Abstract
Bradoriids, among the earliest arthropods to appear in the fossil record, are extinct, ostracod-like bivalved forms that ranged from the early Cambrian to the Middle Ordovician. Bradoriids are notable for having appeared in the Cambrian fossil record before the earliest trilobites, and considering their rapid ascent to high genus-level diversity, provide key data for our understanding of the evolutionary dynamics of the Cambrian Explosion. This paper presents a broad review of bradoriid paleobiology. It is hypothesized here that an allele of Antennapedia determines whether bradoriid shields are preplete, amplete, or postplete. The preplete configuration of the shields of Cambroarchilocus tigris gen. nov. sp. nov. suggests that shield rowing motion may have propelled the animal backwards. Arcuate scars attributed here to a microdurophagous predator (Arcuoichnus pierci nov. ichnogen. nov. ichnosp.) occur on the paratype of Cambroarchilocus tigris gen. nov. sp. nov.
Highlights
Bradoriids (Bradoriida Raymond, 1935 [1]) are small bivalved basal euarthropods that appeared in shallow marine habitats during Cambrian Stage 3
Bradoriids enter the fossil record before the first trilobites [3,4,5], and as such provide important data regarding the dynamics of the Cambrian Explosion
Arthropods are arguments have been presented that bradoriids are polyphyletic [5,23], Bradoriida is considered here to be monophyletic [17], but their astonishingly rapid class-level diversification at the treated asCambrian monophyletic this is the case,magnitude and with of additional information regarding basehere of the adds to theIfperceived and puzzling the Cambrian the limb structure arguments of bradoriids, of a new arthropodan class to accommodate this clade haveerection been presented that bradoriids are polyphyletic
Summary
Bradoriids (Bradoriida Raymond, 1935 [1]) are small bivalved basal euarthropods that appeared in shallow marine habitats during Cambrian Stage 3. Short-lived genera are largely confined to a single Cambrian stage, whereas long-lived taxa have ranges that extend significantly beyond a single stage (see Figure 4 in Williams et al [2]) These successive episodes of diversification, here called “waves”, are interpreted as actual evolutionary events rather than artifacts of monographic swelling, as stratigraphic control [2] seems sufficient to allow recognition of closely spaced diversification events. The proportion of long-lived taxa originating during successive “waves” shows a general (if not monotonic) decrease from Cambrian Stage 3 to Wuliuan, dropping from an initial 45% to 8% This is interesting because it would seem to be the inverse to the usual evolutionary expectation, namely, that later diversifications should generate longer-lived taxa, as more time had elapsed to allow improvements to the general “competitiveness” of the taxa involved. Ordovician and include a Siberian form (with the marvelously alliterative binomial) Chegetella chegitunica [13]
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