Arx Fatalis Surface

[Nico Sadiq]

Skeletaldisease may hamper the behavior of large predators both living and extinct. We investigated the prevalence of osteochondrosis dissecans (OCD), a developmental bone disease affecting the joints, in two Ice Age predators: the saber-toothed cat Smilodon fatalis and dire wolf Aenocyon dirus. As published cases in modern Felidae and wild Canidae are rare, we predicted that subchondral defects resembling OCD would be rare in the extinct predators. We examined limb joints in juvenile and adult S. fatalis: 88 proximal humeri (shoulder), 834 distal femora (stifle), and 214 proximal tibiae. We also examined limb joints in juvenile and adult A. dirus: 242 proximal humeri, 266 distal femora, and 170 proximal tibiae. All specimens are from the Late Pleistocene Rancho La Brea fossil locality in Los Angeles, California, USA. While the Smilodon shoulder and tibia showed no subchondral defects, subchondral defects in the Smilodon femur had a prevalence of 6%; most defects were small (12mm); and five stifles further developed mild osteoarthritis. Subchondral defects in the A. dirus shoulder had a prevalence of 4.5%; most defects were small, and three shoulders developed moderate osteoarthritis. No defects were found in the A. dirus tibia. Contrary to our prediction, we found a high prevalence of subchondral defects in the stifle and shoulder of S. fatalis and A. dirus resembling OCD found in humans and other mammals. As modern dogs affected by OCD are highly inbred, this high prevalence in the fossil taxa may suggest that they experienced inbreeding as they approached extinction. The deep-time history of this disease supports the need for monitoring of animal domestication, as well as conservation, to avoid unexpected surges in OCD under conditions like inbreeding.

Copyright: 2023 Schmkel et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the paper and its Supporting Information files. All data collected during the study are available as Excelfiles, color photographs and radiographs. The data are stored at the La Brea Tar Pits Museum. All bone specimens are housed at La Brea Tar Pits Museum, 5801 Wilshire Blvd., Los Angeles, CA 90036 and can be accessed by everyone after prior arrangement with the Collections Manager. Updated contact information can be found online at www.tarpits.org. The authors did not have any special access or request privileges that others would not have.

The Rancho La Brea asphalt seeps primarily functioned as a carnivore trap: a large herbivore mired in the asphalt inadvertently would attract large carnivores and scavengers, which themselves would become entrapped in great numbers. Studies of Smilodon fatalis and Aenocyon dirus at RLB have reconstructed their dietary specialization on herbivorous megafauna, with Smilodon probably being primarily an ambush predator and Aenocyon a pursuit predator [8, 9]. The abundant specimens at RLB include numerous examples of bone lesions, or pathologies, incurred by trauma to the animals likely as they hunted large prey; the distribution of these traumatic injuries on the skeleton support the locomotor inferences based on comparative morphology [7].

All specimens were excavated at the Rancho La Brea site and processed at the laboratory in the museum. They are housed at the La Brea Tar Pits Museum. The La Brea Tar Pits Museum is a sister institution of the Los Angeles County Museum (LACM). No permits were required for the described study, which complied with all relevant regulations.

The radiographs of two distal femoral epiphyses LACMHC A 72 and LACMHC A 114 show a subchondral bone defect in the condylar joint surface with a surrounding sclerosis looking very similar to OCD defects in modern domestic animals and the published cases of condyle OCD in Panthera uncia (Fig 6) [8, 9]).

116 adult proximal humeral joints and 126 juvenile proximal humeral epiphyses of A. dirus were inspected, and 11 subchondral defects found (Fig 7). Six defects were small, four medium-sized, and one defect was large. Three shoulders developed moderate OA. Eleven cases in 242 shoulders gives a prevalence of 4.5% subchondral defects.

266 femoral stifle joints of A. dirus and 170 proximal tibia joint surfaces were inspected. Seven subchondral defects were found in the femur (2.6%). No subchondral defect resembling OC/OCD was found in the tibial joint surfaces. Of the seven stifle defects, one was small, another was medium-sized, and five defects were large (Fig 8). Five defects were in the lateral condyle, two in the medial. Five stifles with defects developed mild OA; two were without OA. Radiographs of A. dirus LACMHC 89025 and 88678 stifle surfaces show defects very similar to the radiographic changes in modern dogs affected by stifle OCD (Fig 8).

