Triisodon Temporal range: 63.3–61.7 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓ Torrejonian[1]
T. quivirensis skull, National Museum of Natural History
Restoration of T. quivirensis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	†Mesonychia
Family:	†Triisodontidae
Genus:	†Triisodon Cope, 1881
Type species
Triisodon quivirensis Cope, 1881
Species
T. quivirensis (Cope, 1881) T. crassicuspis (Cope, 1882)
Synonyms
T. antiquus (Cope, 1882) T. rusticus (Cope, 1883)

Triisodon (ancient Greek: “Tri” (three),”isos” (equal), and modern Greek: “donti” (tooth/teeth), supposedly describing tritubercular lower cheek teeth) is a genus of extinct mesonychian mammal that existed during the Early Paleocene of New Mexico, North America, from about 63.5-62.0 Ma.^[2] The genus was named by Edward Drinker Cope in 1881 as a member of the Acreodi, a now invalid taxon that encompassed creodonts, mesonychians and certain arctocyonians. Cope described the type specimen of T. quivirensis as "about the size of a wolf."^[3] A smaller species, T. crassicuspis, has also been identified from the same region. Since material from this genus is incomplete, the exact size of adults and whether they showed sexual dimorphism or regional variations in size is unknown.

Triisodon was a rather large and powerful terrestrial mammal, as evident from the robust forelimb morphology. The body mass of Triisodon crassicuspis is estimated at about 32-44 kilograms.^[2] Triisodon was among the earliest placental mammals to have a predominantly carnivorous diet, and had a skeleton that, like many Paleocene mammals, was not uniformly generalized, combining a simple placental bauplan with many specializations for carnivory.

The skull of T. crassicuspis exhibits rostral constriction, particularly above the upper second premolar, similar to Arctocyon primaevus. The maxilla of T. crassicuspis barely extends onto the zygomatic arch laterally. The posterior zygomatic arch was particularly robust. T. crassicuspis had a large and particularly deep mandibular fossa that is more elongated mediolaterally than anteroposteriorly. The depth of the fossa suggests that it formed a relatively tight hinge-like articulation with the condyle of the mandible. As in A. primaevus, the articular surface of the mandibular fossa in T. crassicuspis is smooth and continuous.

The paired hemi-mandibles of T. crassicuspis were not co-ossified towards the front, as indicated by the rugose texture of the mandibular symphysis. The mandibular body of T. crassicuspis is proportionately deeper dorsoventrally and significantly more robust compared to that of Eoconodon coryphaeus, or mesonychids like Sinonyx jiashanensis and Dissacus praenuntius. In T. crassicuspis, a massive and mediolaterally broad coronoid crest forms the anterior surface of the coronoid process. The coronoid crest ends below the lower third molar, as in T. quivirensis, E. gaudrianus, E. coryphaeus and A. primaevus.

The teeth of Triisodon exhibit several morphological similarities to those of other triisodontids including the diagnostic characteristics of the group: basin-shaped talonids and tribosphenic upper molars. The canines of Triisodon are massive and single-rooted, with marked shearing edges. Other features of the teeth reveal that Triisodon and other triisodontids were showing evolutionary trends towards a carnivorous lifestyle – notably the mesiodistally oriented hypoconulid crest, tall trigonid and distinct protoconid of the lower molars, as well as the strongly-developed posterior premolars bearing marked shearing surfaces. ^[2]

While the postcranial skeleton of Triisodon was less robust compared to other condylarths like Periptychus and A. primaevus, it was far from gracile. Much of what is known about the forelimb of Triisodon comes from the humerus, with both the greater and lesser tubercles placed low relative to the massive, hemispherical humeral head. In addition, the humerus also had a prominent, V-shaped deltopectoral region that extended anteriorly. The attachment site for the teres major and latissimus dorsi muscles is poorly defined. The distal humeral epiphysis is anteroposteriorly compressed. The humeral trochlea is asymmetrical, as it is in A. primaevus and other “triisodontids” including G. levisanus, E. gaudrianus and E. coryphaeus. The humeral trochlea and rounded capitulum form a near continuous articular surface, isolated by a zona conoidea that is poorly-developed. The medial crest of the humeral trochlea is very prominent and is slightly flared, protruding mediodistally, while the lateral crest is smaller and protrudes posteriorly.

The ulna is conspicuously more gracile relative to the humerus, and has a gently convex ventral border. The ulna has a large coronoid process, while the trochlear notch has a saddle-shaped anterior surface. The ulnar diaphysis bears a deep fossa for the abductor pollicis longus muscle on the lateral surface, while the distal ulna was relatively thick, with a well-developed pronator quadratus crest on its medial surface. The radius is somewhat robust relative to the ulna, and exhibits an ovoid, shallowly concave radial fovea. The radial diaphysis is also rather straight in its proximal portion, with a bicipital tuberosity that is proximodistally long, but not particularly prominent compared to condylarths including G. levisanus, Ar. primaevus and Ankalagon. Triisodon also had a massive, hemispherical femoral head with a smooth articular surface, and an ovoid, somewhat deep fovea capitis for attachment of the ligamentum teres. ^[2]

Triisodon is the type genus of the family Triisodontidae, one of the three families within Mesonychia (the other two being Mesonychidae and Hapalodectidae). Other North American triisodontid genera, including Goniacodon, Eoconodon, and Stelocyon, have been referred to Triisodon.^[4] Like many very early mammals, the relationship of triisonodontines to other living and fossil mammals has been uncertain, but most paleontologists currently consider them either mesonychids or the sister group of mesonychids, part of the stem group that led to artiodactyls (including whales) and the ancient South American ungulates.^[5]

• Genus Triisodon^[6]
  • Triisodon crassicuspis (= T. rusticus)
    • Lower Paleocene (Torrejonian). Much smaller than T. quivirensis and with more elongate third premolar; distinguished from Eoconodon by having a somewhat reduced third molar. Assigned in the past to the genera Conoryctes and Goniacodon.
  • Triisodon quivirensis (= T. antiquus)
    • Lower Paleocene (Torrejonian), coexisted in the same habitat with T. crassicuspis. Distinguished from T. crassicuspis by much larger size, from Eoconodon as above.
  • Triisodon heilprinianus identified by Cope, 1882^[7] on the basis of a single molar, has since been referred to multiple groups. Referred to Eoconodon coryphaeus by Kondrashov and Lucas 2006.

The molars of Triisodon are very much unlike those of modern specialized carnivorans in being wide, brachydont and bunodont, comparable to those of ursids, inferring a diet comprising seeds, nuts and fruit, as well as carrion, Triisodon potentially had a large bite force for engaging in durophagy or to compensate for unspecialized dentition, as evident from the robust molar cusps and the deep mandibular fossa.

Triisodon may have had strong shoulders, as indicated by the anteriorly prominent deltopectoral region. Triisodon may also have had moderate digging capabilities, as evident from the thick distal ulna and the robustness of the humerus. The fossa for the abductor pollicis longus muscle on the ulna suggests high-powered forelimb extension, which is movement possibly linked to brief bursts of quick terrestrial locomotion including running. ^[2]