Originally posted at Bizarre Zoology on March 27, 2016
Phylogeny, which can defined as the study of the evolutionary history and relationships amongst the animal kingdom, is one of the branches of zoological research which most captivates me with its complex and often unexpected nature. In the earliest article published on the Blogger-based Bizarre Zoology blog, I made brief reference to an obscure phylogenetic hypothesis which I first caught word of in paleozoologist Dr. Darren Naish’s Tetrapod Zoology Book One. This was done with the intention of introducing the sorts of topics which would form the focal point of my writing, yet I did so quite poorly in that I neglected to elaborate beyond a few sentences. To kick things off after this blog’s relocation, I have chosen to return to this topic which has been given scant attention in scientific literature but just enough to grab hold of my taste for all that is bizarre and zoological. This is the fascinating albeit tenuous link proposed between Dinocerata and Lagomorpha, something which may not sound so interesting unless you are familiar with the animals grouped within these taxonomic orders.
Lagomorpha and Its Diversity Dilemma
Lagomorpha is the mammalian clade which includes the rabbits, hares (family Leporidae), and pikas (family Ochotonidae), of which there are about eighty recognized extant species (Myers & Sorin 2002). Leporids are opportunistic herbivores which occupy a variety of habitats across the globe, with some species even being adept swimmers (Myers & Sorin 2002). Most leporids are solitary and have evolved significant anti-predator strategies such as a subdued pelt coloration, burrowing behavior, and musculature adapted for prolonged running (Myers & Sorin 2002). Ochotonids inhabit the mountainsides or steppes of cold regions and are actively aggressive defenders of their individual territories (Myers & Sorin 2002). These mammals possess vocalizations with geographic dialects so distinctive in sound and function that they have been considered a potential tool in the taxonomic differentiation of pika subspecies (Somers 1973). The earliest fossil representatives of Lagomorpha date to more than fifty million years ago, being found in the Early Eocene strata of west-central India and the Middle Eocene strata of central Asia (the Eocene epoch lasted from 56 to 33.9 million years ago) (Rose et al. 2008). These animals were tiny and had not yet developed a hopping locomotion, as is seen in the the taxon Paleolagus which was a mere ten centimeters in length and possessed the short hind legs of a scamperer (Kindersley 2009). The diversity of ancient lagomorphs has been described as being much richer than that present in modern day ecosystems, with 75 genera and more than 230 species having been described from the fossil record (Alves et al. 2008). A study presented at the annual meeting of the Society of Vertebrate Paleontology argued that the paradoxical scarcity of extant lagomorph taxa can be attributed to competitive interactions with contemporary ungulates (hoofed mammals), intensified by climactic change (Tomiya & Miller 2014). The analysis found that the lagomorphs of the Oligocene epoch (33.9 million to 23 million years ago) coexisted with diminutive duiker-like ungulates until climate change prompted transitions in biomes during the early Miocene (23 to 5.3 million years ago), allowing for a radiation of larger grasslands-dwelling hoofed mammals equipped with advanced grazing dentition (Tomiya & Miller 2014). For those familiar with the Speculative Zoology Movement and the celebrated book After Man: A Zoology of the Future, it is worth mentioning that author Dougal Dixon (1981) imagined that the future descendants of rabbits dubbed ‘rabbucks’ would occupy the grazer niches left behind by the ungulates. Such speculation could perhaps be taken as early commentary on ecological rivalry between the two mammalian groups.
