After the massive extinction that wiped out the dinosaurs (known to modern science as the K-Pg mass extinction) the world was brimming with opportunities for the survivors. The group that ultimately took these opportunities and diversified into these new niches were the placental mammals, which had stayed relatively small while the dinosaurs dominated, but with them gone there were endless new lifestyles to explore. One such opportunity was the role of large herbivores, which was a niche the dinosaurs had previously occupied. In the early ages of the cenozoic many mammals experimented with herbivory, in fact a number of our modern large herbivores had their beginnings at this time. But these forebears to our modern horses and deer weren’t the largest plant eaters in the landscape, in fact they were relatively small. One of the first groups to become megaherbivores was a strange group with no living descendants; the dinoceratans. These first big herbivores were an odd and somewhat mysterious group, with many having large horns and tusks sticking out of their faces, and their early evolution is not completely understood. As they have have no descendants it can seem like a baffling task to try and reconstruct their evolutionary history and life appearance, but through the usage of a number of scientific techniques we can start to develop a better picture of these odd mammals.
Evolutionary History: What and where did these things come from?
As discussed above, the evolutionary history of the dinoceratans is murky at best, with numerous conflicting ideas as to which modern groups they are most closely related to. It has been proposed that they may be most closely related to a primitive group that gave rise to the rodents and lagomorphs, the anagalids, or somewhere at the base of the now polyphyletic ungulatomorpha. Both of these proposals proved to be problematic, and the broad scientific consensus is that they seem to be a sister group to the two general groupings of hoofed mammals today, the perissodactyls (which include rhinos and horses) and the artiodactyls (deer, pigs and cows), a sister clade to only the perissodactyls, or they could possibly be descended from or a sister group to a weird and unique ungulate group from South America, the meridiungulatans (for most of it’s history South America was isolated from the rest of the world and so developed some unique animals, but early in the cenozoic there seems to have been a land bridge present). The case for this interpretation is based off of a medium sized meridiungulatan called Carodnia from the middle paleocene to the earliest eocene of South America, which shares many characteristics that are also seen in the most primitive dinoceratans, such as Prodinoceras and Probathyopsis. We really don’t have a great understanding of where the dinocerata originated, but it would be interesting if they were proven to be meridiungulatans, as that group is currently only known from South America, but the most basal dinoceratans are found in North America and Asia, so either dinoceratans evolved in South America and radiated outwards from there (Thewissen and Gingerich 1987), or basal meridiungulatans moved northwards. Once they evolved however, there seem to have been a number of dispersal events, at least three within dinocerata proper, in which members crossed over newly formed land bridges to new continents. Whether this original continent was North America or Asia has yet to be determined however, as the most basal members within dinocerata (united under the family prodinoceratidae) come from both landmasses.
Relationships within Dinocerata: A Big Jumbled Mess
The relationships within dinocerata itself aren’t any clearer than is their origin, as the last cladistic analysis was performed in 1985 by Schoch and Lucas, with an analysis of the gobiatheriidae in 2001 by Lucas (see cladogram for details). Thankfully, in the publication Evolution of tertiary mammals of North America: volume 1, terrestrial carnivores, ungulates, and ungulatelike mammals includes a section on the dinocerata written by Schoch and Lucas (1999) which provides an overview of the North American taxa within dinocerata, helping to clarify their relationships. They do note, however, that there are areas that require some revision, including relationships within prodinoceratidae (which is largly resolved if the Thewissen and Gingerich 1987 analysis is taken into account for Probathyopsis) as well as Bathyopsis sp. ( which is resolved in Schoch and Lucas 1985). In any case, the cladogram below is the most up to date, complete, and (to my knowledge) only one available. The following is a general overview of the current taxonomy within dinocerata; there are two recognised families, the prodinoceratidae and the unintatheriidae, with the prodinoceratidae being the more basal of the two. This clade contains the two earliest known dinoceratans, Probathyopsis and Prodinoceras, which are distiguished by their earlier appearance( thanetian to ypresian stages), smaller relative size, less specialized dentition