Convergence in the Underworld

The Riviera Maya is a wonderful vacation spot. It is a beautiful, resort-filled strip of coastline south of Cancún near the town of Playa del Carmen (along the east coast of Mexico’s Yucatan Peninsula) that boasts excellent snorkeling, great wildlife viewing, and fascinating Mayan ruins. You can also zipline through tropical semi-deciduous forest or go swimming in a cenote (pronounced say-NO-tay), a sinkhole that connects to a water-filled, underground cave.

Figure 1 Cenote Map.jpg
Map of the Riviera Maya region in the Mexican state of Quintana Roo illustrating the location of Cenote Zapote and Hoyo Negro, discussed below. Modified from Stinnesbeck et al. (2017:fig. 1). Reuse permitted under CC BY-NC-SA 2.0.

Swimming in a cenote is very different from snorkeling the Caribbean; the water is chilly, and there isn’t much aquatic life to see other than the occasional cave catfish. But the atmosphere of swimming through the clear waters of a cave is truly unique. Plus, it turns out that some cenotes preserve the remains of fossil mammals that lived during the late Pleistocene epoch, more than 12,000 years ago.

The most famous fossil from one of these cenote caves is a nearly complete human skeleton known as Naia that was described a few years ago in the journal Science. It is the oldest relatively complete human skeleton from North America and is important because the structure of its skull, combined with its ancient DNA, suggest that native North Americans are the descendants of Siberian immigrants. Of course, there are some dissenting opinions regarding the interpretation of this skeleton (see this comment in Science and this brief communication in the American Journal of Physical Anthropology).

Figure 2 Cenote Diagram.jpg
Vertical sections through cenotes, illustrating these water-filled caves. Left, Cenote Zapote (from Stinnesbeck et al. 2017:fig. 2), with blue representing the present water level. Right, the pit known as Hoyo Negro (marked by a red X in the top diagram) and two nearby cenotes that provide access to the Sac Actun cave system (marked by green circles; modified from Collins et al. 2015:fig. 8). The three diagrams at right show changing water levels from the late Pleistocene (bottom) to around 6,000 years ago (top).

Yucatan cenotes and caves were not always filled with water. Rather, they represent the remains of air-filled chambers that formed as water slowly dissolved the limestone bedrock of the peninsula. The fossils they contain are from animals that entered and died in the cave when it was mainly filled with air or that accidentally fell through its ceiling sometime later. When sea levels rose as a consequence of melting glaciers at the end of the Pleistocene Epoch (the last “Ice Age”), these underground networks filled with water, creating the cenotes and submerged caves we know today.

The non-human mammal remains from these cenotes include extinct species, such as saber-toothed cats and elephant-like gomphotheres, as well as modern ones, such as peccaries, cougars, and tapirs. Most of these fossils have not yet been formally described, but two new sloths from these deposits were named in the past couple months: Xibalbaonyx oviceps and Nohochichak xibalbahkah. Both of these sloths belong to the family Megalonychidae, which includes a wide variety of extinct species from throughout South America and the Caribbean as well as living two-toed sloths (genus Choloepus).

Figure 3 Hoyo Negro Bones.jpg
Large mammal remains from Hoyo Negro as they appeared on the floor of the pit. The lower jaw of a megalonychid sloth (likely Nohochichak xibalbahkah) is shown in D (occlusal view, between the tape measure and rib, front toward the lower right). Skulls of a peccary (A), cave bear (E), and saber-toothed cat (F) are also shown, along with a variety of limb bones. From Chatters et al. (2014:fig. S5). 

Xibalbaonyx oviceps was the first of these new sloths to be named. The genus is a bit of a mouthful and combines xibalba (roughly pronounced she-bahl’-bah), the Mayan word for the underworld or place of fear (in reference to the cenote where the remains were found) and onyx, the Greek word for claw (which may sound familiar if you have read my blog on the sloth Megalonyx). The specific epithet (the second part of the name) means “egghead” and refers to its high, domed skull rather than its intellectual abilities.

Only a single specimen of Xibalbaonyx oviceps was described, but it is a real beauty; it consists of a nearly pristine skull that is only missing a few upper teeth and small pieces of bone here and there. To be completely accurate, I should revise that statement; the specimen actually consists of a skull and skeleton, but only the skull has been described thus far. The remainder of the animal remains submerged on the floor of Cenote Zapote, some 50 m (165’) below the water’s surface. Presumably it will eventually be collected and described; collecting specimens in a cenote presents a variety of challenges that we dry land paleontologists don’t have to deal with. In addition, different techniques need to be used to preserve such specimens once they are collected. In the case of  Xibalbaonyx, the skull was soaked in distilled water for 8 months after it was collected, and then it was subjected to another month in a bath of distilled water and polyvinyl acetate (a type of consolidant commonly used in paleontology) to strengthen and stabilize it for study. Some patience needs to be exercised in awaiting description of the other remains.

Figure 4 Sloth Crania.jpg
Left lateral (side) views of the crania of Xibalbaonyx oviceps (left; from Stinnesbeck et al. 2017:fig. 4A) and Nohochichak xibalbahkah (right; from McDonald et al. 2017:fig. 4a). Anterior is toward the left, and specimens are at approximately the same scale.