Lagomorphs Without Those Darned Ungulates
The giant Neogene rabbit of Minorca named Nuralagus rex was a testament to the evolutionary capabilities of Lagomorpha in the absence of ungulates. The gigantism exhibited by Nuralagus was likely the product of evolution in an insular environment lacking substantial predators or competitors and thus allowing the luxury of resulting energy savings (Quintana et al. 2011). This animal was probably slow-moving with low sensory abilities, as is indicated by its small ears and eyes, low degree of flexibility in the vertebral column, flat neurocranium correlating with reduced brain size, decreased pulmonary ventilation capabilities, and lack of aerobic adaptations for running or leaping (Quintana et al.2011). Nuralagus was also the largest member of Lagomorpha ever known, with a body mass twice as large as that of the biggest modern day Flemish rabbits and ten times as large as that of the extant European rabbit (Quintana et al. 2011). The presence of hypercarnivorous lagomorphs in the fossil record has yet to be established, although teams working in the deposits of the Cave of Caerbannog feel that discovery is inevitable (Python 1975…*humor*). If you thought that the lagomorphs are mostly uninteresting with the exception of aberrant species like Nuralagus, think again. The odd anatomy, morphology, physiology, and behavior of modern day rabbits has led Darren Naish (2010) to describe them as “the most freaky of all mammals” (see here for more on this). Thus, it is well arguable that the members of Lagomorpha deserve to stop being regarded as the commonplace small mammals which destroy your garden and receive more appreciation for being the fascinating animals they are. But, enough on lagomorphs for now…
Dissecting the Life of a Dinoceratan
Dinocerata is an extinct clade of Mammalia whose members were first unearthed from fossil beds in the Uinta Basin of Utah dating to the Eocene epoch (Prothero & Schoch 2002). The members of Dinocerata most recognizable to the common zoological enthusiast are those belonging to the family Uintatheriidae. The uintatheres were mammals possessing great body mass and tiny brains, characterized by their displaying six blunt horns on the head and large protruding canine teeth shielded by flanges in the lower jaw. These herbivores filled a niche as the largest land mammals in Asia and North America until the late Eocene when their role was superseded by a diversity of big ungulates like the brontotheres (Prothero & Schoch 2002). Some researchers such as William Turnbull (2002) have proposed that the uintatheres exploited an amphibious lifestyle analogous to that of modern hippos due to traits such as their stocky limb proportions, barrel-shaped rib cage, increased bone density (pachyostosis), structurally simple cheek teeth, and pelvic adaptations for hindgut food fermentation. Turnbull (2002) made significant note of the latter two features in comparison with sirenian anatomy, concluding that the uintatheres and modern sea cows share similar digestive systems with reduced dependence on their dentition.
Interestingly, the canine teeth of the dinoceratans coupled with their lack of upper incisors are also comparable to the condition seen in the dentition of muntjac deer (Wheeler 1961). The enlargement of canine teeth as weapons is typically found in mammals which practice the territorial defense of material resources (Geist 1998), so could one make the behavioral inference that uintatheres may have also exhibited such aggressive social behavior? If so, it is likely that their blunt canines served more of a purpose for sexual dimorphism and the intimidation of rivals rather than use as actual weapons. Instead, uintatheres likely made use of another striking anatomical feature. Dr. Valerius Geist (1998) notes that modern large-bodied mammals acting as resource defenders (e.g. okapis) practice the tactic of parrying, or using the head to counterblow an attack. This combat strategy creates the opportunity of selection for bump-like protrusions on the top of the head, transforming it into a sort of club which is used to damage the facial tissues of an aggressor (Geist 1998). If the uintatheres did indeed act as resource defenders, then the multitude of horns adorning the skull of these animals likely served this function. Geist (1998) notes that the use of the head as a club would have also prompted their evolution of the flanges on the lower jaw, which acted to prevent the dagger-like canines from snapping off. Inferences from the behavior of modern day resource defenders bearing similar combat teeth thus create a tantalizing hint at dinoceratan behavior, although one which is largely speculative considering the lack of data shedding direct light on their habits.
But…Why Doesn’t Anyone Talk About Gobiatherium?
The uintatheres diversified into a variety of forms, from the 2.1 meter tall Eobasileus to the perplexing Gobiatherium of middle-Eocene Mongolia. As noted by Darren Naish (2009), Gobiatherium has an oddly minimal presence on the Internet despite its being a true paleontological puzzle, so here’s a chance to help fill that gap. This Mongolian mammal was one of the latest-living members of the dinoceratan lineage and is often placed in its own ‘subfamily’ as a result of its anatomical peculiarity. Although it lacks the classic knobby horns and flange-gaurded tusks of the American uintatheres, the skull of Gobiatherium is quite striking in its own respect. The cranium of this uintathere is long and narrow with broadly flaring zygomatic arches (cheekbones) and a highly arched nasal region which is bridged to the anterior end of the remaining premaxilla by bony septum (see the diagram below) (Spencer 2001). The prominent cheekbones of Gobiatherium have been described as closely resembling those of the entelodonts (Osborn & Granger 1932), the so-called ‘terminator pigs’ whose evolutionary affiliations actually lie closer to a clade consisting of whales, hippos, and the giant hoofed carnivore Andrewsarchus. Although the known cranial material attributed to Gobiatherium is considered well-preserved, the enlarged nasal region remains a structure of ambiguous function. Osborn & Granger (1932) note that paired bony lumps or small horns are present on the nasal arch of one Gobiatherium specimen, suggesting a potential link to sexual differentiation. Prothero & Schoch (2002) suggest that the nasal structure may have been functionally similar to the proboscis of the saiga antelope which acts to warm inhaled air. However, the authors quite reasonably note the erroneous nature of such a comparison, as this adaptation would have no advantage in the humid climate present during the Eocene epoch. Another problem, as pointed out by biologist Cameron McCormick in correspondence with me, is that the bulbous nose of the saiga antelope is correlated with a lack of bone rather than an enlargement of it. McCormick and others (see the comments section of Naish 2009) have noted a similarity to the nasal aperture of the Pleistocene diprotodontids such as Zygomaturus, which did have a raised nasal region topped by two roughened lumps, but that still doesn’t do much for explaining the dramatic structure’s purpose. Paleoartist Tim Morris has suggested to me that the nasal cavity acted as a sort of resonating chamber for calls, a logical explanation but one which has not yet been subjected to any formal analysis. Thus, the function of the giant nose of Gobiatherium remains a paleozoological enigma. I would argue that sexual display is a strong possibility considering Osborn & Granger’s aforementioned observations regarding the paired bony lumps, and the fact that ridiculous and exaggerated structures in large animals often serve this purpose. What is really needed is a reexamination of the pertinent fossil material under the direction of functional anatomy…if only my scientific training and entering into academia could happen sooner.
