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| Morenelaphus, an extinct South-American giant deer. Illustration copyright: Vitor Silva |
PETER MOON
Brazilian Agency for Scientific Reporting
The megafauna of South America formed a formidable bestiary of majestic and amazing animals that lived during the millions of years immediately prior to the arrival of man on that continent. About 20,000 years ago, when the icy glacial cold began to subside, the extinctions accelerated. Fourteen thousand years ago humans were hunting mastodons in South America. Hence, hundreds of species and genera, dozens of families and several orders of mammals were, one by one, disappearing. Thus we arrive at the current ecological panorama of South America, a continent emptied of medium and large mammals.
To date, no consensus has been reached on what would have been the reasons that led the South American megafauna to extinction. In the academy, opinions are divided. On the one hand, there is a group of scholars that points to climate change and climate warming, with their effects on the various biomes, such as possible suspects. Another group identifies a single devastating agent behind mass extinction: the human being. Although sparse, there is evidence to support both hypotheses. Which alternative is correct? Hundreds of megafauna species have been uniformly extinct? Or not? 700 thousand years ago, the deer family reached its greatest diversity in South America. There were 12 genera with dozens of species. Half of them disappeared. Had they succumbed to starvation? Or were they devoured by us?
The largest among extinct South American cervids is the scientific name of Morenelaphus brachyceros. It was a giant deer, a large cousin of the Marsh deer (Blastocerus dichotomus) which, at around 130 kilos, is the largest living deer in South America. Morenelaphus, however, was much larger - and more imposing. One can imagine him as a Pantanal deer with steroids. An adult male was supposed to reach the size of a horse, maybe 400 kilos. In addition, males boasted superb antlers, as can no longer be seen in any living cervid.
In the Pleistocene, there was in Eurasia another giant deer with huge antlers: Megalocerus (also known as "Irish Elk" or "Giant elk"). The animal was two meters shoulder height, weighed an estimated 500 to 600 kilograms - and boasted unbelievable 3.5-meter antlers from end to end. They were complex, like moose antlers. Morenelaphus' antlers, similarly complex, should not be far behind in terms of size and majesty.
In spite of its many predicates, a magnificent animal like Morenelaphus is curiously never remembered when it comes to megafauna. Morenelaphus remains known only to specialists, vertebrate paleontologists such as Leonardo dos Santos Avilla, or Leo (as everyone calls him), the head of the Laboratory of Mastozoology, Department of Zoology, of the Federal University of the State of Rio de Janeiro (UNIRIO), in Rio.
In his laboratory, Léo and his students devote themselves to investigating how the paleobiology of extinct megafauna was - among other studies. The focus of the research are creatures like mastodons and the extinct American horse. More recently, work began on extinct deer. The goal, in this case, is to understand their evolution and determine which are their closest relatives. How did they live? What did they feed on? Why did they disappear?
"The taxonomy of deer is all based on the morphology of the antlers," said Leo, "and the largest antler of all South American deer was Morenelaphus." The study of antlers helps identify the species of deer to which a certain fossil belongs .
But it is useless to study antlers when what is wanted is to know the paleodieta of the beast. Fortunately, among the collections of Morenelaphus fossils in Argentina, Bolivia, Brazil, Paraguay and Uruguay there are many bones, antlers, fragments of skulls - and especially many teeth.
In the case of extinct mammals, the study of the dentition is the best way to try to infer a past diet. The teeth are the hardest and toughest bones of the mammalian skeleton, because in principle they are made to last the whole life of the animal. Although resistant, as we well know the teeth are not indestructible. They break, chip, lose enamel, wear out and carry the indelible marks of the kind of food they chew, cut, gnaw, tear, or macerate.
In the case of a herbivore such as Morenelaphus, the signature of the constituents of its diet reveals itself in the form of small millimeter marks on the enamel of the teeth. Such marks may assume, in certain teeth, the shape of scratches, pits, or gouges. The existence of more or less scratches, pits and gouges, as well as the association between them, is what indicates the type of food preferentially consumed.
Morenelaphus fossils were found in a vast region, spanning virtually all of central-west Brazil from the fringes of the Amazon to the north, through the south and southeast regions of Brazil, to the south, over part of the Paraguayan territory, and also to the northeast of Argentina.
Morenelaphus inhabited regions now covered by biomes as diverse as the Caatinga, Cerrado, Pampa and Gran Chaco. Such diversity of biomes implies a wide variety of food available to herds of herbivores. Had it been so in the past? Would Morenelaphus be adapted to make the most of the resources that nature provided, or would it be an animal with a more selective diet, adapted to a more specific food niche? To find out, you need to investigate the teeth.
