Bones of humans and animals are occasionally found in the submerged caves of Quintan Roo, but Hoyo Negro is unique in having a high concentration of multiple species in a high state of preservation. As of December, 2017, not including Naia and the untold numbers of fish and bats, we have recognized at least 34 individuals of 13 species on the pit floor and another 8 animals and one additional species in the shallow tunnels leading to and from the pit. Seven species—fully half of the taxonomic assemblage—are extinct and 24 of the 42 individuals belong to those species. These include the highland gomphothere (an elephant-like animal related to mastodons) three species of giant ground sloth, sabertooth tigers, a species of short-faced bear, and an as-yet-to be disclosed species of wolf-like animal.

In the following list, the number of individuals in Hoyo Negro is shown, along with the number in connecting tunnels in parentheses.


  • Highland gomphothere (Cuvieronius tropicus) 2 (3)
  • Shasta ground sloth (Nothrotheriops shastensis) 2 (1)
  • New ground sloth species (Nohochichak xibalbahkah) 1
  • Harlans ground sloth (Paramylodon harlani) (1)
  • Sabertooth tiger (Smilodon fatalis) 3 (1)
  • South American short-faced bear (Arctotherium wingei) 8
  • Canid species (to be announced) 2


  • Cougar (Puma concolor) 3
  • Bobcat (Lynx rufus) 3
  • Coati (Nasua narica) 2
  • Tapir (Tapirus cf bairdii) 3 (3)
  • Collared peccary (Pecari tajacu) 2
  • Opossum (Didelphis sp) 1 [plus one provisional]
  • Canid species (to be identified) 1

Hoyo Negro contains several exciting new discoveries.

  • A never-before seen species of giant ground sloth of the megalonychid family, which includes the modern two-toed sloth Coloepus. We named this sloth Nohochichak xibalbahkah, maya for “Great-claw underworld-dweller.” (LINK TO McDonald et al. 2017)
  • The first report of the extinct South American short-faced bear Arctotherium wingei outside South America.
  • The first complete skulls and limb bones of the south American short-faced bear A. wingei.
  • The first complete skull of Smilodon fatalis from Mexico.
  • Documentation of the bobcat far outside its modern range.

Although many of the skeletons found in shallow tunnels are eroded and extensively damaged by trampling, most bones in Hoyo Negro are almost perfectly preserved, despite most having lain exposed in air for thousands of years before sea level rise inundated them about 10,000 years ago. This remarkable preservation is due to three factors.

  1. They were deep within a cave. Caves have almost-constant climates year-round, unlike the landscape above, where wetting and drying, warming and cooling lead to mechanical breakdown of bone.
  2. Waters that surrounded the bones after inundation were rich in calcium carbonate (lime), which means the water was slightly chemically basic and did not dissolve the mineral part of the bone. In fact, calcium carbonate impregnated the bones and precipitated in the form of calcite cement, which helped hold the bone together.
  3. Animals were not walking around in Hoyo Negro; all those in the cave (besides bats and fish) died when the fell in and did not trample bones of earlier arrivals. Another factor, which has allowed some of the bones to retain their protein matrix and wood and seeds to preserve, is that the water very bottom of the cave is almost completely anoxic, reducing rates of bacterial decay.

A key question we have about the Hoyo Negro fauna is whether the animals represent a single time period and thus single surface ecosystem, or represent a cross-section of distinct ecosystems that occupied the Yucatan as the ice age transitioned into the modern interglacial. Dating the bones is difficult because, although they have held their shapes well, the bones are almost entirely lacking in protein, which we need to obtain the most-trusted radiocarbon dates directly on the bone. We are, therefore, attempting the determine minimum and maximum possible ages—radiocarbon dating wood and seeds or uranium-thorium dating calcite formations found on or in the bones of a species for the minimum age and radiocarbon dating the mineral portion of bones or teeth for maximum ages.

Thus far we have dates on one gomphothere of between 19,000 and 36,000 years and one bear at about 13,000 years. The bear date is on bone protein, so is an accurate age for the animal.