Osteochondrosis (OC) is considered as a developmental disease, which can manifest as osteochondrosis dissecans (OCD) [2]. Involvement of diet, growth rate, certain hereditary factors, trauma, and vascular disorders makes OCD a multifactorial disease [4, 19].

Genetic predisposition is a risk factor in the development of OC including OCD [4, 20]. Several cases of OC have been reported to be genetically connected in humans [20, 21]. Two pairs of siblings were among five Panthera uncia treated for stifle OCD, possibly forming small family clusters of OCD as in humans [16, 17, 20, 21]. In dogs and horses, some breeds were observed to be more affected by OCD [22, 23]. Many reports have been published about the genetic background of OC/OCD, and the disorder is complex and controlled by several genes [21].

Only nine of 30 subchondral defects in adult S. fatalis showed mild to moderate osteoarthritic pathologies (Fig 5). A possible explanation for the low incidence of severe osteoarthritis found in the stifles with a defect is the clinical experience that, in humans and modern dogs, early stages of OC (OC latens) can heal in patients before it develops into severe OCD and clinical osteoarthritis [24, 26]. When OC development induces cartilage lesions, the cartilage progenitor cells within the articular tissues will initiate the formation of cells that will regenerate and repair the damaged tissues [27]. Another explanation could be the fact that the victims of the RLB asphalt seeps did not reach their natural life expectancy, their lives cut short by asphaltic entrapment: dental examination shows that most fossilized carnivores at RLB were young adults [28]. It remains unknown how OCD would have affected the joints in older animals. We can assume that more joints would have developed advanced osteoarthritis during a longer life span, especially the joints with large and deep subchondral defects in the weight-bearing cartilage surface.

The current study is limited by the fact that only disarticulated and unassociated bones with the joint surfaces could be examined, not the joint function and the cartilage or the surrounding soft tissues. We can only compare the subchondral joint surface defects in the extinct S. fatalis and A. dirus with similar joint surface defects found in living animals today. Despite this limitation, our results do not support our prediction that OC/OCD was a rare pathology in the Pleistocene S. fatalis and A. dirus. We found a high prevalence of subchondral defects in the femoral and humeral joint surfaces resembling OC/OCD found in humans and other mammals. Research is needed to determine if these defects also affected S. fatalis and A. dirus in other areas of North and South America, illuminating potential pathological patterns as these two species approached extinction.

Well children... the old man coughed. The world as you see and know it today has not always been our world. A long time ago, this is what the tradition tells us, all people lived outside this fortress on the surface of the earth. They had something called a blue sky with crystal white smoke called clouds floating around above their heads.

You better believe me! Back then we had a beautiful sun warming everything with its magical beams. People were able to grow crops without the use of magic but only with the help of soil and sun. Sure there were also goblins, trolls, dragons and demons but everything lived so far apart that one rarely encountered any of these. The thing that really hit our civilization was how the sun suddenly vanished and died. From one day to the other it was just gone. The shadow of the night came and stayed forever. Since that damned day, the icy hand of frozen death reigns on the surface. All living things either froze to death or retreated into the womb of mother earth, just like our folks did in the old days. This ancient dwarf mine was the home to all those refugees from then on. The dark age began.

From that fateful day on, only the traveler guild has been bold enough to set foot on the surface again and to travel the surface in constant search for other refugees out there. Down here everything kind of settled with the years. Each race occupied certain areas of the mine for themselves. Today we grow food only with the help of magic and without that we would have already wasted away a long time ago.

Some levels below the goblins and the ratmen survive only by eating the nastiest things and stealing from us humans. Everyone down here has found his or her place by now. Lunshire the Just is a good and rightful king to us; protecting us from all the evil lurking behind the walls of our fortress. You are raised here and are happy with your lives since you do not know better. Hear me now and imagine how beautiful a life would be in an outside world with light from the sun instead of tunnels and caverns lit by flickering torches or magical spells!

As if on cue, all the torches suddenly flickered and their glow crept up the walls towards the ceiling of the room. Frightened by the flickering lights, the children moved closer to each other looking up to where this strange surface was supposed to be.

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