TRANSITORY PYGIDIA IN HINTZEIA SP. NOV. AND THE DYNAMICS OF THE POSTERIOR “TAGMA” IN TRILOBITES

The transitory pygidium in trilobites accreted exoskeletal segments posteriorly while releasing them anteriorly during the meraspid growth phase. This dynamic passage of segments through the transitory pygidium has been interpreted to indicate that the structure is an unique “tagma” within arthopods. During the meraspid phase, many trilobites had an accumulation phase in which the rate of accretion of segments in the transitory pygidium exceeded that of their release into the thorax. This was followed, in some trilobites, by an equilibrium phase in which accretion rate equaled release, and then some trilobites had a shedding phase in which accretion rate declined but release continued until the onset of the holaspid phase. Here we describe the meraspid pygidial development of Hintzeia sp. nov. (Pliomeridae) from the Lower Ordovician Broken Skull Formation of Mackenzie Mountains of the western District of Mackenzie. Hintzeia sp. nov. underwent an accumulation phase followed directly by a shedding phase. The terminal trunk segment in mature Hintzeia sp nov. was distinctive from its first appearance at the posterior of the transitory pygidium; its first appearance was coincident with the beginning of the shedding phase. Other segments of the mature pygidium of Hintzeia sp nov. do not appear to differ in shape from those destined to become thoracic segments. We can therefore conclude that the holaspid pygidium itself in Hintzeia sp. nov. may not constitute a distinct tagma in the conventional sense, but the distinct terminal piece of the pygidium may do so. The tagmata of extant arthropods, once organized, apparently remain stable both over the life of an individual and over evolutionary time. Our findings suggest that use of articulation as the sole criterion for recognizing tagma in trilobites hinders comparison with the body plans of living arthropods.