The Cambrian (Sunwaptan, Furongian) agnostoid arthropod Lotagnostus Whitehouse, 1936, in Laurentian and Avalonian North America: systematics and biostratigraphic significance.

Abstract

Restudy of type and other material shows that the Cambrian (Sunwaptan, Furongian) agnostoid arthropod <i>Lotagnostus americanus </i>(Billings, 1860), far from being distributed globally, can be identified with confidence only at its type locality of Quebec. As such, it is ill-suited as an index for international correlation. <i>Lotagnostus obscurus </i>Palmer, 1955, from Nevada is not a synonym of <i>L</i>. <i>americanus </i>and is most similar to other effaced species from Australia. Sclerites from Avalonian Canada are represented by perhaps as many as two species, one of which is <i>L</i>. <i>germanus </i>(Matthew, 1901), and the other is similar to the type species, <i>L. trisectus </i>(Salter, 1864). However, loss of information due to compaction makes <i>L. trisectus </i>difficult to interpret in its type region of Avalonian Britain, and possible occurrences in other areas are therefore questionable at best. At the current state of knowledge, this species name should not be applied outside of Avalonian Britain. Aside from <i>L</i>. <i>obscurus</i>, which may possibly appear in slightly older strata, all Laurentian occurrences are confined to Upper Sunwaptan successions. Most are in the <i>Illaenurus </i>and <i>Prosaukia pyrene </i>zones, although review of material assigned to <i>L. hedini </i>(Troedsson, 1937) in western Newfoundland suggests that this species may occur, albeit questionably, only in the basal Skullrockian <i>Phylacterus saylesi </i>Fauna; indeterminate species occur in the <i>Keithia schucherti </i>Fauna in the latter region. Occurrences in Avalonian Canada are all from the upper <i>Peltura </i>Zone, and are likely somewhat younger (<i>Saukiella junia </i>Subzone and correlatives) than those in Laurentian successions

Intraspecific variation and taphonomic alteration in the Cambrian (Furongian) agnostoid Lotagnostus americanus: new information from China

“…In our view, the morphologic and geographic limits placed on L. americanus and other agnostoids cited by Landing et al (2010 and Westrop et al (2011) are overly restrictive, and have resulted in an over-splitting of species. Intraspecific morphologic variation in agnostoids has been documented by numerous authors, as emphasized by Rowell et al (1982), Pratt (1992), and Peng & Robison (2000).…”

“…This paper responds to the arguments raised by Westrop et al (2011) on the concept of Lotagnostus americanus and by on the purported diachroneity of the first occurrences of Lotagnostus. The present study focuses on material from large collections made in three regions of China, but draws on comparative material from elsewhere in the world.…”

“…Troedsson's (1937) sclerites, which are from a single locality in the Charchaq Range, vary in having a weakly to strongly furrowed cephalon with normal sized basal lobes and a faint to distinct trisection of the posteroaxis of the pygidium. As such, these specimens exhibit nearly all the features used by Westrop et al (2011) to split L. amer-icanus into a variety of "species". Troedsson (1937, p. 25) remarked on the morphologic variation of L. asiaticus (= L. americanus), stating "I have found it impossible to keep those forms [weakly and strongly furrowed sclerites] separate, and am convinced that they all belong to one and the same species".…”

“…Westrop et al (2011) refigured specimens previously assigned to or synonymized with L. americanus and split the taxon into several species, most of which were previously synonymized. In resurrecting the synonymized forms, Landing et al (2010 and Westrop et al (2011) emphasized narrow limits of morphologic variability and downplayed the possibility of intraspecific variation and variation introduced by taphonomic processes. They also employed definitions of species characterized by highly restricted geographic ranges, single localities in most cases.…”

“…Species filling such ecologic niches can be expected to show wide morphologic variation across broad geographic space, in contrast to, for example, nektobenthic polymerid trilobite species that inhabited more variable habitats of restricted inner-shelf environments. Innershelf-dwelling polymerids can be expected to show higher levels of endemicity, although the species definitions of agnostoids advocated by Landing et al (2010 and Westrop et al (2011) are extremely narrow even compared to most definitions of endemic polymerids.…”