Revealing the true identity of Zenkerella insignis, one of Africa’s most mysterious mammals

PictureThe enigmatic Cameroon flightless
anomalure, Zenkerella insignis.
My fascination with Africa’s endemic anomaluroid rodents — also known as "anomalures", or by the misnomer “scaly-tailed squirrels” (they do have scaly tails, but are not closely related to squirrels) — began about 15 years ago, when my colleagues and I first started to discover fossil bones and teeth of 37 million-year-old anomaluroids at what was then a newly-discovered site, “Birket Qarun Locality 2” (BQ-2), in the Fayum area of northern Egypt. At the time I couldn’t claim to have any expertise on anomalures, or even rodents generally, but I knew just enough about the fossil record of rodent evolution to be intrigued by the new specimens. For some reason the BQ-2 rodent fauna was radically different from those documented at younger fossil sites in the Fayum area — many of which were exceptionally rich, but none of which had ever yielded fossil anomaluroids. Instead, those younger sites are dominated by early “hystricognaths”, a large rodent group that ultimately gave rise to familiar species such as guinea pigs, porcupines, and mole-rats (1,2). Clearly, something had happened in northern Africa toward the end of the Eocene epoch (around 34 million years ago), that led to the successful diversification of hystricognaths and the local extinction of anomalures, the latter of which are still alive today in the equatorial forests of West and Central Africa, including those on Bioko, an island off the coast of Cameroon that is part of Equatorial Guinea. 

PictureHesham Sallam at BQ-2.
Fortunately it wasn’t long after we had found the new anomalure fossils that an Egyptian student, Hesham Sallam (image to the right, now Director of the Mansoura University Vertebrate Paleontology Center in Egypt, and a postdoctoral researcher at Duke University), applied to work with me on fossil mammals from the Fayum area. Hesham enthusiastically agreed to analyze the rodents from BQ-2 (and other sites) for his DPhil thesis at University of Oxford, and over the next several years, we worked together to describe the teeth and cranial parts of the two BQ-2 anomalures — a large species that we named Kabirmys (Arabic kabir = large, Greek mys = mouse, roughly translating to “big mouse” or “mighty mouse”) and a smaller species, Shazurus, whose teeth more closely resembled those of fossil anomaluroids known from early Miocene (~20 million-year-old) sites in Kenya and Uganda (3, 4). The teeth of Kabirmys and Shazurus were quite different, suggesting that they were probably derived from a very ancient common ancestor.

PictureAnomalurus pusillus
While the teeth of Kabirmys and Shazurus provided some good clues that helped us to figure out how these ancient species were related to other living and extinct anomaluroids, in some ways the most fascinating anomaluroid fossils that had been found at BQ-2 were the bones from these species' arms, hands, legs, and feet. This is because two of the living anomalure genera (the relatively large-bodied Anomalurus, see image to the left, and the tiny Idiurus) have anatomical adaptations that allow them to glide between trees, including a “patagium” (or gliding membrane), and we hoped that the fossil bones from BQ-2 might allow us to determine whether this remarkable mode of locomotion had already appeared by 37 million years ago. Among other things, if we could infer that the BQ-2 anomalures were gliders, they would be the only other mammals (aside from primates and possibly bats) that could be definitively tied to an arboreal habitat in this ancient community; indeed, they could have been hanging out in the trees right alongside our ancient monkey-like ancestors.

As research proceeded it became obvious, however, that the limb bones of Kabirmys were quite different from those of Anomalurus and Idiurus, suggesting that it probably was not a glider. But when we went to make comparisons with the third living anomalure, Zenkerella insignis, we ran into a major problem: despite having been discovered over a century ago, the species was still largely a mystery to scientists. Zenkerella was known to lack a patagium, and so was clearly not capable of gliding, but it had never been seen alive by trained mammalogists, and there was no information in the scientific literature about how it moved, what it ate, or when and where it spent its waking hours. Only eleven specimens were known in museum collections worldwide, and none of those specimens had been cleaned to reveal what the bones of the arms, legs, pelvis, and spinal column looked like. Awkwardly, we suddenly knew more about the postcranial anatomy of 37 million-year-old Kabirmys than we knew about a living species! To make matters worse, Zenkerella's DNA had never been sequenced, and so nobody knew for sure how it was related to the two gliding anomalurids. Some researchers had argued on the basis of skull and tooth anatomy that Zenkerella was most closely related to Idiurus, but this implied that either Zenkerella had lost its patagium (and its ability to glide), or that Anomalurus and Idiurus had evolved patagia and gliding behavior independently (see image below). Both of these evolutionary scenarios frankly seemed unlikely to us, but without DNA or postcranial skeletons of Zenkerella, we were stuck -- we could describe the postcranial bones from BQ-2, but there would be no solid evolutionary context in which to place them.
    