The second species, Nohochichak xibalbahkah, was collected in the same spot as the famous human skeleton, Hoyo Negro, which is located about 65 km southwest of Cenote Zapote. Hoyo Negro is part of a giant underground cave system known as Sac Actun that extends underground for some 350 km and is accessible by a variety of cenotes. The genus name Nohochichak is the Mayan equivalent of Megalonyx and means “great claw,” a particularly appropriate name considering its familial affinities. The specific epithet combines xibalba (as in Xibalbaonyx) with the Mayan word for “dweller,” resulting in the combination “underworld dweller.”

Only a single specimen of Nohochichak has been discovered thus far, as is the case for Xibalbaonyx, and it also consists of a skull. The lower jaw of Nohochichak is in very good shape, but the remainder of the skull is much more fragmentary than that of Xibalbaonyx and basically consists of the snout, palate (sans teeth), and the left zygomatic arch (cheek bone). Other parts of the skeleton remain to be collected and are located on the floor of Hoyo Negro some 40-50 m (130-165’) under water.

Based on radiocarbon dating the bones of Xibalbaonyx are 10,647–10,305 years old, and those of Nohochichak are 11,264–11,183 years old, making them roughly the same age. However, it is thought that both of these dates probably slightly underestimate the true ages of these fossils because some of the other mammals associated with these sloths went extinct some 12,000 years ago. Submerging bones in water for thousands of years is not a great way to preserve the collagen in them, which is the material that is actually dated when using the radiocarbon method. Therefore, it is certainly reasonable to take the dates results with a grain of salt. Moreover, dating methods used to date other remains from Hoyo Negro confirm that those specimens are at least 12,000 years old (see Chatters et al. 2014).

Xibalbaonyx and Nohochichak have a lot in common in addition to their age, where they were found, and the material that has been described thus far. Both sloths have a snout with parallel sides that is slightly narrower than the braincase and that slopes downward when viewed from the side. The lower teeth are very similar in size and shape, and the two specimens are remarkably similar in size; the length of the lower tooth row (including the caniniform and all three molariforms) is 131 mm in Xibalbaonyx (Stinnesbeck et al.:table 1) and 131.8 mm in Nohochichak (McDonald et al. 2017:table 2). You can’t get much closer than that.

Figure 5 Sloth Jaws.jpg
Occlusal views of the lower jaw of Xibalbaonyx oviceps (left; from Stinnesbeck et al. 2017:fig. 13A) and Nohochichak xibalbahkah (right; from McDonald et al. 2017:fig. 5c). Anterior is toward the top, and specimens are at approximately the same scale.

The two sloths do differ in some respects. For example, the bony flange that extends downward from the cheekbone appears to be slightly longer in Xibalbaonyx than in Nohochichak. The lower jaw of Xibalbaonyx appears to be broader than that of Nohochichak, and its front end, which is known as the predental spout, also appears to be somewhat broader and to subtend a slightly greater angle. With only one specimen of each animal, it is difficult to assess whether these are real (diagnostic) differences between the two species or simply reflections of individual variation or differences in preservation. Moreover, comparisons like this really need to be done with the actual specimens (or casts) in hand to be accurate. But it makes me wonder if they might represent the same species.

From an ecological perspective, it seems unlikely that two closely-related sloths such as these would be able to coexist in the same place at the same time. A basic tenet of ecology is that two species cannot fill the exact same ecological niche, and the many attributes shared by Xibalbaonyx and Nohochichak make it difficult to envision how they might have differed in what they ate, how they moved, or some other aspect of their paleobiology. Nevertheless, ecological arguments can’t really be marshaled as evidence that two animals represent the same species. Such decisions must fundamentally be based on anatomy, so I’ll leave it to the slothologists to determine whether or not that is the case.

In the mean time… let’s speculate that Xibalbaonyx oviceps and Nohochichak xibalbahkah really do represent the same species. Then what would happen? Someone would have to publish a paper formally stating (and demonstrating) that the two species are synonymous. In that case, the two names would be synonyms of one another, and one of them, the senior synonym, would be the valid name. In taxonomy, the senior synonym is the one that was published first. The name Xibalbaonyx oviceps was published on May 22, 2017 and Nohochichak xibalbahkah was published on June 8, 2017. Therefore, Xibalbaonyx oviceps would be the valid name, and the name Nohochichak would be banished to the underworld, never to be heard of again. The specific epithet, xibalbahkah, could be paired with another genus, and in fact this could easily happen in a third scenario; if specialists were to decide that the two species are very closely related but not the same, the second species could be transferred to the first genus, creating the new combination Xibalbaonyx xibalbahkah. That is certainly a name that would please the gods of the underworld!

What really struck me as remarkable about this pair of papers is that the two groups of researchers were apparently working on their respective specimens in parallel, unbeknownst to one another. They don’t appear to have been in a race to publish – Xibalbaonyx was collected in 2009, and Nohochichak was recognized as new prior to 2014 – but the papers happened to be submitted to their respectively journals within six weeks of one another (in the summer of 2016) and published scarcely two weeks apart. Furthermore, both groups independently decided to name their species after the Mayan underworld, Xibalba, though it formed part of the genus of one and the specific epithet of the other. Maybe that was an obvious choice, but given the myriad possibilities, it still surprises me. I am hoping I can glean some additional details at the upcoming annual meeting of the Society of Vertebrate Paleontology.

Plenty of fossils remain to be collected from these and other Yucatan cenotes, and I eagerly await seeing what will emerge from the underworld next!

References Cited:

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