# Tyrannasorus rex

Tyrannasorus rex is an extinct species of hybosorid beetle and the sole member of the genus Tyrannasorus. The species is known from a single, presumably female fossil specimen found in the Dominican Republic. She was trapped in the amber resin of Hymenaea protera, a species of tree which is also now extinct. The most recent studies date Dominican amber to the Miocene epoch (15 to 20 million years ago). T. rex is broadly similar to modern-day hybosorids of genera Apalonychus and Coilodes, sharing their reddish-brown color and a rounded shape. The specimen is deposited at the American Museum of Natural History, New York. The name of the species is a word play on the Tyrannosaurus rex, another, much larger extinct species.

## Taxonomy

Tyrannasorus rex is a species of hybosorid beetle from the subfamily Hybosorinae. It is the sole species of the genus Tyrannasorus. The species and the genus were described in 2001 by the entomologists Brett C. Ratcliffe and Federico Carlos Ocampo. It was the first fossil hybosorid described from the Dominican Republic, a country in the Caribbean islands. Manuel Iturralde-Vinent and Ross MacPhee propose that hybosorids may have colonized the islands by walking across a hypothetical land bridge from South America.
Tyranna, which Ratcliffe and Ocampo describe as "the loosely formed stem" of the genus name, comes from the Latin tyrannus, meaning "master" or "tyrannical". The second part of the genus name comes from the Latin sorus, which means "hump" or "pile"; the same suffix is used for the name of the family Hybosoridae and its type genus, Hybosorus. The resulting name, "tyrannical hump", refers to the "possible state of 'mind'" of the holotype specimen when she became fatally trapped in sticky tree sap.
The specific epithet rex means "king" in Latin. Ratcliffe and Ocampo state that the generic name is similar and the specific epithet identical to those of "another famous, and much larger, species of extinct animal". The naturalist John Acorn explains that both Tyrannasorus rex, the beetle, and Tyrannosaurus rex, the dinosaur, can correctly be called T. rex, citing this as an example of confusion that may arise from abbreviating generic names.

## Description

Tyrannasorus rex is known from a single specimen, the holotype. It is most similar to the genera Coilodes and Apalonychus, particularly in its reddish-brown color and slightly rounded shape. Ratcliffe and Ocampo surmised that the specimen is female based on similarities to the female specimens of Apalonychus, extant hybosorids from the West Indies. She measures 5.8 mm in length and 2.7 mm in width. Some parts of her body, especially the abdomen, are obscured by the dark pigmentation of the amber. A pair of prolegs, a pair of middle legs, and a pair of posterior legs are all clearly visible. The specimen was deposited in the American Museum of Natural History, New York.
The frons (upper part of the face) is irregularly pitted, and its edges curve upward. The clypeus, a hardened plate on the lower face, has a truncate front edge with curved sides. The flap-like mouthpart called labrum extends beyond the clypeus, forming a trapezoidal shape and featuring a row of bristles at its center. The mandibles—tooth-like structures used for chewing—are large, with a smooth, concave surface and a sharp tooth. The eye canthus, which divides the compound eye, is elongated and rectangular.
The maxillary palps, sensory appendages near the mouth, consist of four segments that increase progressively in length. The labium, or lower lip, is equipped with long bristles. Unlike other hybosorid beetles from the West Indies, which typically have ten-segmented feelers called antennae, Tyrannasorus is distinguished by its nine-segmented antennae. The first segment is widened at the tip and bears long bristles, the second is broad, the third is twice as long as it is wide, and the fourth segment is equally long and wide. The fifth and sixth segments are wider than they are long, with the fifth having two bristles. The club (the end part of the antenna) is three-segmented, with the first segment slightly hollowed to fit the others. All segments are covered in fine, velvety hairs.
The pronotum (a shield-like plate covering the first segment of the thorax) is slightly convex and widest just behind its middle. It features moderate pitting, a sinuous front edge with projecting corners, and a curved back edge with a central projection. The scutellum, a triangular structure on the back of the thorax, has a rounded tip. The hardened wing covers, known as elytra, are smooth, with minute pits but no distinct ridges. They are widest at the middle, with slightly raised bumps near the base of the elytra, called humeral umbones. The margins are bordered with a bead-like edge.
The legs have simple claws. The front tibiae—leg segments between the knees and feet—are equipped with three sharp denticles (small teeth) along their edge, with smaller denticles between them. The middle and hind tibiae feature a ridge lined with bristles and end in two spurs. The outer spur is longer than the first foot segment, while the inner spur is shorter. The spur on the front tibiae reaches the second foot segment. The foot (tarsus) consists of five segments, with the first and fifth being the longest. Each segment ends with a pair of bristles.

## Provenance

The specimen was found enclosed in amber in the Dominican Republic in the region of West Indies. The amber came from the resin produced by Hymenaea protera, a leguminous tree which is also extinct. Joseph B. Lambert, James S. Frye, and George Poinar, Jr. dated the Dominican amber to Oligocene or Eocene, but later research by Iturralde-Vinent and MacPhee found this to be erroneous and dated all Dominican amber to 15 to 20 million years ago, which corresponds to Miocene.
The exact provenance of the amber could not be determined because researchers bought it from dealers. Ratcliffe and Ocampo presume that it came from the mountain range north of Santiago de los Caballeros, an area where most of the amber mines were situated. The amber-rich layer is found within the top 300 meters of the La Toca Formation.