Ontogenetic stages of trilobites have traditionally been recognized on the basis of the development of exoskeletal segmentation. The established protaspid, meraspid, and holaspid phas- es relate specifically to the development of articulated joints between exoskeletal elements. Tran- sitions between these phases were marked by the first and last appearances of new trunk segment articulations. Here we propose an additional and complementary ontogenetic scheme based on the generation of new trunk segments. It includes an anamorphic phase during which new trunk seg- ments appeared, and an epimorphic phase during which the number of segments in the trunk re- mained constant. In some trilobites an ontogenetic boundary can also be recognized at the first appearance of morphologically distinct posterior trunk segments. Comparison of the phase bound- aries of these different aspects of segment ontogeny highlights rich variation in the segmentation process among Trilobita. Cases in which the onset of the holaspid phase preceded onset of the epimorphic phase are here termed protarthrous, synchronous onset of both phases is termed syn- arthromeric, and onset of the epimorphic phase before onset of the holaspid phase is termed pro- tomeric. Although these conditions varied among close relatives and perhaps even intraspecifically in some cases, particular conditions may have been prevalent within some clades. Trilobites displayed hemianamorphic development that was accomplished over an extended series of juvenile and mature free-living instars. Although developmental schedules varied mark- edly among species, morphological transitions during trilobite development were generally reg- ular, limited in scope, and extended over a large number of instars when compared with those of many living arthropods. Hemianamorphic, direct development with modest change between instars is also seen among basal members of the Crustacea, basal myriapods, pycnogonids, and in some fossil chelicerates. This mode may represent the ancestral condition of euarthropod de- velopment.

https://bioone.org/journals/paleobiology/volume-32/issue-4/06017.1/The-ontogeny-of-trilobite-segmentation-a-comparative-approach/10.1666/06017.1.pdf

The ontogeny of trilobite segmentation: a comparative approach

http://arago.elte.hu/sites/default/files/TrilobiteEye_ASD.pdf

The eyes of trilobites: The oldest preserved visual system

Paleogeography and paleoenvironments of the Late Devonian Kellwasser event: A review of its sedimentological and geochemical expression

A diverse fauna of phacopid trilobites is described from the Late Devonian (middle Frasnian to early Famennian) of the northern Canning Basin, Western Australia. One new genus and four species in two genera are described from zones 11, 13a and 13b of the middle and late Frasnian: Trimerocephaloides sinevisus gen. nov. and sp. nov., T. ? linguiformis sp. nov., Acuticryphops acuticeps (Kayser, 1889) and A. klapperi sp. nov. Late Frasnian phacopines are either blind, as shown for the first time in Trimerocephaloides sinevisus, or show trends to decreasing eye size up to the Frasnian-Famennian 'Kellwasser' mass extinction event. This evolutionary trend in Acuticryphops is demonstrated to have been global at this time. One new genus and six species of early Famennian phacopids are described, from the Upper triangularis, crepida and rhomboidea zones: Houseops gen. nov. with the new taxa H. canningensis sp. nov., H. beckeri sp. nov. and H. sp. A, Babinops planiventer Feist & Becker, 1997, B. minor sp. nov., Trimerocephalus tardispinosus Feist & Becker, 1997 and T. mimbi sp. nov. In contrast to European sections where exclusively blind phacopids are known in earliest Famennian sites, initial recovery following the mass extinction event in Canning peri-reefal environments is characterized by oculated forms. These trilobites must have evolved from conservative ancestors with normal eyes that had succeeded in surviving the Kellwasser biocrises in reef-related shallow water niches. Thus the origin of post-event phacopids from shallow water environments is demonstrated for the first time. Descendant lineages show increasing eye size, increased cephalic vaulting and effacement during the early Famennian. Of the five orders of trilobites that are present during the Frasnian Stage at the beginning of the Late Devonian, only two, the Proetida and the Phacopida, survived into the Famennian. The presence of a conformable sequence of trilobite-bearing fore-reef limestones, the Virgin Hills Formation, that form part of the Late Devonian reef system in the northern part of the Canning Basin in Western Australia, allows the patterns of evolution and extinction of the late Frasnian and early Famennian trilobites to be assessed. In these deposits the three orders that became extinct during the Frasnian-Famennian biocrises were the Corynexochida (McNamara & Feist, 2006), the Lichida (Feist & McNamara, 2007) and the Harpetida (McNamara, Feist & Ebach, in press). Here we focus on one of the groups that survived this event, the Phacopida. Although, with a single exception, they are rare elements of the trilobite fauna in the Canning Basin, the phacopids are generically the most diverse of the

Patterns of extinction and recovery of phacopid trilobites during the Frasnian–Famennian (Late Devonian) mass extinction event, Canning Basin, Western Australia

Paleogeographic differences in temperature, water depth and conodont biofacies during the Late Devonian

Exceptionally rich and well-preserved silicified exuviae comprising numerous immature growth stages allow the tracing of detailed morphological and morphogenetical features of the reduced-eyed phacopid species Cryphops ? ensae (Richter and Richter, 1926) from the latest Famennian of Thuringia (Central Germany). Ontogenetic features of this species are more similar to those of Phacops than to Trimerocephalus.

Larval morphology and ontogeny of a Late Devonian phacopid with reduced sight from Thuringia, Germany

This study characterizes the stratigraphical context as well as the palaeoenvironmental and sea-level trends during the Late Frasnian and Famennian in the Buschteich section (Thuringia, Germany). An integrated approach combines conodont biostratigraphy, chemostratigraphy based on carbon and oxygen isotopes , conodont biofacies analyses and investigations of the morphotypes of foraminifera that occur in the section was applied. Conodonts provided a fine-scale biostratigraphical framework. All the Famennian con-odont biozones were identified, with the exception of the three lowest. The Frasnian-Famennian transition was further identified by a positive carbon isotope excursion. This continuous Famennian record is developed in a condensed limestone succession. The reddish Griotte facies, typical of strata of this period, were not documented. Black shales were not developed except for an interval of dark colored mud rocks corresponding to the Hangenberg event at the top of the section. The carbonate succession of Buschteich was deposited in an outer ramp environment, pelagic organisms are the dominant fauna. This is corroborated by the overall dominance of the deep-water conodont genus Palmatolepis. Microfacies, conodont biofacies and foramini-fera indicated a deepening trend from the Lower crepida to the Lower rhomboidea Zone, followed by a shal-lowing trend from the Upper rhomboidea to the praesulcata Zone. This sequence overall matches the sea level reference curve for Euramerica. The timing of maximum water depth at Buschteich, as well as the general sea-level reference record, differs from the Col des Tribes (Montagne Noire, France). Differential uplift or overprinting of the long-term eustatic changes may be the cause of the discrepancy in the local sea-level records.

K. Bartzsch

Papers

Larval morphology and ontogeny of a Late Devonian phacopid with reduced sight from Thuringia, Germany

Conodont biostratigraphy and palaeoenvironmental trends during the Famennian (Late Devonian) in the Thuringian Buschteich section (Germany)