Paradoxides is a group of large trilobites that lived during the Middle Cambrian period and are found in many parts of the world. A famous example, Paradoxides davidis, was described by John William Salter in 1863 and measured 37 cm (15 in) in length. Its head, called the cephalon, was semicircular, with free cheeks that ended in long, narrow, recurved spines. The eyes were crescent-shaped, allowing the trilobite to see nearly 360° horizontally. The thorax, or middle body section, had 19 to 21 segments and was covered in long, recurved pleural spines. The pygidium, or tail section, was much smaller in comparison. Paradoxides is a key trilobite of the 'Atlantic' (Avalonian) fauna. Avalonian rocks were formed near a small landmass called Avalonia in the Paleozoic Iapetus Ocean. These rocks are now found in a narrow strip along the East Coast of North America and in Europe.

Description

The exoskeleton of Paradoxides is large to very large and relatively flat. It is about one and a half times longer than it is wide, with the widest part across the genal spines. The cephalon is nearly half-circle shaped, with long genal spines extending from the back corners of the cephalon. The facial sutures vary in length behind the palpebral lobes depending on the species. The part of the facial suture near the eyes follows a slight S-shaped curve and meets the front edge of the cephalon in front of the eye. Usually, four pairs of glabellar furrows are present. The two pairs at the back (1p and 2p) cross the glabella. The furrows 3p and 4p are short, with 4p often pointing forward and outward to meet the axial furrows where the glabella is widest. Palpebral lobes are short and vary in length. In some species, the hypostoma is joined to the rostral plate, such as in Paradoxides davidis. This feature helps distinguish the genus from other trilobites, except for some Cambrian Corynexochida like Oryctocephalus and Fieldaspis.

The thorax has 19 to 21 segments. The axis is about as wide as each pleura, but the pleural spines curve backward and grow slightly longer toward the back. The spines on the last thoracic segment are twice as long as the pleurae and extend beyond the pygidium.

The pygidium is small and has one or two axial rings. It may be partially or fully joined to the last thoracic segment. The axis does not reach the back edge of the pygidium, forming a U-shaped pleural field.

Ontogeny

The larval development (or growth stages) of Paradoxides was first described by Barrande in 1852. The earliest stage, called the protaspis, is a round disc with three pairs of spines along its edge. Genal spines are located halfway along the disc's edge, pointing outward and backward at a 45° angle, curving slightly further backward and measuring about one-third the length of the protaspis. Intergenal spines are sharply pointed, straight, and positioned at the back of the future cephalon. These spines are about half the disc's diameter in length, pointing backward and slightly outward at a 15° angle. These spines are not present in adult specimens. The first spines that will later form the thorax are placed next to the intergenal spines and curve toward the midline. The front part of the glabella nearly reaches the edge of the disc and has four sets of lobes. These lobes are divided by a furrow along the midline in the front two-thirds of the glabella, with furrows separating the lobes. The most backward set of lobes includes two central and two lateral lobes. Further back, the glabella ends with a central occipital lobe that has a small node and two lateral occipital lobes. The axis ends with three rings that gradually decrease in width. Along the sides of the most forward glabellar lobes, semicircular eyelobes run parallel to the edge of the exoskeleton, ending near the base of the intergenal spines.

Behaviour

The thorax of Paradoxides has special segments that help the animal roll, offering protection from the front, back, top, and bottom. These segments leave the soft underside of the animal exposed on both sides.

Complete fossils of Paradoxides show the librigenae and a fused rostral-hypostomal plate. During moulting, the body curved upward above the ground, with the front edge at the front and the back pleural spines pressing into the sediment. Stretching the body caused the cephalon's sutures to break, allowing the librigenae and rostral-hypostomal plate to detach. After moulting, the animal moved forward to leave its old exoskeleton.

Some Paradoxides fossils contain intact Peronopsis trilobites located between the glabella and hypostome, in the area where the gut would have been. These trilobites are not believed to have been food. Instead, they may have been scavenged by Paradoxides or used the large trilobite as shelter.

Reassigned species

Many species that were once placed in the genus Paradoxides have been moved to different genera:

Distribution

Fossils of Paradoxides have been found in many places around the world, including Canada (New Brunswick, Newfoundland and Labrador), Colombia (Duda Formation, El Dorado, Meta), the Czech Republic, Denmark, France, Italy, Morocco, Norway, Poland, the Russian Federation, Spain, Sweden, Turkey, the United Kingdom, and the United States (Alaska, Massachusetts, South Carolina).