Dissacus

Not to be confused with Dissopsalis.

Dissacus Temporal range: Early Paleocene-Early Eocene (Danian to Ypresian), Da. S T Ypr. Lut. B Pr. Rup. Ch.
Skull of Dissacus zanabazari
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Placentalia
Order:	†Mesonychia
Family:	†Mesonychidae
Genus:	†Dissacus Cope, 1881
Type species
†Dissacus navajovius
Other Species
†D. argenteus O'Leary and Rose, 1995 †D. bohaiensis Tong and Wang, 2006 †D. europaeus Lemoine, 1891 †D. indigenus Dashzeveg, 1976 †D. magushanensis Yan and Tang, 1976 †D. praenuntius Matthew and Granger, 1915 †D. rotundus? Wang, 1975 †D. saurognathus? Wortman, 1897 †D. serior? O'Leary and Rose, 1995 †D. serratus Chow and Qi, 1978 †D. shanghoensis Chow et al., 1973 †D. zanabazari Geisler and McKenna, 2007 †D. zengi Ting, 2004

Dissacus is a paraphyletic genus of extinct carnivorous jackal to coyote-sized Mesonychidae, an early family of hoofed mammals that consisted of carnivore-omnivore. Their fossils in Early Paleocene to Middle Eocene aged strata in Eurasia and North America. Dissacus europaeus may have evolved in North America migrated to Europe, and first appeared in European strata in the Thanetian (c. 58-57 MYA). A phylogenetic analysis of European mesonychids suggests that Dissacus was part of an early diversification of species in the Paleocene, and other genera diversified in a later episode in the early Eocene. D. europaeus survived the Paleocene-Eocene Thermal Maximum, a period of severe global warming, which suggests it had flexible habits. Orientation patch analysis of the molar teeth of the North American D. praenuntius suggests its omnivorous with consisted diet consisting of meat, suggesting it wasn’t a hypercarnivore such a felid or weasel. It shared its environment with more omnivorous mammals of a similar body size.

Originally, it was believed Dissacus was present in both Northwestern and Southwestern Europe, however all European species, with the exception of D. europaeus, was reclassified as Hyaenodictis. This reclassification restricted European Dissacus to Southernwestern Europe during the latest Paleocene to earliest Eocene.

Taxonomy

Phylogenetic modeling recovered Dissacus as a paraphyletic genus. Spaulding et al. (2009) recovered D. praenuntius being more closely related to Pachyaena and Harpagolestes orientalis.

	†Hapalodectes †Sinonyx †Mesonyx †Harpagolestes uintensis †Dissacus zanabazari †Dissacus praenuntius †Harpagolestes orientalis †Pachyaena Mesonychia †Protungulatum Perissodactyla Artiodactyla Tylopoda Suina Ruminantia †Entelodontidae †Anthracotheriidae Hippopotamidae †Raoellidae Cetacea Ungulates

Solé et al. (2018) recovered D. praenuntius, D. europaeus, and D. argenteus as basal mesonychids. D. serratus and D. zanabazari were recovered as members of the Asian Dissacus clade, with D. willwoodensis being recovered as close relative to the clade. The European Dissacus clade consisted of D. rougierae, D. raslanloubatieri, D. blayaci, and D. filholi. D. navajovius was recovered as a sister taxon to the monospecific Ankalagon, questioning the validity of the latter genus. Within the same analysis recovered D. serior within the Mesonyx clade.

Solé et al. (2023) revived the genus Hyaenodictis finding it distinct from Dissacus. In addition, they reclassified D. rougierae and D. raslanloubatieri as European species and D. willwoodensis as the North American species of Hyaenodictis. Additionally, D. blayaci, D. progressus, and D. filholi were also reclassified as species of Hyaenodictis. Their phylogenetic analysis included D. rotundus, D. zengi, and D. magushanensis. D. rotundus was found to be closely related to D. navajovius and Ankalagon. D. zengi was recovered as a sister taxon to the Asian Dissacus clade and D. magushanensis was recovered as sister taxon to D. argenteus. Current Dissacus species also saw some revised positioning, with D. bohaiensis was moved to the Asian Dissacus clade.

Description

Size comparison between male and female D. saurognathus

D. navajovius was the smallest North American species, length and width regressions of m2 suggests the species weighed 9.92 kg (21.9 lb) on average, with the largest specimen, AMNH 3361 weighing 11.33 kg (25.0 lb). D. praenuntius was significantly larger than the afromentioned species, with Clarkforkian D. praenuntius averaging 18 kg (40 lb). During the PETM, D. praenuntius saw a decrease in size averaging at around 14.28 kg (31.5 lb). Possible Dissacus species, D. saurognathus, was the largest species with a head length of 30 cm (12 in) and an estimated body mass of 46.9 kg (103 lb). ^{Including supplementary materials}

D. europaeus was the smallest European mesonychid weighing 8–14 kg (18–31 lb) prior to the thermal maximum. During the thermal maximum, D. europaeus shrunk in size significantly with dwarfed sized specimens weighing 4–5 kg (8.8–11.0 lb).

Paleobiology

Life reconstruction of D. zanabazari

Locomotion

A morphological study of limb bones of D. europaeus suggests this animal was digitigrade and more cursorial than is usually assumed for the genus. Analysis of the elbow joint shows it was specialized for extra flexion and extension, an adaptation usually found in running species; the amount of specialization is unusual for a Paleocene mammal. This does not mean Dissacus species were swift runners by modern standards, but that D. europaeus was more adapted for running than other mammals of its time. It may be part of a trend among early Cenozoic mammals toward developing more specialized bodies to fill different ecological niches.

Diet

Prior to the Palaeocene-Eocene Thermal Maximum (PETM), D. praenuntius primarily ate meat. During and after the PETM, however, bone featured more prominently in its diet compared to before the hyperthermal event.

Extinction

Mandible of Pachyaena gigantea, one of the mesonychids that replaced Dissacus in Europe

D. europaeus would go extinct roughly 55.5 million years ago contemporary with the appearance of Pachyaena and Palaeonictis in Northwestern Europe. However, it was unlikely Pachyaena ecologically replaced Dissacus, due to the former's much larger body size. Palaeonictis occupied a different niche than Dissacus, with the latter having a greater preference for meat. D. praenuntius, would go extinct in or shortly after Wa-2, which dates to 54.7-54.4 million years ago. Its extinction coincided with the diversification of Pachyaena and Hyaenodictis willwoodensis. Schwartz et al. (2025) suggested the possible chance of competitive replacement, however dental microwear on the two mesonychids is required. The last North America species, represented by D. serior was found within Wa-6, which dates to 53.0-52.4 million years ago.