While it may seem that I have strayed from the main point of this article by providing such background, the usual perception of lagomorphs as commonplace small mammals and dinoceratans as obscure fossil taxa has led to their receiving very little prior attention in the zoological blogosphere. Being that I have made it my goal to raise awareness and talk about the lesser known bizarre side of the animal kingdom, I have felt obliged to at least briefly write about them. So, returning to the main point, what is the supposed relationship between these two seemingly evolutionary disparate groups?
Despite the relatively well documented nature of their fossil records, both Dinocerata and Lagomorpha have posed an identity crisis for interested zoologists. Part of this issue certainly arises from these animals being misunderstood and rarely studied, but also from their overall morphological peculiarity. As was mentioned before, paleozoologist Darren Naish has described lagomorphs as “the most freaky of all mammals”, and many people are surprised to learn that these animals are indeed not classified as rodents. Paleontologist Dr. James Gidley, the first researcher to formally distinguish the lagomorphs as separate from Rodentia, noted that some lagomorphan characteristics such as their lateral chewing style, spacing and width of dentition, limb and foot anatomy, and dorso-lumbar vertebrae structure are similar to those of cetartiodactylan ungulates (Gidley 1912). In earlier writings, Gidley (1906) proposed that Lagomorpha owes its origins to the clade Eutriconodonta which existed during the Jurassic and Cretaceous periods. That’s right, while there never was a Precambrian rabbit, there may still have been Mesozoic ones…well, not really. Gidley (1906) attempted to establish this relationship through a diagram depicting the derivation of the teeth of Paleolagus from the eutriconodontid Triconodon through a hypothetical intermediate form. However, comparative tooth morphology has since proven unreliable for phylogenetic purposes (Sansom et al. 2014; see related discussion in Naish & Conway 2013 at time 6:40). Perhaps most interesting to the zoologically-inclined Homo sapiens is the protein sequence-based conclusion of Graur et al. (1996): that Lagomorpha is more closely allied to fellow primates such as treeshrews than to the members of Rodentia. The most recent and best supported analyses evaluating data from morphology, paleontology, and mitochondrial and nuclear DNA sequences have placed the lagomorphs within a clade known as Glires (Douzery & Huchon 2004). Ironically, considering the previous emphasis which zoologists have placed on the distinction between Lagomorpha and Rodentia, the two are grouped within this clade (Douzery & Huchon 2004).
The phylogenetic classification of the Dinocerata has been a matter of continued controversy, with a few researchers having taken some shots in the dark at the closest relatives of this lineage. The uintatheres were originally interpreted by Edward Drinker Cope as ‘antlered elephants’, and were described by William D. Gunning as “the ruins of an animal composed of Elephant, Rhinoceros, Ruminant, Marsupial, and a something unknown to the world of the living” (Shuker 2015). More recent and better established studies have placed Dinocerata amongst the ungulates due to specialized anatomical similarities, but the evidence for this evolutionary affiliation has been described as slim by Prothero & Schoch (2002).
As can be inferred from some of these rather eccentric notions, attempts to determine these group’s evolutionary relationships through comparative anatomy have fallen short. The insight offered by molecular studies has shed crucial light on the phylogeny of Lagomorpha, but the Eocene age of dinoceratan fossils prevents such enlightening analyses. Recent systematics work has formed a solid hypothesis for the phylogenetic placement of the uintatheres and kin, but before I discuss this, one more peculiar hypothesis demands to be explored as the ultimate purpose of this article.
