Ontogenies of some Ordovician Telephinidae from Argentina, and larval patterns in the Proetida (Trilobita)

Journal of Paleontology, Mar 1999 by Chatterton, Brian D E, Edgecombe, Gregory D, Vaccari, Norberto E, Waisfeld, Beatriz G

Type C protaspides are interpreted by us to be adultlike and to represent benthic growth stages.

Life history patterns of Proetida.-One of the most useful characters for demonstrating monophyly of higher taxa is the presence of a single, derived life history pattern among the early growth stages (e.g. Asaphina in Fortey and Chatterton, 1988; Calymenina in Chatterton et al., 1990). We have shown in Figure 2 the known distribution of larval types described above among Proetida, including the number of instars within each type. A number of different patterns are apparent in this diagram. Suggested patterns include: 1) entirely benthic stages; 2) a single pelagic stage, followed by benthic stages; 3) an extensive series of benthic stages (protaspides and early meraspid stages), then pelagic stages; and 4) entirely pelagic stages.

Life history patterns represented entirely by benthic stages include those ontogenies where type B larvae are followed by type C larvae [e.g., Aulacopleuridae, Bathyuridae, Dimeropygidae, Hystricuridae, Scharydae, Toernquistdae, and some Telephinidae (Goniophrys and perhaps Carrickia)].

The second life history pattern is represented by species where a single pelagic type A larva is followed by benthic stages, including type C metaprotaspid larvae (e.g., Proetidae and Tropidocoryphidae).

The third life history pattern is represented by species with benthic larval stages (types B and C) followed by benthic stages into the late meraspid period and then, after a radical metamorphosis, the later stages are all pelagic (e.g., species of Telephina described in this work).

The fourth life history pattern is represented by other telephinid trilobites (e.g., some species of Telephina), where a bulbous type A larval stage is followed by large and somewhat unusual type B larval stages, with eyes that may see better ventrally than dorsally. Small meraspid stages of this group of taxa are not well known. However, it is possible that most, if not all, of the postembryonic life cycle was pelagic. These forms could have molted, however, on the sea floor.

The situation for Carolinites is not clear. The shapes of the small, rather unusual, type B protaspides could be interpreted as benthic or pelagic. They vary considerably in morphology between different species. No type C protaspides have been found for this genus. Small meraspides appear benthic in form and include only moderate sized eyes. Large holaspides are likely epipelagic in form (Fortey, 1985; McCormick and Fortey, 1998), although some partly articulated molts demonstrate that adult stages must have molted on the sea floor (Fortey, 1975a, p. 114, fig. 15, pl. 36, fig. 19, pl. 37, fig. 19). This genus could have had a life cycle similar to that of the species of Telephina described here, consisting of a long benthic stage, followed by pelagic mature stages.

One obvious difference among Proetida is the presence or absence of a type A larva. The type A protaspis appears to be highly plesiomorphic, because small globular larvae with three pairs of marginal spines, an anterior pair of pits, incurved rather than inturned doublure, anteriorly constrained facial sutures, and large, marginally spinose hypostomes occur in a wide range of orders and suborders (Asaphina, Calymenina, Lichida, Ptychoparda, Scutelluina). However, these small protaspides are not identical for all of these groups, and they have not yet been found in the earliest and most primitive Proetida. They may be derived for the Proetida. This type of protaspis is present in some Telephina but absent in the apparently closely related Carolinites and in the species of Telephina we describe in this work. The reappearance of this primitive looking larva in two different groups of Proetida could be controlled either by the timing of onset of calcification after hatching or by the stage of maturity when the first larva hatched from the egg. In other words, this stage could be passed through while the trilobite was still an embryo. The presence or absence of a type A larva affects whether or not the earliest part of the life cycle is considered to be pelagic. It is possible that some species had uncalcified pelagic stages immediately after hatching that have not been preserved.