"The micro-wear analysis of tooth enamel is an inexpensive and non-destructive method," explains Leo. Their use, therefore, has advantages when compared to the chemical or molecular techniques with the same purpose, which necessarily depart from the extraction of small portions of the bone, which always damages the fossil.
Alline Rotti, Biological Sciences student at UNIRIO was the one who faced the challenge of studying the Morenelaphus teeth in the various collections. The objective was to identify micro-wear patterns capable of indicating what would have been the diet of those giant cervids.
According to Alline, microwear analysis of teeth may indicate three possible diets. Like most living deer, Morenelaphus could be a browser, a herbivore that feeds preferentially on leaves, bark, and green stems from plants. Morenelaphus' teeth could also reveal that it was a grazer, a herbivore that feeds preferentially on grasses at or near ground level, such as horse and cattle.
The third hypothesis would be an intermediate diet, which associates branches and grasses, leaves of trees and grass, in order to take advantage of the food that is available in the environment, or the vegetation available in a certain season of the year.
The results of the research have just been published in the article "Diet reconstruction for an extinct deer (Cervidae: Cetartiodactyla) from the Quaternary of South America," in Palaeogeography, Palaeoclimatology, Palaeoecology.
The microwear analysis technique used in the study was developed by the American biologist Gina Semprebon, a researcher at Bay Path University. Semprebon signs the article with Alline Rotti, Leo Ávila, and the paleontologist Dimila Mothé.
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| Morenelaphus skull, with detail of molar teeth |
Scratches, pits and gouges
According to Alline, the technique of microwear analysis stipulates the use of second uppr molars (M2), which have an intermediate dental wear, and are not fragmented or damaged. "Why analyze M2 molars and not the others? M1 molars suffer a lot of wear during the life of the animal. M3 molars suffer little wear. M2 molars have an intermediate wear. They are, therefore, ideal for research."
There are eleven M2 molars of Morenelaphus in museum collections. Of these, eight molars were suitable for micro-wear analysis, because they allowed the recognition of enamel borders, exposed dentine, and microwear marks.
The studied specimens are from three localities. Two specimens are from Gruta do Urso cave, Aurora do Tocantins municipality, Tocantins state, Northern Brazil; five specimens are from fossiliferous sites along the Carcarañá River, Santa Fé province, Argentina; and one specimen is from Buenos Aires, Buenos Aires province, Argentina. The studied specimens are housed at the collections of Laboratório de Mastozoologia of Universidade Federal do Estado do Rio de Janeiro; Museo de La Plata, and Museo Argentino de Ciências Naturales “Bernardino Rivadavia”.
"First the specimens were cleaned. Next we made molds of each tooth enamel surface using high-precision silicone, the same one used by dentists," explains Alline. "The goal was to produce replicas that could be analyzed." Obviously, we were not able to make good quality replicas right away, but after gaining a certain practice the result started to look good. Only then did we start the count".
Once the surface replicas of all eight M 2 molars were obtained, it was time to go under the microscope to identify and count the scars on the enamel. But this did not happen on the entire surface of the specimens. The research was restricted to a very specific area. The recognition and scoring of enamel scars were made on the second band of the mesiovestibular region of the paracone.
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| Examples of scratches, holes and grooves in M2 molars of Morenelaphus |
"Each food leaves a kind of mark on the enamel. But how to determine which mark was produced by what kind of food? One works by comparison," says Leo. "There is a bank of images on the internet with photos of each type of mark, scratch or scar, and the indication of the probable food whose consumption caused each form of wear."
According to Alline, the scratches are elongated scars, with straight and parallel margins, and could be fine or coarse. Fine scratches are thin and superficial. Coarse scratches are wide and deep. Cross-scratches are fine scratches oriented perpendicularly among each other. The pits are rounded scars that can be small and large. Gouges are the largest and deep perforations with irregular borders
"We determine the type of food by counting the number of scratches and pits and by the patterns they form," says Alline. "Even under the microscope, many scratches are not easy to see. You have to turn the tooth a little under the light, in order to distinguish the scratches by their refractive properties. In relation to the recognition of eating patterns, scratches are more informative than pits. For example, low numbers of scratches designate a browsing habit, whereas high values are indicative of a grazing diet. Intermediate values (between grazer and browser values) are often suggestive of the mixed feeder dietary category.
