Andrewsarchus — Giant of the Eocene Riverlands

Where Andrewsarchus Is Found

In the campaign present, wild Andrewsarchus are not part of ordinary surface fauna. Their strongest surviving populations live in the Hollow Earth, inside isolated warm Paleogene refuges separated from the dinosaur basins and Ice-Age valleys.

They favour broad interior river systems, reed-choked floodplains, open woodland and scrub where large mammals gather around water. These territories lie mainly in the great inner-world refuge networks beneath Asia, although connected Black Roads, faults and ancient waterways can carry individual animals far from their usual range.

Rare Andrewsarchus encountered in the Threshold Depths are usually displaced wanderers following prey, carrion or reopened passages. A surface sighting should therefore be unusual and significant: it normally means a deep route has opened, an animal has been deliberately moved, or something below is forcing prehistoric wildlife outward.

Behaviour

Andrewsarchus is usually encountered alone. It follows broad feeding circuits between water, carrion sites and places where large animals cross rivers or narrow ground. It investigates smell before sight and often knows that a wounded animal, butchered carcass or travelling pack train is present long before anyone sees it.

Its first response to competitors is normally assessment rather than an immediate charge. It may stand broadside, expose its jaws, make short rushes or push forward deliberately until weaker animals give ground. A creature that abandons food or opens an escape route may end the confrontation without bloodshed.

If Violence Starts

Once committed, Andrewsarchus closes hard and tries to break formation. It prefers an exposed creature at the edge of a group, especially one already prone, injured or separated. The animal does not divide its attention neatly among opponents; it keeps pressure on the easiest target until pain, fire, magic or a stronger threat forces it to disengage.

A hungry animal can be driven away if the expected meal becomes too costly. A cornered Andrewsarchus is much less predictable because escape, not feeding, becomes the priority. It attacks whichever creature blocks the clearest route out.

Signs an Andrewsarchus Is Near

• Enormous hoofed tracks pressed deep into soft riverbank mud.
• Large carcasses with heavy bite damage concentrated around the ribs, neck and limbs.
• Scavengers waiting at an unusual distance from a fresh kill.
• Low, repeated grunts or jaw-clacking carried across open ground.
• Scrub flattened along the same route between water and feeding sites.
• Old bones scattered from a carcass after repeated feeding visits.

Using Andrewsarchus in Play

At a River Crossing

The animal is already using the ford when the party arrives. It may ignore travellers who keep their distance, but pack animals, blood, food or a panicked mount can turn a tense crossing into a confrontation. The encounter has choices before initiative: wait, reroute, distract it or force passage.

At a Carcass

Andrewsarchus has claimed a dead animal that the characters also need to examine. Evidence, equipment or a missing person may lie beneath the body. The creature wants control of the carcass rather than a pointless fight, giving the party room to bargain with distance, noise, fire or another food source.

As a Predator

Use a predatory individual when it has learned that livestock, wounded mounts or isolated travellers are easier meals than healthy wild prey. It follows scent, waits for separation and commits only when it believes the target cannot escape.

As an Ecological Clue

Its presence tells the party that the region can support extremely large mammals and a functioning scavenger guild. Smaller predators alter their routes, carcasses disappear quickly and local people learn which riverbanks are unsafe at dusk.

Challenge Rating and Encounter Scale

Across these rules sets, Andrewsarchus is built as a straightforward CR 5 bruiser. It has enough durability to survive focused attacks, enough speed to close with a party and a bite that matters, but it does not rely on supernatural defences, legendary actions or complicated control effects.

The fifth-edition versions reach their threat through two solid melee attacks and a knockdown opportunity after a charge. Pathfinder adds a bull rush rider to the same idea. The 3.5e block preserves the established CR 5 conversion and its deliberately simple single-bite routine.

Motivation and Reactions

Adventure Hooks

• The Ford Is Closed. A huge Andrewsarchus has begun resting beside the only safe river crossing for miles, trapping merchants on both banks while something worse follows the stranded caravans.
• The Wrong Killer. Villagers blame Andrewsarchus for mutilated livestock, but the feeding pattern shows it arrived after death. The real predator is still hunting nearby.
• Skull in the Shrine. A fossilised Andrewsarchus skull is stolen from a ruler’s collection. The thieves believe it marks the route to a surviving population hidden beyond the known valleys.
• The Empty Kill. Hunters find a freshly slain giant herbivore stripped of every useful possession but barely eaten. Andrewsarchus has driven the true killers away and now guards the scene.
• River Teeth. Repeated attacks on ferry animals are blamed on one giant individual. Killing it is easy to demand; discovering why it has abandoned its normal range is harder.

Natural History and Fossil Context

Andrewsarchus mongoliensis is known securely from an enormous skull discovered in 1923 in Mongolia and described the following year. The American Museum of Natural History notes that it remains the only known specimen of the species and dates the animal to roughly 45 million years ago in the Eocene.

That matters because the famous full-body image is much less certain than the skull. The shoulders, limbs, feet, coat, exact mass and overall proportions are reconstructed by comparison rather than preserved as a complete skeleton. Older restorations often made Andrewsarchus look like an immense wolf-like mesonychian. Later phylogenetic work placed it within the broader even-toed ungulate and whale radiation, closer to hippopotamus-and-whale relatives than those old reconstructions implied.

The jaws and teeth establish a very large animal capable of powerful feeding, but they do not prove one narrow lifestyle. Hunting, scavenging and opportunistic omnivory have all been discussed. For play, the most useful interpretation is a dominant large-bodied opportunist that can hunt, displace other feeders and exploit carrion without behaving like a monster that attacks everything it sees.

Research and Further Reading

• American Museum of Natural History — Andrewsarchus and the original skull.
• Spaulding, O’Leary & Gatesy (2009) — relationships of Cetacea and fossil artiodactyl relatives.
• Dmitry Bogdanov reconstruction — public-domain image and palaeoart notes.

After the Encounter

Andrewsarchus carries no deliberate treasure. A feeding site may contain torn packs, damaged tack, weapons or valuables belonging to previous victims, while the animal’s teeth and skull attract collectors who value rare prehistoric trophies. Removing a dominant individual can also open the territory to smaller predators and scavengers that had been kept away.

Sources and Licensing

D&D 5.5e / 2024 and D&D 5e / 2014: Andrewsarchus is presented here as an original fifth-edition adaptation using the relevant edition’s monster conventions. It is not an official SRD monster.

Pathfinder 1e: The Pathfinder stat block is an original conversion using Pathfinder First Edition rules conventions.

D&D 3.5e: The legacy CR 5 statistics preserve the established conversion attributed to Shade of the EN World forums, with calculations retained and presentation cleaned for play.

Earlier tabletop appearance: Andrewsarchus appeared in David Howery’s “Back to the Age of Mammals,” Dragon Magazine #167 (1991). That publication is identified for history only; its prose and artwork are not reproduced here.

Artwork: Original site hero illustration. As with every full-body reconstruction of Andrewsarchus, the body remains necessarily speculative because the animal is known principally from cranial material.