Evidence of False Thumb Evolution
Roughly 75% of all the types of mammals that have ever lived are now extinct. We tend to think of extant organisms as all there is. We know dinosaurs once lived and are now extinct. An article explored the routes of diversification taken by early mammals, only those lineages in the far right based on figure 1, in Cenozoic Era have survived to the modern day. Each era group showed how common it was in various time period. The multituberculates were very common until the early Paleocene. In history, they were the dominant group of mammals, existing in the largest numbers for the longest time and are now extinct. Their generations, the spalacotheriods and eutriconodonts, were successful in the Early Creataceous but its population declined by the end of that Period. At the root of the tree showed (figure 1) the groups of mammaliaforms, not true mammals, but have some similarities. This group first appeared in the Jurassic era and didn’t survive the Cretaceous era. The Cretaceous ended with a global extinction that wiped out the non-avian dinosaurs, and several stem-mammals suffered the same fate. Others lingered into the Paleocene Epoch only to become extinct, possibly outcompeted in the rapid radiation of the therians.
We tend to think of extinct groups as more “primitive” than extant groups, but the mammals radiated rapidly and many niches occupied today were occupied by now-extinct forms in the past. In the phylogentic tree some show niches available and how they have been filled at various times. The presence of a gliding mammaliaform in the Jurassic is especially interesting
A few certain niches seem to be filled by one species or another at any time period. For instance, in mammalian evolution, Castorocauda, a Jurassic mammaliform seemed to have fit into the niche now occupied by beavers or otters (figure 2). But from time to time we run across an animal that seems to be adapted for a unique role in its ecosystem, like the animal Simocyon, puma-sized caniform that lived about 14 million years ago, and died out by four million years ago. Simocyon has a variety of unusual adaptations, like having a false thumb, and indicates as its closest living relative is the red panda, Ailurus.
Geographical diversity sometimes exhibit groups of animals and plants of unusually similar appearance, despite the fact that the organisms may be only vaguely related. It is challenging to explain the similarities as the result of coincidence. Instead, natural selection appears to have favored parallel evolutionary adaptations in similar environments. Because selection in these instances has tended to favor changes that made the two groups more alike, their phenotypes have converged. This form of evolutionary change is referred to as convergent evolution, or sometimes, parallel evolution.
How do we know when two similar characters are homologous and when they are analogous? As we have seen, adaptation favoring different functions can obscure homologies, while convergent evolution can create analogs that appear as similar as homologs. There is no easy and fast answer to this question. The determination of homologs is often a questionable issue in biological classification. Looking at the morphological evidence of the two organisms would be a good start, comparing the functional anatomy of the skeleton based on evidence, and compile the major evidence to clarify the phylogenetic history of both species. Based on non-molecular evidence, Simocyon’s rare false thumb has similar characteristics to the thumb of giant pandas and red pandas. Since they shared this unusual feature and an odd diet of bamboo it used to be thought that red pandas and pandas are closely related, but it was later discovered that pandas are bears, and red pandas are more closely related to raccoons. The red panda, genus Ailurus, is placed in the family Ailuridae, and Simocyon likewise falls into this family. The false thumbs of the giant panda (Figure 3.1) and Simocyon (Figure 3.2) are shown. The false thumb is produced by an enlarged radial sesamoid bone and serves as a relatively immovable surface that the other digits can be brought into opposition with, allowing a pinching or grasping motion.2
In the giant panda and red panda, the false thumb is a convergently evolved trait and is used to grasp bamboo while eating. Both the pandas have teeth that are adapted for this vegetarian, bamboo-based diet, but Simocyon has more generalized canine-form dentition, which would prey on small vertebrates but also eat fruits, seeds, and eggs.3
The reason for the evolution of false thumb was due originally for tree climbing and was pre-adapted for bamboo eating. Red pandas are primarily arboreal animals, and Simocyon has skeletal adaptations for climbing. The common ancestor of the two was perhaps primarily an arboreal carnivore that evolved the false thumb to aid in gripping branches. Salesa and other researchers, for the first time, have analyzed the postcranial skeleton of Simocyon in detail to determine what its lifestyle was like.3
