON, B.D.E., G.D., EDGECOMBE, N.E., VACCARI & B.G. WAISFELD. 1999. Ontogenies of some Ordovician Telephinidae from Argentina, and larval patterns in the Proetida (Trilobita). Journal of Paleontology, 73(2): 219-239.
CRONIER, C., R., FEIST, K., BARTZSCH, & D. WEYER. 1999. Larval morphology and ontogeny of a Late Devonian phacopid with reduced sight from Thuringia, Germany. Journal of Paleontology, 73(2): 240-255.
EDGECOMBE, G.D., B.D.E., CHATTERTON, N.E., VACCARI & B.G. WAISFELD. 1997. Ontogeny of the proetoid trilobite Stenoblepharum, and relationships of a new species from the Upper Ordovician of Argentina. Journal of Paleontology, 71(3): 419-433.
LEROSEY-AUBRIL, R. 2006. Ontogeny of Drevermannia and the origin of blindness in Late Devonian proetoid trilobites. Geological Magazine, 143(1): 83-104.    Abstract   pdf version
LEROSEY-AUBRIL, R. & R. FEIST. 2005. First Carboniferous protaspid larvae (Trilobita). Journal of Paleontology, 79(4): 702-718.    Abstract
LEROSEY-AUBRIL, R. & R. FEIST. 2005. Ontogeny of a new cyrtosymboline trilobite from the Famennian of Morocco. Acta Palaeontologica Polonica, 50(3): 449-465. Abstract    pdf version
WHITTINGTON, H.B. 1957. Ontogeny of  Elliptocephala, Paradoxides, Sao, Blainia, and Triarthrus (Trilobita). Journal of Paleontology, 31: 934-946.
(LARVAE & TRILOBITE ORDERS) (MY ABSTRACTS 1)
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