Bradoriids

Bradoriids are an extinct order of bivalved arthropods that lived during the Cambrian and Ordovician periods. Their fossils are characterized by a carapace composed of two overlapping valves, similar in appearance to modern bivalves like clams, but they were arthropods, possessing segmented bodies and jointed appendages. These arthropods are particularly notable for their small size, typically ranging from a few millimeters to a couple of centimeters in length.

The exact phylogenetic placement of bradoriids has been a subject of debate among paleontologists. Historically, they were often grouped with ostracods, a diverse group of crustaceans that also possess bivalved carapaces. However, more recent studies, particularly those examining internal anatomical features preserved in exceptionally fossilized specimens, suggest a more complex relationship. Some researchers propose they represent an early, independent lineage of arthropods, potentially related to the ancestors of crustaceans or even chelicerates. Others maintain their close affinity to ostracods or related crustacean groups.

Bradoriids are found globally in marine sedimentary rocks of Cambrian and Ordovician age. Their presence in these ancient seas indicates they were a common component of the paleozoic benthos, likely inhabiting soft substrates on the seafloor. The precise ecological role and lifestyle of bradoriids remain largely speculative, though their morphology suggests they were likely deposit feeders or scavengers. Their relatively simple structure and widespread distribution make them useful index fossils for dating and correlating these early Paleozoic strata.