with the retention of the upper incisors and the lack of cranial protuberances. P. sinyuensis has been excluded from this cladogram as it is only known from Asia and is likely synonymous with one of the two species of Prodinoceras. It should be noted that P. harrisorum and P. praecursor provide good evidence for anagenesis, as they are found in successive deposits and show a trend towards smaller overall body size (see Thewissen and Gingerich 1987 for details). The status of P. lysitensis is questioned as the remains are fragmentary in nature and could belong to either Probathyopsis or Bathyopsis sp. but in this analysis it has been tentatively assigned to Probathyopsis based on the work of Thewissen and Gingerich (1987) and its location in successive faunal assemblages to P. harrisorum. The second family within dinocerata, the uintatheriidae, contains two subfamilies; the gobiatheriinae and the unitatheriinae. The gobiatheriinae (which some authors recognize as a family, the gobiatheriidae, but due to the lack of any other known genera is considered a subfamily in this analysis) is only represented by one genus with two species from the bartonian and priabonian stages of the late eocene of China, Kazakhstan, and Kyrgyzstan. They are an incredibly specialized group, with an elongated rostrum with a large bulbous nasal opening, as well as a lack of upper canine teeth and cranial protuberances. This extremely odd and specialized anatomy suggests that there is a potential ghost lineage of Asian uintatherids leading to the more derived gobiatheriinae. Hopefully potential transitional species will be found in the future to fill out the lineage and potentially elevate gobiatheriidae to a family ranking. The unitatheriinae is much more speciose and widespread then the known members of gobiatheriinae, with a total of four genera and six species. The members of uintatheriinae are the “classic” uintatheres, with prominent cranial protuberances in the form of up to three pairs covering the upper region of the skull (for more information see functional morphology) as well as large and prominent deep rooted tusks emerging from the maxilla. The basal most member of the uintatheriidae, Bathyopsis, was proposed to contain two species by Wheeler (1961); B. fissidens and B. middleswarti, although the latter species has been omitted here as they are likely synonymous due to the sexually dimorphic nature of the dinocerata and the diagnostic features of the species falling within that range (Schoch and Lucas 1985). Uintatherium sp., the namesake of the subfamily, is a very widespread genus, both geographically and temporally, with U. anceps occurring in the early to middle eocene (lutetian to bartonian stages) of western North America from as far south as south Texas to the coast of California (J. A. Wilson 1986 and S. L. Walsh 1991 respectively). U. insperatus is known from the middle to late eocene (lutetian to priabonian stages) of east Asia, specifically China. The North American uintatheriinae also spawned the largest of the dinocerata in the form of the sister genera Tetheopsis and Eobasileus, which grew to an estimated size of approximately seven feet at the shoulder and around four tons in weight, making them by far the largest dinoceratans known.
If it can be believed, the taxonomy within the dinocerata used to be much more problematic, until the work of Schoch and Lucas (1985) synonymized a number of North American taxa. This overabundance of genera is the work of two infamous paleontologist; Othniel Charles Marsh and Edward Drinker Cope. These two men competed in the early days of paleontology for the right to have named the most number of prehistoric animals from the western United States. Most famous are their dinosaur discoveries, but they also discovered and named a number of dinoceratan taxa, including Eobasileus, Bathyopsis, and Tethyopsis. These are only the remaining genera after a number of cladistic analyses however (Wheeler 1961 and Schoch and Lucas 1985) as Cope and Marsh (as well as the original descriptor of Uintatherium, Joseph Leidy) named a ridiculous number of genera and species; Octotomus Cope (1885), Tinoceras Marsh (1872), Uintamastix Leidy (1872), and Dinoceras Marsh (1872). All these genera were considered distinct, but these descriptions didn’t account for the sexual dimorphism present in all known genera within dinocerata, as well as individual variation. This also happened with the sole remaining prodinoceratan genus in North America, Probathyopsis. Thewissen and Gingerich (1987) synonymized Bathyopsoides and Prouintatherium into Probathyopsis as junior synonyms and only recognizing three species, P. harrisorum, P. praecursor, and possibly P. lysitensis. A similar event also occurred with Asian taxa; at one point there were six or more distinct genera with each having a number of species assigned Tong (1979), Zhai (1978), Flerov (1952). All these taxa have been synonymised into three genera; Uintatherium, Gobiatherium, and Prodinoceras.
Functional Morphology: What were these things doing?
