The Case of the Wrong Femur

The right femur on the Greater Cuban ground sloth Megalocnus rodens on exhibition at the American Museum does not belong to Megalocnus!



Figure 1: Greater Cuban sloth Megalocnus rodens Leidy 1868 (AMNH no.16876).

In the Hall of Primitive Mammals at the fourth floor of the American Museum (AMNH), close to where the dinosaurs are mounted, there is a large sloth specimen on walking position. This specimen belongs to the Greater Cuban ground sloth Megalocnus rodens described by Joseph Leidy in 1868, probably the first rock star of the Caribbean vertebrate paleorecord.

But how and why did this specimen get the wrong femur? How did such detail escape meticulous and experienced scientists? Well, that story is very interesting, and the theme of this blog post.

A Bit of Background

AMNH specimen number 16876 (fig. 1) is a composite skeleton. By composite, we mean that it is mounted with remains of multiple individuals. As the story goes, the first Megalocnus was discovered by the young university student Jose Figueroa in April 1860 at the thermal baths of Ciego Montero, central Cuba. Figueroa sent his fossil specimen, a mandible in this case, to Cuba's foremost zoologist Felipe Poey then professor of comparative anatomy at the University of Havana, who at first thought it belonged to a large rodent.

That initial discovery deeply incited the serious interest of Dr. Poey's padawan, Dr. Carlos de la Torre y Huerta, who was on his way to becoming Cuba's most prominent naturalist for all times (fig. 2 left).



Figure 2: Dr. Carlos de la Torre (left) and Barnum Brown (right). Brown's picture is from his file in Wikipedia. Each photograph is circa the late 1890s.

Dr. de la Torre spent several decades collecting more specimens in other localities, later inviting the AMNH to participate in his quest. His idea was to find specimens that could be better described and composed into whole skeleton mounts. In response to his request, the AMNH sent Mr. Barnum Brown, one of their best diggers/researchers (fig. 2 right). His reputation was impressive, for he was the discoverer of the now iconic Tyrannosaurus rex!

Between 1910 and 1918 Brown collected fossil specimens from Ciego Montero, the same locality from where the first specimen originated. Their efforts resulted in enough material to mount three specimens, of which only two were completed by Adam Hermann under the supervision of Dr. Henry F. Osborn, then director of the American Museum. That third specimen was prepared by Charles Long but never mounted. The last two remain today at the AMNH.




Figure 3: Megalocnus rodens composite skeleton  prepared by A. Hermann, and sent to Dr. Carlos de la Torre, in Havana. Photograph available from Wikipedia.

As agreed by the collecting parties, one specimen (fig. 3-4) was sent to Dr. de la Torre and is now on display at the National Museum of Natural History in Havana. [ Trivia: Dr. de la Torre was so fond of his Megalocnus mount that he used to take pictures of it and exhibit it everywhere he could, like that on fig. 4]. The other became a walking mount displayed as a separate exhibit at the AMNH. That specimen is now specimen no. 16876, and it bears the right thigh bone of Parocnus browni; another extinct Greater Antillean ground sloth of similar proportions (fig. 4).





Figure 4. Dr. Carlos de la Torre and the climbing mount of  the Megalocnus rodens. Photograph circa 1920. The bust on the left is Felipe Poey, luminary naturalist of the 19th century.

The Case

The specimens sent by Dr. de la Torre and those later collected by Brown comprised the main Megalocnus collection at the AMNH (plus other fossil verts). This collection was first studied by the renown paleontologist  Dr. William Diller Matthew along with de la Torre (1915). By 1919, Dr. Matthew had published two quick articles on the subject, but the main corpus of work was never finished. That responsibility later fell upon Dr. Carlos Paula Couto, who completed the quest (Matthew and Paula, 1959; Paula, 1967). As a posthumous note (Matthew, 1931) had installed Mesocnus browni, who figures into the story as Parocnus browni mentioned above. So named browni in the honor of Mr. B. Brown.

There seemed to be several good reasons for that erroneous mount that escaped Dr. Osborn, Hermann, Matthew, and later Paula Couto. The most reasonable are that both Megalocnus and Parocnus are anatomically similar in skeletal morphology and body size. The differences we understand now to be of specific value to them, such as the presence of a bony crest on the femur shaft known as the third trochanter, were plausibly masked by their similarity. This equivocal assignment is exemplified in plate 20 of Matthew and Paula's (1959) publication, which illustrates a Parocnus left femur (no. 49977) identified as Megalocnus rodens rodens. In this specimen, the third trochanter is clearly visible (Acevedo et al., 1972).




Figure 4: Left, Megalocnus rodens at the Cuban MNHNCu. Right, the femur of Parocnus browni mounted on the M. rodens no. 16876 at the AMNH. Note the presence of a third trochanter on the P. browni specimen on the Megalocnus rodens on the right. The photograph on left from Hernandez-Pacheco, 1947.  

But I am not the first to make these observations. That credit we owe to Manuel Acevedo and one of Cuba's most prolific paleontologist Oscar Arredondo, who committed similar observations in their book "Cueva del Tunel" (1972). A similar note was later published by Carlos Arredondo Antunez, son of the former Oscar Arredondo, in an article on the book "Mammals in Cuba" (Arredondo, 2011:32-33). However, for whatever reason, the observations of Acevedo et al did not figure into later publications on Cuban ground sloths.

