Journal
PAPERS IN PALAEONTOLOGY
Volume 7, Issue 2, Pages 931-950Publisher
WILEY
DOI: 10.1002/spp2.1323
Keywords
Trilobita; ontogeny; morphometrics; Cambrian; Redlichiida; Estaingiidae
Categories
Funding
- Australian Government RTS PhD scholarship
- Constance Fraser Scholarship from the University of Adelaide
- Australian Research Council [LP0774959, FT120100770, FT130101329]
- South Australian Museum
- Beach Energy
- SeaLink
- Australian Research Council [FT130101329, FT120100770, LP0774959] Funding Source: Australian Research Council
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Trilobites, particularly the ellipsocephaloid Estaingia bilobata, play an important role in studying early animal evolution and developmental processes. This study provides insights into the post-embryonic ontogenetic series of E. bilobata, with new information on growth patterns and morphological features. The findings further support the phylogenetic relationships between the Estaingiidae, Ellipsocephalidae, and Xystriduridae.
Trilobites are one of the most diverse and abundant fossil groups from the early Palaeozoic, and as such are useful for answering important questions about early animal evolution, including developmental processes. Ontogenetic information for a large number of trilobite species has been published, but cases where multiple articulated specimens are known across the full range of developmental stages are rare. The early Cambrian (Series 2, Stage 4) Emu Bay Shale biota from Kangaroo Island (South Australia) is numerically dominated by trilobites, particularly articulated specimens of the ellipsocephaloid Estaingia bilobata, which are present in densities of >600 individuals per square metre on certain bedding planes. Here we describe the essentially complete post-embryonic ontogenetic series of E. bilobata from the Emu Bay Shale, and investigate patterns of growth relating to articulation and segmentation in this early Cambrian arthropod. Estaingia bilobata exhibits the hypoprotomeric mode of growth, with the epimorphic phase (the cessation of trunk segment generation) reached prior to the onset of the holaspid period. The meraspid pygidium had an extended equilibrium period in which anterior segment release into the thorax was matched by subterminal segment generation. Previously undocumented morphological features of E. bilobata, including the hypostome and bispinose pleural tips in holaspides, are also described. The growth characteristics and morphological features of E. bilobata documented herein strengthen close phylogenetic relationships between the Estaingiidae, Ellipsocephalidae and Xystriduridae.
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