Picture
Possible relationships among anomalures, and scenarios for the gain or loss of gliding adaptations. Modified from Coster et al. (5).
PictureDavid Fernández (left) of The
University of the West of England
Photo: G. McCabe
Our first major breakthrough came when I was attending the doctoral dissertation defense of Spanish primatologist and conservationist David Fernández, at Stony Brook University in New York in 2013 (David is now at the University of the West of England in Bristol). I was surprised to hear that David would soon be moving to Bioko (one of the areas where Zenkerella occurs) to become the co-Director of the Moka Wildlife Center, as part of the Bioko Biodiversity Protection Program (BBPP) that is run by Drexel University and the National University of Equatorial Guinea. We talked briefly about Zenkerella and the need to obtain new specimens and DNA sequences, and fortunately David was very excited to search for the species on Bioko, and to interview locals to find out more about the Zenkerella's distribution and habits. Not long after arriving on Bioko, David was able to confirm, after talking with village elders and soldiers, that Zenkerella had recently been trapped in ground snares near a village called Ureca, but that the bodies had been discarded because they are not valued for their meat. He was even told that in the local Bubi language some call Zenkerella “musuló”, which means “inferior to all squirrels”. The few people who were familiar with the species indicated that Zenkerella is nocturnal. David asked the local trappers in Ureca to hold on to any specimens that they found in their ground snares. And we waited.

PictureFernández with the first whole
-body specimen of Zenkerella
Photo: G. McCabe
For eight months, there was no news. But finally, only a few days before he was scheduled to leave Bioko for good, David went back to Ureca and was told by the chief of the village that a specimen had been found in one of their traps. He sent me images of the specimen and I could barely contain my excitement — indeed, there it was (image to the left), the first whole-body specimen known for the entire species whose preservation would finally allow us to easily study its DNA, postcranial bones, muscles, brain, gastrointestinal system, etc. Several months later, David wrote again with even better news — two more specimens had been found (including the specimen figured below), giving us a total of three whole-body specimens! Finally, as of just a few weeks ago, David has confirmed that we have yet another two individuals, male and female. All have been found in ground snares during the rainy season. With five specimens, including male and female individuals, we will be able to study intraspecific variation and sex differences as well. The individual figured below is a male.

Picture
The second male specimen of Zenkerella insignis, from the near the village of Ureca on Bioko. From Heritage et al (6).
PictureSteven Heritage, Stony Brook
University Ph.D. student.
Enter Steven Heritage (image to the right), my Ph.D. student at Stony Brook University who is studying the phylogenetic relationships of another fascinating and obscure group of small African mammals — macroscelideans (also known as elephant-shrews or “sengis”) — for his dissertation, using both DNA and morphology. Fortunately Steven was happy to lend his expertise and take on the task of sequencing the new specimens' DNA and comparing it to that of other rodents, so that Zenkerella could finally be placed securely in the anomaluroid family tree. After several months of hard work, Steven successfully sequenced five genes (three from the mitochondrial genome, and two from the nuclear genome) from two of the Zenkerella individuals, pulled hundreds of other homologous rodent DNA sequences off of GenBank (a public repository for DNA sequences), aligned all of those sequences, and ran multiple analyses to determine the placement of Zenkerella among a large sample of 66 other rodent species. 