Deer are generally browsers, therefore their teeth show low numbers of scratches. The high number of scratches indicates a pasture diet, with the consumption of grasses. Its leaves are rough, abrasive and sharp. They wear out teeth because they contain silica (or silicon oxide), the main component of the sand and the raw material of glass.
A high number of scratches indicates a pasture diet, with the consumption of grasses. Its leaves are rough, abrasive and sharp. They wear out teeth because they contain silica (silicon oxide), the main component of the sand and the raw material of glass.
But there is a third possibility. When the researcher identifies an intermediate number of scratches and pits in tooth enamel in relation to what one expects to find in the teeth of a browser or a grazer, this suggests that it is an animal with a more diversified diet, that it feeds on branches, bark, sprouts, fruits, seeds, hard grains and grasses. "Normally, strict grazers can in certain situations feed on branches or bark," says Leo. "The opposite is more difficult. Browsers do not have teeth adapted to the consumption of grasses.
Only grazers have teeth adapted to the consumption of grasses. These are called hypsodontic teeth: molars and premolars of continuous growth. To prevent premature loss of teeth due to wear caused by consumption of abrasive grasses, hypsodonty in grazers is an adaptation that keeps teeth growing continuously throughout the life of the animal.
When it came time to count the marks on Morenelaphus' teeth, Alline detected a high number of pits (8 to 42 depending on the molar) and a not so high number of fine scratches (between 11 and 32), accompanied by few (on average 3) scratches and gouges, thick or crossed.
Once the mean values of the holes and scratches for Morenelaphus were calculated, such values were plotted on a graph with reference values for browsers and grazers. What happened? The average values of pits and scratches for Morenelaphus fell between the spaces on the chart with many scratches (grazers) and space with few scratches (browsers).
Although living South American deer usually have browsing habits, the study of the Morenelaphus molars pointed toward an intermediate diet. That is, those giant deer were neither strict browsers nor grazers. The work suggests that Morenelaphus fed on both branches and tree bark, as well as grasses and hard grains. How to interpret this result?"
"The tissue of antlers is the fastest growing among all vertebrates," explains Leo. Deer males lose their antlers every year. After the molting period, to support the rapid growth of new antlers, animals need to feed more and more frequently. "Imagine the adaptive cost for a giant deer like Morenelaphus that needs to sustain the growth of his huge antlers?"
During the growth of the antlers, in order not to suffer malnutrition, males had to consume all the food available to a deer in central South America during the Pleistocene period. The temporal emphasis here is important, given that, during the several glaciations that marked the last two million years, drier and cooler climate has made savanna areas currently restricted to the Brazilian Cerrado expand in the north to regions formerly dominated by Amazon forest, and to the south for the current domains of biomes such as the Atlantic Forest, the Pampa and Gran Chaco.
Now, the savannah is par excellence the biome dominated by grasses. It follows that, in order to survive, Morenelaphus was a mockingbird adapted to include grass in his paleodieta. The alternative to this would be to migrate to other warmer and humid regions of the continent where the forests with the more tender leaves prevailed in the Pleistocene. No records of Morenelaphus have yet been found in any of those areas.
Adaptation is no guarantee of survival. Morenelaphus was adapted to take advantage of the existing resources (grasses) while the savannah conditions prevailed. When the glaciation ended at the end of the Pleistocene, the Holocene period began. It is the last 10,000 years, when the climate has become warmer and wetter, and the savannah area on the continent has decreased noticeably.
Would Morenelaphus' ability to consume grasses cease to be an adaptive advantage to become a disadvantage, a maladaptation? "Morenelaphus extinction may be related to the reduction of open, dry and grass-dominated areas in South America during the late Holocene," says Leo. "It may have been a case of negative selection of these animals."
To the climatic maladaptive condition that pressed Morenelaphus was added another (terrible) variable: the entry into the continent of the first hunter-gatherers. We know how this story ended. Megafauna, bye bye.
Reference:
Alline Rotti, Dimila Mothé, Leonardo dos Santos Avilla, Gina M. Semprebon. 2018. Diet reconstruction for an extinct deer (Cervidae: Cetartiodactyla) from the Quaternary of South America. Palaeogeography, Palaeoclimatology, Palaeoecology 497:244-252. https://doi.org/10.1016/j.palaeo.2018.02.026
Press contact for interviews:
Leonardo S. Avilla
Laboratório de Mastozoologia, do Departamento de Zoologia, da Universidade Federal do Estado do Rio de Janeiro (UNIRIO) Rio de Janeiro-RJ
e-mail: leonardo.avilla@gmail.com
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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