Angustidontus

Angustidontus was a predatory angustidontid crustacean, measuring about 6 centimeters in length (9 centimeters if the large maxillipeds are counted).[1] It had one pair of grasping maxillipeds and seven pairs of pereiopods. The first to fifth pair of pereiopods were subchelate and short, ending in a hooked and enlarged dactylus. The sixth and seventh pairs were thinner and longer, ending in simpler and smaller dactyli. The fifth and sixth pleonal somites expanded laterally, with some partial overlap over the telson (the posteriormost division of the body).[2]

Angustidontus had large grasping appendages, modified from the first or second thoracopods, which represent some of the earliest maxillipeds within the Eucarida. These maxillipeds were likely used through being folded downwards to strike at prey, then hold the prey and prevent it from wriggling itself free.[2] Schramidontus, the only known close relative of Angustidontus, also possessed a second smaller pair of maxillipeds that it could use to bring prey to the maxillae, maxillulae and its large mandibles, but these are absent in Angustidontus.[2] Instead, Angustidontus used the next four pairs of thoracopods (which were short and possessed strong claws and serrated gnathobases) to tear apart prey and transport it to the mouth.[1]

The genus Angustidontus was named by Chalmer L. Cooper in 1936, together with the family "Angustidontidae". Cooper described the fossils, consisting of the serrated appendages, as fossil jaws of actinopterygian fish.[3] Since then, the appendages of Angustidontus have been the subject of much debate on its classification. In the 1950s, it was suggested that the fossils instead represented eurypterid chelicerae, possibly of something closely related to Pterygotus.[4]

Copeland & Bolton (1960) considered the fossils to instead represent gill rakers of fish or be "claws similar to those on the second maxilliped of the stomatopod Squilla", furthermore the appendage was noticed to have had some kind of "ball and socket" joint type of articulation.[5] Though it was noted several times by prominent researchers that Angustidontus did likely not represent a eurypterid, it was subsequently treated as a eurypterid (albeit dubiously so) for decades by many authors.[1] This was not universal however, with Jean Berdan concluding in 1964, after studying multiple Angustidontus appendages, that the "rami of Angustidontus are almost certainly part of an arthropod rather than a vertebrate but are not necessarily part of a eurypterid".[6]

Angustidontus often occurs together with Concavicaris, another Devonian crustacean. Concavicaris fossils tend to lack the appendages, whilst Angustidontus fossils often lack the cephalothoracic shield because of its weak sclerotisation. This caused some confusion, and some researchers have suggested that the two would represent different parts of the same animal.[7] Expeditions to fossil localities in Nevada where Concavicaris and Angustidontus were reported to have occurred together by Cooper in 1936 yielded more information on the appendages of Concavicaris and allowed it to be determined that the appendages of Angustidontus did not represent appendages of Concavicaris. With hundreds of specimens being collected, proper research could be conducted on Angustidontus for the first time with the discovery of the first complete specimens.[1]

The new specimens allowed researchers to determine that Angustidontus was a peracarid malacostracan crustacean, and that Concavicaris simply represented a separate animal that was part of a larger Late Devonian fauna including a large amount of different invertebrates, such as worms, cephalopods, bivalves, brachipods and sponges. The appendages which had caused confusion in the past were revealed to be the first thoracopods, but greatly elongated and adapted to be used in feeding.[1]

A generalized bauplan of a malacostracan crustacean. The cephalon and thorax was fused in Angustidontus as a single cephalothoracic shield and the first thoracopod had developed into a maxilliped.

Angustidontus is classified as part of the extinct family Angustidontidae together with the freshwater genus Schramidontus from Belgium. This family is the only family classified as part of the eucarid order Angustidontida. Angustidontids are diagnosed as eucarids that possess carapaces and stalked eyes with "scale-like exopods" on the second antennae, an elongated pleion and a tail fan. These features make the group distinct from most eumalacostracan crustaceans and they are classified as part of the Eucarida due to their carapace being fused to thoracic segments 1-7.[2]

Some additional species of Angustidontus have been named other than the type species (A. seriatus), such as "A. weihmannae", but they were all considered synonymous with A. seriatus by Rolfe & Dzik (2006), with only the type species remaining as valid. Based on the morphology of different fossil mandibles, fossil material from the Early Famennian of Poland may represent two possible additional species.[1]

The cladogram below is based on the relationships of the Eucarida assumed by Gueriau, Charbonnier and Clément (2014),[2] based on the gradual modification of the first thoracopods into the maxillipeds seen in the Decapoda.