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Abstract
BackgroundRadiodonta, large Palaeozoic nektonic predators, occupy a pivotal evolutionary position as stem-euarthropods and filled important ecological niches in early animal ecosystems. Analyses of the anatomy and phylogenetic affinity of these large nektonic animals have revealed the origins of the euarthropod compound eye and biramous limb, and interpretations of their diverse feeding styles have placed various radiodont taxa as primary consumers and apex predators. Critical to our understanding of both radiodont evolution and ecology are the paired frontal appendages; however, the vast differences in frontal appendage morphology between and within different radiodont families have made it difficult to identify the relative timings of character acquisitions for this body part.ResultsHere we describe a new genus of hurdiid, Ursulinacaris, from the middle Cambrian (Miaolingian, Wuliuan) Mount Cap Formation (Northwest Territories, Canada) and Jangle Limestone (Nevada, USA). Ursulinacaris has the same organisation as other hurdiid frontal appendages, with elongate endites on the first five podomeres in the distal articulated region and auxiliary spines on the distal margin of endites only. Unlike all other hurdiid genera, which possess a single row of elongated and blade-like ventral endites, this taxon uniquely bears paired slender endites.ConclusionThe blade-like endite morphology is shown to be a hurdiid autapomorphy. Two other frontal appendage characters known only in hurdiids, namely auxiliary spines on the distal margin of endites only, and elongate endites on the first five podomeres in the distal articulated region only, predate this innovation.
Highlights
Radiodonta, large Palaeozoic nektonic predators, occupy a pivotal evolutionary position as stemeuarthropods and filled important ecological niches in early animal ecosystems
We describe a new genus of hurdiid, Ursulinacaris, known only from frontal appendages, possessing elongate endites with auxiliary spines on the distal margin only on the first five podomeres in the distal articulated region
Trilobite biostratigraphy identifies the level from which the frontal appendage was obtained as the Mexicella mexicana zone (Miaolingian, Wuliuan), slightly older than the Mount Cap Formation specimens, which correlate with a level at the very top of the Jangle Limestone Member [35]
Summary
Radiodonta, large Palaeozoic nektonic predators, occupy a pivotal evolutionary position as stemeuarthropods and filled important ecological niches in early animal ecosystems. Radiodonta, as large nektonic predators in Palaeozoic oceans, were an important member of marine ecosystems and played a pivotal role in structuring these early animal communities. These stem euarthropod predators with raptorial appendages [1, 2], are a diverse and disparate group with over 25 species and 10 genera known from Africa, Australia, China, Europe, Greenland, and North America [3,4,5,6,7,8,9,10,11]. Frontal appendages (2019) 5:18 are critical for understanding radiodont internal relationships, as they have the highest preservation potential of all radiodont body parts (numerous taxa are only known from frontal appendages) and are character-rich, making them useful for phylogenetic analyses and taxonomy
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