As mentioned previously, Tetrapod Zoology: Book One was a volume which largely helped to form my insatiable interest in the anatomy, behavior, ecology, conservation, and evolutionary history of animals. Among the particularly interesting extracts I held onto after reading the book multiple times, one stood out in my mind. In the work’s chapter on the peculiarity of rabbits, Naish (2010) mentions in brief the connection made by some paleozoologists between the lagomorphs and dinoceratans. Remarkably, considering the seemingly stark differences between these two groups of mammals, a few other authors throughout the zoological literature have commented on discernible similarities, with some even advocating a phylogenetic relationship. In their work on the taxonomic revision, phylogeny, and classification of Dinocerata, paleozoologists Dr. Tong Yongsheng, Dr. Spencer Lucas, and Dr. Robert Schoch proposed that the clade Uintatheriamorpha shares a close common ancestry with the anagalids (Lucas & Schoch 1998). The anagalids were burrowing rabbit-like mammals from the Paleocene of China, grouped within the clade Glires alongside modern lagomorphs and rodents (Prothero & Schoch 2002). Lucas & Schoch (1998) specifically cite the anagalid Pseudictops as a taxon sharing key features with the dinoceratans, and make the startling assertion that, “It is tantamount to identifying uintatheres as giant horned bunnies if Lagomorpha and Pseudictops are closely related”. Despite the clear contrast between the giant uintatheres and lagomorph-like anagalids, such a hypothesis is sound considering an Asian origin for Dinocerata (Lucas & Schoch 1998). Just let the idea of dinoceratans as ‘giant horned bunnies’ in a phylogenetic sense sink in for a moment…notions like these are part of the reason why I love zoology and phylogeny.
Unfortunately for those like myself, there are issues with the hypothesis in regard to the data bolstering it. Prothero & Schoch (2002) note that the proposed Lagomorpa-Dinocerata relationship rests largely upon morphological similarities in dentition. Dinoceratan teeth are unusually small considering their body size, and they exhibit a V-shaped crest pattern similar to that of the anagalids (Prothero & Schoch 2002). In his keystone paper discussing the evolutionary affiliations of rabbits, fossil rodent specialist Albert Wood described marked tooth pattern similarities between Uintatherium and Paleolagus, but rightfully described these correlations as indicating a faint relationship at best (Wood 1957). Further casting a poor light on the ‘giant horned bunny’ hypothesis, these features are also found in numerous other primitive mammals (Prothero & Schoch 2002) and, as mentioned before, dental characters have been declared unreliable for phylogenetic analyses in recent studies (Sansom et al.2014). Thus, the hypothesis of dinoceratans as being affiliated with the lagomorphs was a spectacular idea, but one which is able to be rejected after further scrutiny.
Despite the attention given by some paleozoologists to the rather romantic notion of the dinoceratans as ‘giant horned bunnies’ affiliated with the gliriforms, the issue of utilizing dental characteristics in phylogenetic analyses proves to be its greatest fault. It seemed that, without access to genetic material, the precise evolutionary affiliations of the dinoceratans would remain unclear. However, recent systematics work revealed at the 75th Annual Meeting of the Society of Vertebrate Paleontology has made a strong case for their placement amongst Euungulata. Paleontologist Dr. Benjamin Burger presented a paper titled, “The systematic position of the Saber-toothed and Horned giants of the Eocene: The Uintatheres (Order Dinocerata)” in which he conducted a cladistic analysis through the large character matrix of mammals published in MorphoBank. Burger’s paper is not yet available for reference to my knowledge but, thankfully, he has uploaded a video to YouTube which details his work and conclusions. Rather than elaborate, I have linked to it below for the benefit of readers.
So it seems that the dinoceratans don’t have a particularly close relationship with Lagomorpha after all, but are actually relatives of the perissodactyls and South American xenungulates like Carodnia. Even with the conclusions of the latest research, I continue to find myself in the unintelligent position of wanting to look at every rabbit as a kin of the uintatheres. A zoological nerd can dream, can’t he? Regardless, have no fear, for plenty of other incredible examples of phylogenetic relationships exist between what seem to be distant zoological groups. After all, whales are giant marine hoofed mammals closely related to hippos, and humans are ‘naked apes’ which share a common ancestor with ‘giant African monkeys‘. Excellent reviews of other phylogenetically-problematic animals can be found in the articles here and here, which I was tipped off to by a recent episode of the podcast Incidents and Reflections where the newly determined identity of Tullimonstrum as a stem-lamprey (a truly unexpected finding I couldn’t resist mentioning) was discussed. Despite the attitude of some people who regard the animal kingdom as being static, boring, and irrelevant to ‘the human world’, Zoology has shown it to be a plethora of diverse and truly bizarre species, anatomies, evolutionary histories, and behaviors. If you somehow remain of the former opinion, or simply found this article to be of interest, please stop back for more Bizarre Zoology.
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