The unusual thing about Simocyon is that it retains many tree-climbing adaptations while being much larger than a practical arboreal carnivore. Certainly, other features of its anatomy are more suited to a ground-hunting, more like a general carnivore. The researchers suggest that this was Simocyon‘s usual mode of behavior, but that it retained its tree-climbing ability because of its hostile environment. Simocyon lived alongside amphicyonids , known as bear-dogs, and saber-toothed cats. While Simocyon at the size of a puma was not small, weighing about 120 lbs, these other carnivores could weigh twice that or more. Simocyon may have scavenged their foods and prey, and it’s likely these carnivores would have come into contact with Simocyon not infrequently. Large carnivores tend to be aggressive to smaller carnivores, especially those big enough to provide some competition for foods. Like lions and hyenas have a mutually antagonistic relationship, wolves kill coyotes, and Great Horned Owls kill smaller owls. Therefore, it would not be surprising if most of these confrontations ended up in Simocyon bolting up a tree. As the Salesa says, “In this scenario, the strong muscles of the shoulder, forearm and lumbar region of this ailurid would produce the necessary force to propel its body in the vertical faster enough to escape from these encounters.” 3 And I’m sure Simocyon would have been thinking “run FASTER!”, as far as it was able to think. These assumptions were drawn from features of the forelimbs and lumbar spine (figure 4). The forelimbs possess paws with false thumbs, an increased ability to turn the wrist and hand inward, useful for hugging tree trunks, increased brachiating to swing from tree limb to tree limb, and strong muscles in the shoulders. The lumbar spine has unusual adaptations similar to those allowing bounding locomotion in mustelids, 3 similar to how ferrets move, but this type of locomotion is inefficient in an animal, this size and this adaptation is probably related to stabilizing the trunk for vertical climbing. At this time the features of the pelvis and hind-limbs are unknown. A careful look at the details of the false thumb, the evolutionary history of these two lineages is different from each other. Initial morphological, anatomical, behavioral, and paleontological studies disagreed about the placement of the species. Striking similarities in pseudo-thumb morphology, and eating behaviors indicated a close relationship between the two pandas, and the relatively lacking fossil record offered no further clarification. Based on these similarities, and the relative similarity of these traits to those of Procyonidae, the families including raccoons, both pandas were sometimes classified as a subfamily Ailurinae within the family Procyonidae. However, based on the morphological similarities between pandas and the bears (Ursidae), the two species were sometimes classified with the ursids.4 In the Simocyon, the false thumb evolves in a terrestrial bear which becomes an herbivore. In the red panda the false thumb evolves in an arboreal animal. This showed that false thumb is an example of convergent evolution.
Specifically, the schematic tree summarizing the phylogenetic relationships presents a strong argument for the creation of a new family, Ailuridae. This study, which constructed a supertree for the order Carnivora, combines previously-constructed trees in a method called matrix representation using parsimony analysis (MRP). This new type of tree, which can incorporate different terminal taxa, for example can incorporate multiple, incongruent data sets, reduces homoplasy while often indicating new clades that were not previously proposed due to incomplete taxon sampling. Here, the figure 5 propose raising the genus Ailurus to family level, one that is a sister taxa to Musteloidea (Procyonidae + Mustelidae).5

Work Cited (1) Luo, Z.-X. “Transformation and diversification in early mammal evolution.” Nature 2007, 450, 1011-1019.

(2) Salesa, M. J.; Antón, M.; Peigné, S.; Morales, J. (2006). “Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas”. Proceedings of the National Academy of Sciences, 103, (2), 379-382.

(3) Salesa, M.J., Antón, M., Peigné, S., Morales, J. (2008). “Functional anatomy and biomechanics of the postcranial skeleton of Simocyon batalleri from the Late Miocene of Spain”. Zoological Journal of the Linnean Society, 152(3), 593-621.

(4) Davis, D. D. (1964). “The giant panda: A morphological study of evolutionary mechanisms”. Fieldiana: Zoological Memoirs 3, 1–339.

(5) Bininda-Emonds, O. R. P., J. L. Gittleman and A. Purivis. 1999. “Building large trees by combining phylogenetic information: A complete phylogeny of the extant Carnivora (Mammalia)”. Biological Reviews 74:143–175.