What is curious is that Carlos de la Torre  had already noted the usefulness of the third trochanter by 1910!. In an article regarding the restoration of the Megalocnus specimens, he notes (my translation) "...the femur of Megalocnus rodens completely lacks a third trochanter..." (Torre, 1910:30). By then Parocnus remains were already known to de la Torre but were under the name Mesocnus  (see footnotes on Paula Couto, 1967). Thus, this statement suggests that by 1910 de la Torre was, at least, familiar with the femoral variation in Megalocnus, and to a lesser degree with fossils from other species. So how come to Matthew, who was renown for fossil mammals, and had worked with de la Torre, moreover Paula Couto who studied the  completed Megalocnus collections did not make that distinction?
It is reasonable then to think, and I am going on a limb here (pun intended), that these differences in the appendicular skeleton were originally dismissed to individual variation or sexual dimorphism as asserted later (de la Torre, 1916; Acevedo et al., 1972; White and MacPhee, 2001; Silva et al., 2007). These likely extended to the presence or absence of the femur's third trochanter. Today, such differences are considered a distinctive characteristic that helps to separate the sloth's species.

...So What? 

But doesn't this seems a bit trivial? Why does it matter or why is this important? Well, bits make up a whole, and in science every little bit of information is useful. Paleontology, like most of the historical sciences, depends on evidence that is too often partial or incomplete. We rarely find whole, well-preserved specimens from which we can compare whole skeletons, skin or feather color, or DNA, in order to understand what constitutes a species in any given group. In the practical sense, detailed anatomical distinctions allow us to separate and name which organisms belongs to what group, making our communication and distinction of such organisms easier. Therefore, by understanding the little anatomical details we can induce from which group did the fossil belong to and what was its way of life, and its evolutionary history. All these are basic clues to understanding prehistoric life in general.

This is not to emphasize on the error, but on the fact that science is always evolving; growing and relearning from its own mistakes and previous knowledge. This example highlights the importance of details and continuation of research in science, even in minutiae that seemed to have been resolved. The extinct sloths were an integral part of the insular ecosystems in our most recent past and provide a window into the understating on the diversity and extinction of mammalian life in the prehistoric Caribbean.

...Gist on Ground Sloths

Ground sloths are mammals of the order Pilosa, where they are related to armadillos, anteaters, and tree sloths. The Antillean sloths belong to the Megalonychidae family. This is a family that includes the behemoths Megalonyx from North America and the Antillean species that reached well over 50 kg body weight like the Megalocnus and Parocnus of our story. The fossil record indicates their presence in the Greater Antilles during the Oligocene (~ 32 million years ago) and the Miocene (~ 23-5 mya). These oldest records come from extraordinary fossil finds in Cuba and Puerto Rico. Greater Antillean ground sloths seem to have experienced great diversity during the Quaternary, the last several million years or Earth's history, only to disappear about 4000 years ago (MacPhee et al., 2007). Incidentally, their extinction seems to coincide with the time of the Amerindian population of the Antilles. Whether they were already endangered when the Amerindians arrived in the Greater Antilles or if they were killed off by them is still a matter of debate. But stay tuned, we may have more on this issues very soon!



References

Acevedo Gonzalez, Manuel, Oscar Arredondo y Noel Gonzalez Gotera. 1972. Cueva del Tunel. Editorial Pueblo y Educacion.

Arredondo Antunez, Carlos. 2011. Los peresozos extinctos. 29-37 pp. En Mamiferos en Cuba (Eds. R. Borroto-Paez y C. A. Mancina). UPC Print, Vaasa.

Hernandez-Pacheco, Eduardo. 1947. Paleontologia, 3rd ed. Int.Gallar, Spain.

MacPhee, R. D. E., M. Iturralde-Vienent, and Osvaldo Jimenez Vazquez. 2007. Prehistoric sloth extinctions in Cuba: Implications of a new "Last Appearance Date". Caribbean Journal of Science, 43: 94-98. 

Matthew, William D. 1931. Genera and new species of ground sloths from the Pleistocene of Cuba. American Museum Novitates, 511.

Matthew, William D. and Carlos de Paula Couto. 1959. The Cuban Edentates. Bulletin of the American Museum of Natural History, 117.

Paula Couto, Carlos. 1967. Pleistocene Edentates of the West Indies. American Museum Novitates, 2304.

Silva Taboada, Gilberto, William Suarez Duque, and Stephen Diaz Franco. 2007. Comependio de los Mamiferos Terrestres Autoctonos de Cuba Vivientes y Extinguidos. Edicion Bologna, La Habana.

Torre, Carlos de la. 1910. Excursion a la Sierra de Jatibonico: Osamentas fosiles de Megalocnus rodens o Mymoprhus cubensis. Sesion del 10 de Junio de 1910. Imprenta Militar, La Habana.

Torre, Carlos de la., and W. D. Matthew. 1915. Megalocnus and other Cuban ground-sloths. (Abstract). Bulletin of the Geological Society of America, vol. 26, p. 152.

White, Jennifer L. and Ross D. E. MacPhee. 2001. The sloths of the West Indies: A systematic and phylogenetic review: 201-235pp". Chapter 14 in Charles E. Wood and Florence E. Sergile (Eds) Biogeography of the West Indies: Patterns and Perspectives. CRC Press, Boca Raton.