Evidence for segment polarity during regeneration in the Devonian asteropygine trilobite Greenops widderensis

A complete molted exoskeleton of the asteropygine phacopid trilobite Greenops widderensis Lieberman and Kloc, 1997 from the Middle Devonian (Givetian) Widder Formation in southwestern Ontario, Canada that has suffered predatory trauma provides insights into the sequence of regeneration of segments. The molt configuration is such that it is possible to interpret the molting technique used by the trilobite. Predatory trauma affected four areas of the exoskeleton. The pygidium shows loss of the spinose margin on one side and damage to a single spine on the other; one genal spine has been broken and partially regrown; and the posterior of the glabella has been removed. It is thought that the first three traumas occurred during life, as these areas affected show signs of exoskeletal regeneration. The fourth trauma probably occurred to the exuvium. Analysis of the degree of regeneration of the pygidial pleurae indicates that there was an anteroposterior polarity to the regeneration. Other examples in the literature suggest that this regeneration polarity pattern may have been widespread in trilobites. It is suggested that, as in modern arthropods and annelids, this sequential regeneration was under the control of segmentation polarity genes.

Ontogeny, systematics, and evolution of the effaced Early Cambrian trilobites Peachella Walcott, 1910 and Eopeachella new genus (Olenelloidea)

Although used in biostratigraphy and in studies of early Cambrian trilobite evolution, the olenelloid genus Peachella has received little research attention. The ontogenetic and evolutionary origins of its derived features—an effaced cephalon and grossly inflated genal spines—have remained mysterious. Based on examination of new and existing collections, P. iddingsi and P. brevispina are here described in detail, including aspects of their respective ontogenies and the first description of the thorax of P. iddingsi. A new monotypic genus, Eopeachella, is also described from recently collected material from the Delamar Member of the Pioche Formation, Nevada. Eopeachella angustispina n. gen. n. sp., is less derived and stratigraphically older than both Peachella species and bridges the morphological gap between Peachella and typical olenelloids. The study reveals that cephalic effacement was progressively attained during both ontogeny and phylogeny in the Eopeachella + Peachella clade. Comparative ontogeny with other olenelloids reveals that progressive effacement was a trend superimposed upon and independent of a conserved pattern of ontogenetic shape change in the glabella and did not represent a peramorphic “extension” of glabellar ontogeny. Genal spine inflation was also achieved progressively (in a proximal-to-distal direction) through both ontogeny and phylogeny in the Eopeachella + Peachella clade. Genal spine inflation that convergently arose in later trilobite groups may have been similarly ontogenetically dynamic. Discovery of E. angustispina and P. brevispina in the Delamar Member raises olenelloid diversity in this member to at least 20 species; a higher diversity than in any coeval unit.

Spine function in the odontopleurid trilobites Leonaspis and Dicranurus from the Devonian of Oklahoma

The Lower Devonian Haragan Formation of Oklahoma has well preserved odontopleurid trilobites found in repeated calcarous mudstones. These layers (5–9cm. thick) represent distal tempestites that rapidly buried and preserved completely articulated odontopleurids which provide important paleobiological information. The genal and pleural spines of Leonaspis williamsi and Dicranurus hamatus elegantus of the Haragan Formation are examined and compared.In Leonaspis the ventrally directed genal and pleural spines are straight and lay flat. The first pleural spine is the longest followed by progressively shorter pleural spines. Several collected and prepared enrolled specimens have the spines radiating outward. In an enrolled position the spines would protect Leonaspis from a large predator.The pleural spines of Dicranurus have a very different pattern from Leonaspis. The first three spines are short and laterally directed, the fourth, sixth and seventh are very long, curved and directed ventrally, the eighth, ninth and pygidial spines are short and decrease in length toward the pygidium. There is no pleural spine on the fifth thoracic segment and the genal spine is very long and curved. On these spines at many growth stages there is direct and indirect evidence of being encrusted based on the following: (1) Epibionts such as bryozoans, brachiopods and a crinoid; 2) Endoliths (25–150 microns in diameter) of unknown origin are found all over Dicranurus but are predominantly on the spines; 3) Pyrite (now limonite) coating the spines apparently formed from the decay of organic matter. If the source of the organic matter came from the trilobite, then pyrite would be found all over Dicranurus. However, it is only found randomly on the spines. This pattern is like some of the encrusting bryozoans suggesting that the source of the organic matter may be from an encrusting organism.It is doubtful that the pleural spines of Dicranurus would be adapted for resting on the bottom when bryozoans are found encrusting the entire surface of the spine. However, the very long pleural and genal spines with attched organisms would break up the outline of Dicraurus making it less conspicuous to a predator.

Revision and subdivision of the polyphyletic ‘Leonaspis’ (Trilobita)

ABSTRACTA phylogenetic analysis is performed on all species previously assigned to the odontopleurid trilobite genus Leonaspis. The PAUP analysis shows this genus, as conventionally defined, to be polyphyletic, composed of four groups of equal taxonomie rank. Leonaspis (s.s.) is here restricted to one of these groups only, a monophyletic set of species characterised by ‘four-spined’ pygidia (i.e. with four spines between the major border spines) and a thorax of nine segments. Kettneraspis is recognised for the largest of these four groups, and is composed of ‘two-spined’ species with nine thoracic segments. Leonaspis and Kettneraspis belong to the Odontopleurinae. A third group of previous ‘Leonaspis’ species constitute a new genus of the Acidaspidinae. Its species are characterised by being ‘four-spined’ and having ten thoracic segments. For these the new genus Exallaspis is erected, with type species E. bufo. A fourth group of ‘Leonaspis’ species, being ‘four-spined’ with nine thoracic segments, belongs to Eoleonaspis, an Ordovician odontopleurine genus. Leonaspis and Exallaspis are temporally and spatially non-overlapping, Leonaspis being exclusively Gondwanan post-Wenlock, whereas Exallaspis is restricted to areas north of the Rheic Ocean and ranges from basal Llandovery to Ludlow. Kettneraspis is pandemic. The condition of five epiborder spines and ten border spines on the free cheek is proposed as plesiomorphic for odontopleurids, and the border spines are shown to originate as two separate rows with alternating spines. A shift in position of the facial suture is shown to transfer the genal spine from the cranidium to the free cheek in Kettneraspis meraspides of degree 1 or 2, and the previously suggested co-occurrence of a fixigenal and librigenal spine in these stages is refuted. Earlier proposed dimorphism is rejected, and two mechanisms for reduction in the number of pygidial border spines are proposed. Kettneraspis reetae sp. nov. is described.