PicturePhylogenetic analysis of mitochondrial and
nuclear gene sequences showing the
placement of Zenkerella in anomaluroid phylogeny.
The results were as surprising as they were decisive (image to the left). As detailed in the study of Heritage et al. (6), which was published today in the open access journal PeerJ, Steven’s analyses showed that Zenkerella was unequivocally not closely related to Idiurus as had previously been suggested, but it also wasn’t closely related to the other anomalurid glider (Anomalurus). Instead, Anomalurus and Idiurus were found to be “sister taxa” — that is, they were more closely related to each other than to Zenkerella. This grouping of Anomalurus and Idiurus had perfect statistical support, and sat at the end of a long common branch, suggesting an extended period of common ancestry to the exclusion of Zenkerella. This result has profound implications for understanding anomaluroid evolution and the origin of gliding adaptations: given these relationships, it is highly probable that the last common ancestor of Anomalurus and Idiurus was a glider, and since Zenkerella diverged from the Anomalurus-Idiurus lineage long before gliding evolved along their “stem lineage”, there is no reason to believe that there were any reversals back to the primitive condition as some had envisioned for Zenkerella — that is, gliding probably had an ancient single origin within anomalures, and (so far as we know based on the fossil record) it was never lost once it appeared. Interestingly, these results aligned perfectly with the relationships that had been proposed for anomaluroids well over a century ago by a Swedish zoologist named Tycho Tullberg, in his seminal study of rodents published in 1899 entitled “Ueber das System der Nagethiere: eine Phylogenetische Studie” (7). Tullberg’s hypothesis was, of course, based solely on anatomy.

PictureIsolated teeth of Prozenkerella saharaensis, a
30 million-year-old species from Libya.
From Coster et al. (5).
We still didn’t know what the time-scale of these evolutionary changes might have been, but available evidence from the fossil record gave us good reason to suspect that Zenkerella was an exceedingly ancient branch in the anomalure family tree. It had been known since 1973 that fossils attributed to the genus Zenkerella were present at ~20 million-year-old sites in East Africa (8), and, remarkably, a new piece of evidence emerged mid-way through our project — late in 2015, a team of paleontologists led by the French scientist Pauline Coster announced the discovery of an ancient extinct relative of Zenkerella in ~31 million-year-old rocks in central Libya (5). The fossil species was only known from isolated teeth (see image to the right), but they were so similar to those of Zenkerella that Coster and colleagues named the new species Prozenkerella (“before Zenkerella”) saharaensis. When we analyzed the anatomical data from these and other living and extinct anomalurid species alongside the DNA evidence, using a relatively new Bayesian phylogenetic approach known as “tip-dating”, which takes into account rates of evolution to provide estimates for divergences between living and extinct species, we found that Zenkerella was estimated to have diverged from other anomalures about 49 million years ago. Our results further suggested that Anomalurus and Idiurus last shared a common ancestor near the close of the Paleogene, about 26 million years ago.

To put such ancient divergences into context — we last shared a common ancestor with chimpanzees about 6-to-8 million years ago, and all living anthropoids (including New World monkeys, Old World monkeys, apes, and humans) last shared a common ancestor between 35 and 45 million years ago (image below). If the Zenkerella lineage diverged 49 million years ago, then it was likely distinct and evolving independently in Africa long before that anthropoid common ancestor even existed, and maybe even before anthropoids had appeared in Africa! If we look to other primate groups, the ancient divergence and persistence of Zenkerella is somewhat analogous to the case of the strange aye-aye (Daubentonia) of Madagascar that probably also diverged from its lemur relatives around the same time, early in the Eocene, and is now represented by only one species. And, if the teeth of fossil relatives of Zenkerella are a reliable guide, by ~31 million years ago Prozenkerella had already acquired the feeding strategies of modern-day Zenkerella; the teeth of the living form and the ~31 million-year-old form only differ in slight details. Available genetic and fossil evidence seem to be converging on the conclusion that Zenkerella is a “living fossil” — that is, a species that differs little in its anatomy from its ancient fossil relatives, and that, in this particular case, may well have persisted in its current niche for well over 31 million years.

