Actiocyon is an extinct genus of ailurid that lived in western North America during the Middle Miocene. It was named by Chester Stock in 1947 for the type speciesActiocyon leardi. A second species, Actiocyon parverratis, was described in 2016. No other species have been assigned to Actiocyon in the past, and fossil material assigned to the current species is rare.
Both species of Actiocyon were small carnivores, with A. leardi known to live in a coastal environment while A. parverratis lived in a high-altitude temperate forest. They differ from the closely-related European genus Alopecocyon in the precise characteristics of the teeth, and from Simocyon in their overall smaller size. A. leardi may have weighed about 7 kilograms (15lb), and A. parverratis was even smaller.
History and naming
In 1938 Robert Leard collected mammalian fossils in Ventura County, California. Among them was a partial carnivoran skull which was identified as a canid belonging to a new genus. American paleontologist Chester Stock in 1947 described the new genus and species Actiocyon leardi based on the specimen. He noted that the precise location of the fossil's origin placed it as coming from the Caliente Formation. The genus name comes from the Greek aktlos, pertaining to the coast, and κύων/kúon, meaning dog. The specific name honored Leard for collecting the specimen.[1]
The second species Actiocyon parverratis was described in 2016 based on several fossils from the lowermost section of the Monarch Mill Formation in Middlegate Basin, Nevada. The specific name comes from the Latins words parvus, "small", and erratis, "wanderer", together meaning "small wanderer".[2]
Description
The holotype and only specimen of A. leardi (LACMlCIT 2747) is a fragment of a skull with several attached teeth (upper right canine, second through fourth premolars, first and second molars). Stock noted that when the specimen was collected there were three small, poorly preserved incisors present on the right side of the snout that were apparently lost prior to the specimen being prepared in the laboratory.[1] No other fossils have been assigned to A. leardi.
Stock compared A. leardi in size to Cynodesmus thomsoni,[1] which is estimated to have weighed approximately 7 kilograms (15lb).[3]
Jon Baskin, in his 1998 review of fossil procyonids (which included ailurids at the time), listed the following characteristics as diagnostic of A. leardi: four premolars, with the fourth only slightly longer than the first molar and possessing a very small parastyle, a narrow internal shelf that extends posteriorly as a cingulum, and a very small protocone. The first molar being subtriangular with a low protocone connected to small metaconule, and a prominent posterointernal hypercone. The canine tooth has the same lateral groove also present in Simocyon and Alopecocyon.[4]
Three specimens from three different localities are assigned to Actiocyon parverratis. The holotype (UCMP 141928) is a pair of associated left and right mandibles, both with attached complete teeth (second through fourth premolar, first and second molars). Of the other two fossils, one is part of the left maxilla with the fourth premolar attached, while the second is partial right dentary with the third and fourth premolars and the first molar attached.[2]
Smith et al, when describing A. parverratis, noted it as differing from Simocyon in its much smaller size, the presence of non-reduced second and third premolars, in the lack of a protocone and possibly a small parastyle on the fourth premolar, in the second molar having joined trigonid crests that form a small, subcircular structure, and in the trigonid on the second molar being narrower than the talonid. They further described A. parverratis as differing from Alopecocyon again in trigonid crest structure of the second molar, as well as that tooth having a larger protoconid than metaconid, the talonid longer and wider than the trigonid, and prominent cusps composing a median talonid ridge. Finally, Smith et al distinguished A. parverratis from A. leardi by its smaller overall size, the maxilla having a relatively larger and lower-set infraorbital foramen, and by the fourth premolar having a less-developed and undivided hypocone on the internal cingulum.[2]
Classification
When Stock described A. leardi in 1947, he considered it a close relative of the then-canidAlopecocyon, and called it an aberrant genus that had acquired procyonid traits.[1] Webb, in his 1969 work on Pliocene canids, considered Actiocyon a junior synonym of Alopecocyon, which he also classified as a canid, though he noted that De Beaumont had suggested in 1964 that Alopecocyon was a musteloid descended from Broiliana and the lack of material made its position difficult to resolve.[5]
Jon Baskin, while writing the chapter on Procyonidae for a book by Cambridge on Tertiary mammals in 1998, listed Actiocyon as closely related but distinct from Alopecocyon, and included both in the clade Simocyoninae in "Ailuridae or Unnamed Group" that he stated was controversially placed in Procyonidae.[4]
In 2010, paleontologists Michael Morlo and Stéphane Peigné published a complete review of what they stated was the family Ailuridae, in which they included A. leardi as a species of Alopecocyon in the subfamily Simocyoninae.[6] The 2016 description of A. parverratis unambiguously considered Actiocyon a separate genus in the subfamily Simocyoninae in the family Ailuridae.[2]
The 2025 review of ailurid taxonomy included both A. leardi and A. parverratis as valid species, with the genus placed in the subfamily Simocyoninae, but noted the inclusion of A. parverratis in Actiocyon was tentative given the overall lack of fossils for both species.[7]
Paleoecology
Actiocyon leardi is part of the Cuyama fauna, a late Clarendonian or early Barstovian assemblage of vertebrates that also includes the ground squirrel Citellus quatalensis, the pocket mouse Perognathus furlongi, the rabbit Hypolagus apachensis, the equids Merychippus, Protohippus, and Hipparion, the pronghorn Merycodus, a mastodont, a camelid, and various avians and testudines.[1] The Caliente Formation in the Cuyama Basin is a series of fluvial and lacustrine sediments representing very wet coastal environment.[8]
Actiocyon parverratis is part of the Barstovian-aged Eastgate Local Fauna, a highly diverse assemblage include fish, reptiles, amphibians, birds, and at least sixty species of mammals from seven different orders. Paleobotanical studies of the fossils sites indicate that the area was a mixed conifer-hardwood forest in north-facing canyons; known flora species include the firAbies concoloroides, larchesLarix cassiana and Larix nevadensis, Amelanchier grayi, and Aesculus preglabra. The canyons are thought to have been a mesic habitat with a mean annual temperature of 10.2°C (50.4°F), at an altitude of 2,700–2,800 metres (8,900–9,200ft).[2]
↑Webb, S. D. (1969). "The Pliocene Canidae of Florida". Bulletin of the Florida State Museum: Biological Science. 14 (4): 279. doi:10.58782/flmnh.fxqk6360.
↑Morlo, Michael; Peigné, Stéphane (2010). "Molecular and morphological evidence for Ailuridae and a review of its genera". In Goswami, Anjali; Friscia, Anthony (eds.). Carnivoran Evolution: New Views on Phylogeny, Form, and Function. pp.92–140. doi:10.1017/CBO9781139193436.005. ISBN978-0-521-73586-5.
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