While dinocerata is not an incredibly speciose clade it is worth noting that many genera existed for many millions of years, making them very long lasting as far as large animals go. This shows that the dinocerata were successful at what they were specialized for, but their exact ecological niche as well as many elements of their strange morphology remain mysterious. Through the usage of careful study of existing specimens as well as comparisons to modern animals however, we can attempt to reconstruct their life histories as best as possible. The bizarre skulls of the uintatheriidae may seem like a daunting task to try and comprehend, but by comparing them to extant species we can begin to understand some of these structures in more detail. The members of the prodinoceratidae have the least extravagant skulls of the dinocerata, with only a pair of tusks with their corresponding flanges on the mandible and no upper cranial ornamentation. Their premolars and molars are also less specialized than those of the later uintatheriidae, which show a trend towards higher and more divided tooth cusps, to the point where in the most advanced uintatheriines they are almost bilophodont (Schoch and Lucas 1999) which suggests uintatherids were browsers feeding on woodland vegetation. The prodinoceratidae retained their upper incisors, a feature that was lost both in the gobiatheriinae and the uintatheriinae, suggesting a fleshy mouth pad as seen in modern ungulates that have lost their upper incisors as well as a shift in diet that is supported by other elements of their dental anatomy, such as the relatively small tooth size and lack of visible wear on the surface of dinoceratan molars and premolars (although no microwear analyses have been performed to my knowledge). This suggests that the dinocerata were adapted for the consumption and processing of soft, grit free vegetation, such as freshly sprouted leaves or potentially aquatic plant life (no carbon isotope analyses have been done for this group so consumption of aquatic flora is uncertain). These very limited dietary preferences as well as their presence in eocene forest and shrubland deposits adds credence to the idea that dinoceratans were selective browsers, moving through the landscape and only selecting the freshest leaves and shoot. In extant large selective browsers, such as the black rhinoceros (Diceros bicornis) and the okapi (Okapia johnstoni) this behavior is associated with the presence of large mobile lips, and in some genera a small trunk. The members of dinocerata show no cranial evidence for a trunk, but a pair of thick mobile lips is likely. The tusks of dinoceratans have been interpreted in a number of ways, but some of these theories seem unlikely when all information is taken into account, such as their usage in rooting for subterranean food sources (which would leave much more wear on both the tusks and the rest of the teeth from particles in the soil scraping on enamel) or for the collection of plant matter (their placement in the mouth is problematic for this). Unfortunately there has been little to no work on the composition and wear patterns found in uintatherid tusks. The most likely modern analogues for the purpose of these tusks can be found in the extant family moschidae, or the musk deer, in which males use their characteristic fangs in male on male intraspecific conflict. This is likely how the tusks of the dinocerata were used, as they show evidence of sexual dimorphism with the tusks of males being larger than perceived female (this assignment of gender is based on the general trend in mammals for males to take the larger role in sexual dimorphism) although whether the tusks were used for actual combat or simply display is unknown. The most unusual aspect of dinoceratan (specifically uintatherine) cranial anatomy is the presence of some sort of cranial adornment. This is minimal in Bathyopsis, which only displays small maxillary knobs, and extravagant in the more derived members such as Uintatherium, Eobasileus, and Tetheopsis, which all display prominent maxillary and parietal horns and in some cases nasal bumps. These horns were unlike those found on most modern ungulates in that they were not covered in a layer of keratin; instead they were likely covered in skin like the ossicones of the giraffids (extant examples include giraffes and okapis). Like their tusks, the function of their horns is unknown, but they display the same sexual dimorphism as the tusks, with males having larger and more pronounced knobs. These may have been used simply for display, or they may have been used in intraspecific combat, either locking horns or perhaps a behavior similar to necking in giraffes. If they were indeed used for combat it implies a number of things about uintatheriid social behavior, perhaps suggesting that they would act much like male deer, competing for control of females and territory. The known members of uintatheriidae display the trend towards stout, graviportal limbs, which has been interpreted as evidence for a semi aquatic lifestyle, although the evidence to support this interpretation is somewhat scant, as these robust limbs are often seen in large, heavy animals. The environmental evidence from the time in which the dinocerata thrived suggests an environment that was changing from dense jungle to a more open woodland, and although a cooling trend was beginning (which likely caused this environmental shift) the dinocerata seemed to thrive in this new environment (Prothero 2006). This seems to contradict the idea that the dinocerata were slow, swamp dwelling beasts and suggests a more terrestrial lifestyle, possibly akin to that of large, forest dwelling bovids of today, such as water buffalo. As mentioned earlier, the members of gobiatheriinae display very unique traits found nowhere else in the dinocerata, which suggests a shift in behavior and possibly diet, although the specifics are unknown. It may have been that being one of the last known members of the clade Gobiatherium was faced with more competition from other early herbivorous mammals, such as pantodonts and brontotheres, which forced it to become more specialized as not to directly compete with these groups. The dinocerata may have been at a disadvantage, as they are known to have had one of the smallest brain to body size ratios of any mammal, with a concave skull roof leaving little room for the braincase. This suggests that they were limited in their cognitive ability and may have been restricted to a simple set of behaviors and may have been less flexible to changes in their environment, which would have been problematic as the cooling that opened up their preferred open woodland habitat continued and eventually caused the Asian and North American continents to become much more temperate instead of subtropical as they had been for much of the paleocene and eocene. This had drastic implications for the dinocerata, as their preferred food sources would have gone extinct, and with competition from new herbivorous clades as well as their lack of adaptability would have caused them to slip into extinction by the beginning of the late eocene. With no surviving descendants they slipped into obscurity, but now we are working to recover and understand how these most unusual beasts survived and thrived at the dawn of the cenozoic, and hopefully future discoveries will clarify some aspects of their evolution and life history.
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