Picture
A comparison of major divergences within the family trees of primates (left) and anomaluroid rodents (right).
The estimated 26 million-year-old divergence between Anomalurus and Idiurus is also surprisingly ancient (image above), roughly equal in age to the time period when Old World monkeys diverged from apes, in the late Oligocene. Our tip-dating analyses suggest that the extinct early Miocene glider Paranomalurus might have diverged from the lineage leading to Anomalurus and Idiurus about 30 million years ago; if this is the case, then gliding must have appeared at some point between 49 and 30 million years ago in Africa. We speculated that such adaptations might have evolved in response to the dramatic environmental changes that occurred world-wide around 34 million years ago, at the beginning of the early Oligocene, when Antarctic glaciation accelerated and environments in the northern latitudes became much more seasonal. This was also when anomaluroids stopped showing up in the fossil-bearing rocks of northern Egypt, suggesting that their latitudinal distribution had been constricted. This hypothesis can be tested through future paleontological work in the Eocene and Oligocene of Africa. 
  
Such ancient divergences were difficult to reconcile with the prevailing taxonomy, which placed all anomalures in the single family Anomaluridae. We proposed that the gliding forms (Anomalurus and Idiurus) be retained in the family Anomaluridae, and that Zenkerella be placed in its own family, Zenkerellidae. Together, the living anomalures were placed in a higher-level group, Anomaluroidea, that would also include at least one extinct family known from the Miocene, Nonanomaluridae, if not also the older and phylogenetically basal Nementchamyidae — the family to which Kabirmys belongs.

PictureLandscape around Ureca on Bioko (photo by David Fernández).
Despite the important progress that we have made in figuring out Zenkerella's position in anomaluroid phylogeny, we still have much to learn about this genus. Research is currently underway on the anatomy of this fascinating rodent, and is revealing some interesting primate-like features that presumably reflect a largely arboreal lifestyle. Yet we know that Zenkerella comes to the ground, at least occasionally, because it has only been found in ground traps. David Fernández recently returned to Bioko, and, in collaboration with our co-authors Drew Cronin (Drexel University and BBPP), and José Manuel Esara Echube (National University of Equatorial Guinea and BBPP), will soon be making important headway on new studies of Zenkerella’s distribution, with the goal of ultimately studying the species’ behavior, locomotion, and feeding ecology. With any luck, in the coming years there will be lots of interesting new information to report about this remarkable "living fossil" that has eluded scientists for well over a century.

References (with links to the original papers, if available):

(1) Wood, A. E. 1968. Early Cenozoic mammalian faunas, Fayum Province, Egypt, Part II: the African Oligocene Rodentia. Peabody Museum Bulletin 28: 23-205.


(2) Sallam, H.M., Seiffert, E.R. 2016. New phiomorph rodents from the latest Eocene of Egypt, and the impact of Bayesian “clock”-based phylogenetic methods on estimates of basal hystricognath relationships and biochronology. PeerJ 4:e1717.

(3) Sallam, H.M., Seiffert, E.R., Simons, E.L. 2010. A highly derived anomalurid rodent (Mammalia) from the earliest late Eocene of Egypt. Palaeontology 53:803-813.

(4) Sallam, H.M., Seiffert, E.R., Simons, E.L., Brindley, C. 2010. A large-bodied anomaluroid rodent from the earliest late Eocene of Egypt: Phylogenetic and biogeographic implications. Journal of Vertebrate Paleontology 30:1579-1593.

(5) Coster, P.M.C., Beard, K.C., Salem, M.J., Chaimanee, Y., Jaeger, J-J. 2015. New fossils from the Paleogene of central Libya illuminate the evolutionary history of endemic African anomaluroid rodents. Frontiers in Earth Science 3:56.

(6) Heritage, S., Fernández, D., Sallam, H.M., Cronin, D.T., Esara Echube, J.M., Seiffert, E.R. (2016) Ancient phylogenetic divergence of the enigmatic African rodent Zenkerella and the origin of anomalurid gliding. PeerJ 4:e2320.

(7) Tullberg, T. (1899) Ueber das System der Nagethiere: eine Phylogenetische Studie. Uppsala: Akademische Buchdruckerei.

(8) Lavocat, R. 1973. Les Rongeurs du Miocène d’Afrique Orientale, Miocène inférieur. Mémoires et Travaux de l’Institut de Montpellier de l’Ecole Pratique des Hautes Etudes 1:1-284.