Paleontologists have identified a new species of extinct alligatorid that lived in what is now Colorado just a few hundred thousand years after the asteroid impact that wiped out the non-avian dinosaurs.
The skull of Mandrasuchus milleri, a species of alligatorid that lived nearly 65.4 million years ago in what is now Colorado, the United States. Image credit: Lessner et al., doi: 10.1080/02724634.2026.2712566.
The newly identified species lived during the earliest Paleocene, between roughly 200,000 and 350,000 years after the end-Cretaceous mass extinction.
Named Mandrasuchus milleri, it is the earliest confirmed member of Alligatorinae, the subfamily that includes modern alligators.
“Among crocodyliforms, Alligatoroidea is the only group to increase in diversity immediately following the Cretaceous-Paleogene mass extinction event,” said first author Dr. Emily Lessner from Bureau of Land Management and her colleagues.
“The first appearance of this clade is in the Late Cretaceous and today the crown group includes the extant Alligatorinae (Alligator spp.) and extant Caimaninae (Paleosuchus spp., Caiman spp., and Melanosuchus niger).”
“Early alligatoroids (stem alligatorids) include a number of small, blunt-snouted, globe-toothed taxa, and this morphology is sometimes interpreted as the plesiomorphic condition for alligatorids, though recent analyses indicate the morphology may be more appropriately described as plesiomorphic to caimanines.”
“In general, there is not a clear division at the base of Alligatoridae between alligatorines and caimanines, with little resolution amongst the basal members of these clades.”
The fossilized remains of Mandrasuchus milleri — skulls and fragmentary postcranial material from several individuals — were unearthed at the Corral Bluffs study area in the southwestern portion of the Denver Basin east of Colorado Springs, El Paso County, the United States.
“The large number of Mandrasuchus milleri specimens thoroughly documents the ontogenetic changes in alligatorid anatomy and the impact such changes have on understanding evolutionary relationships,” the paleontologists said.
Osteoderms and articulated tail of Mandrasuchus milleri. Scale bars – 5 cm in (A-I) and 10 cm in (J). Image credit: Lessner et al., doi: 10.1080/02724634.2026.2712566.
The ancient predator was relatively small, reaching up to about 1.5 m (5 feet) in length and roughly 35 kg, with a short, blunt snout and rounded, button-like teeth toward the back of its jaw.
According to the team, this dental structure points to a generalist diet rather than one specialized for crushing hard-shelled prey, allowing the animal to feed on soft prey as well as hard-bodied creatures like mollusks and arthropods, while still being capable of scavenging larger carcasses.
Mandrasuchus milleri shared its habitat with a larger crocodylian called Borealosuchus sternbergii, suggesting the two species divided up the available food resources rather than competing directly for the same prey.
A third semi-aquatic predator, the fish-eating reptile Champsosaurus, was also part of this community.
“It was a dog eat dog world after the extinction of the dinosaurs, but one group that thrived were crocodilians,” said Dr. Tyler Lyson, curator at the Denver Museum of Nature & Science.
The authors concluded that the survival and diversification of alligator relatives across the extinction boundary — including apparent burrowing behavior inferred from fossilized burrow structures at the site — underscores how freshwater terrestrial ecosystems were comparatively insulated from the devastation that reshaped life elsewhere on Earth.
“The proximity of Mandrasuchus milleri to the Cretaceous-Paleogene boundary expands knowledge of early alligatorids and morphological and behavioral traits (i.e., burrowing) necessary to survive such catastrophic events and thrive in the earliest Paleocene,” they said.
Their paper was published this month in the Journal of Vertebrate Paleontology.
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Emily J. Lessner et al. New alligatorid, Mandrasuchus milleri, from the Early Paleocene (Danian) of the Denver Basin supports earliest Paleocene North American alligatorid radiation. Journal of Vertebrate Paleontology, published online September 2, 2026; doi: 10.1080/02724634.2026.2712566
