Arundelconodon

Arundelconodon is an extinct genus of mammal of the family Triconodontidae, containing the species Arundelconodon hottoni. It is known from multiple dental remains from the Arundel Clay in Maryland, United States, dated to the Early Cretaceous. The remains consist jawbone fragments with premolar, molar, and canine teeth. Its anatomic features are intermediate between Jurassic and later Cretaceous triconodonts. The deposits from which it is known represent either a fringe swamp or a floodplain, likely near a coast.

Discovery and naming

The type specimen of the genus Arundelconodon was described by paleontologist Richard L. Cifelli and colleagues in 1999. The type species was named Arundelcodon hottoni. Their description was based on a "remarkably complete" jaw from the Arundel Clay of Maryland, United States. The genus name comes from the words Arundel, in reference to the Arundel Clay where the remains were found, and, which is Greek for "cone-tooth". The specific name, hottoni, was given in recognition of the contributions of Nicholas Hotton to vertebrate paleontology. In 2001, paleontologist Kenneth D. Rose and colleagues described additional mammalian remains from the same fossil beds, which were tentatively assigned to A. hottoni.

Description

The holotype of Arundelconodon consists of a right mandibular ramus with five teeth: two premolars and three molars. The two premolars are large in relation to the molars, with each possessing four cusps. The teeth range from to in length and to in width, all possessing two "well-divided" roots. The only preserved alveoli are those containing the teeth. The entirety of the preserved portion of the jaw is less than long. Cifelli and colleagues assigned the genus to the family Triconodontidae, with its distinguishing features including the size and shape of its cusps; its weak, discontinuous lingual cingulid ; an interlocking system between molars, where the front and back sides of the molars have a groove and a ridge, respectively, that extend from the crown to the root; and the presence of a Meckelian groove, the last of which they described as noteworthy, as it is a basal feature found in many early mammals and their relatives that is absent from other North American Cretaceous triconodonts.
The remains described in 2001 consist of two pieces of a jawbone, which the researchers believed could most likely be rejoined at the lower margin. Seven complete alveoli are preserved, representing canine teeth, molars, and premolars. As with the holotype, a distinct Meckelian groove is present. The researchers interpreted the specimen as representative of an immature individual. Despite its young age, the condition of the alveoli indicates that the premolar teeth are permanent.

Classification

Cifelli and colleagues described Arundelconodon as an intermediate between Jurassic and Cretaceous triconodontids, with some of its features advanced in comparison to genera from the Jurassic and earliest Cretaceous, while others are primitive in comparison to later Cretaceous triconodontids. Since then, multiple phylogenetic studies have supported the placement of Arundelconodon as the most basal member of Alticonodontinae, a subfamily of Triconodontidae.

Paleoecology

terrestrial vertebrates are poorly known globally. The discovery of Arundelconodon represented the first recorded Early Cretaceous mammal from the East Coast of [the United States|Eastern Seaboard] of the United States. Its discovery supports the idea that the terrestrial fauna of North America remained consistent throughout the Early Cretaceous, as it demonstrates no affinities with European or Gondwanan groups. The Arundel Clay, where it was first found, was dated to the Aptian age in its original description, but later authors have labelled the formation as more likely early Albian. Other known vertebrates include hybodonts, crocodylomorphs, bony fish, dinosaurs, turtles, and another genus of mammal. Aside from the hybodonts, which were likely tolerant of saltwater and consequently widely distributed, all taxa identified at the species level and several genera are endemic to the site. The deposits belong to the Potomac Group. The area has been interpreted as either a fringe swamp or, more recently, a floodplain where fossils were preserved in the mud of isolated channels and oxbow lakes. The presence of remains of various groups associated with coastal environments and the absence of lissamphibians, which are generally not saltwater tolerant, have been interpreted as indicative of a near-coastal environment.