Evolution Luo, Z., Chen, P.,

Evolution

Luo, Z., Chen, P., Li, G., Chen, G. & Chen, M. "A New Eutriconodont Mammal and Evolutionary Development in Early Mammals." Nature, (2007) Vol 446; 288-293

Evolutionary research tells us that one of the keystones in the development of modern mammals was the adaptation over the millennia of the mammalian middle ear. The theory being that as the species evolved from its reptilian origins, the delicate bones of the middle ear made the transition from the reptilian lower jawbone and evolved into what we now know as the three small bones in the middle ear. They are the hammer (malleus), anvil (incus) and stirrup (stapes) which are connected to one another in what anatomist term the ossicular chain. While this transition has been theorized about, for the first time a fossil has been found that actually shows signs of the transitional period occurring. This fossil also presents similar transitions occurring in lower lumbar vertebra and thoracic vertebra of the spinal column, another hallmark of mammalian evolution.

This fossil is Yanoconodon allini gen, Class Mammalia, Order Eutriconodonta,

Family Jeholodentidae, or know by his friends as just Yanoconodon. Named for the Yan mountain range in China where he was discovered, and condon which is a typical suffix associated with the naming early mammals referring to the teeth, Yanoconodon is a very tiny mammalian ancestor that lived during the Jurassic period. Weighing in at around 2 grams and about 20 to 30 centimeters long, it is extremely rare that any fossils have survived intact.

The researchers meticulously examined this fossil, which is imbedded in a slab of siltstone and fastidiously mapped all the bone structures. There were certain missing portions so some approximations had to be made regarding those areas, but none that would have impeded the validity of their research. After analyzing all of the dental and skeletal characters of Yanoconodon the researcher determine its place along the chain of evolution and cite it as a close relative of species known as Jeholodens. By doing so they can then make further assumptions regarding its mammalian characteristic and its place in the evolutionary transition of mammals. Jeholodens had some characteristics of early reptiles such as well-muscled and broad rear legs, but its front legs were beneath its belly as in modern mammals. More than likely with this configuration it was able to maintain an upright posture at times, another mammalian transitional trait. This point goes to further demonstrate the reasoning behind another find in the species Yanoconodon, that of the changing vertebrae and spinal structure during this period of mammalian evolution.

The results of their thorough examination of Yanoconodon showed some very tantalizing evidence of a transitional species development from reptile to mammal that had not been seen before. The authors themselves do not explicitly say they have found a "missing link," though they do imply it has that kind of importance. Other researchers are debating the point but they all seem to agree that finding this transitional mammal has many important aspects regarding the evolution of the species.

For one, the researcher discovered that there was a structure called Meckel's cartilage that was present between the incus, or anvil bone, of the middle ear and the lower jawbone of Yanoconodon. Why is this so important? Well, coincidentally, in the developing human embryo the lower mandible transforms and produces the bones of the middle ear. As the embryo further evolves, the identical Meckel's cartilage is produced as the bones migrate from the jaw to the middle ear. Afterwards the cartilage is dissolved into fibrous membranes and becomes part of the jawbone again and dissapears. This is the first time the presence of Meckel's cartilage was found in a fully developed mammal. This also adds some credence to an old saying "ontogeny capitulates phylogeny," or in English the stages in the development of the embryo mirrors the evolutionary stages in the development of the species. While intriguing and also not altogether true, it certainly becomes another one of the fascinating aspects of this find. It is also a rare and previously undocumented discovery of finding the existence of Meckel's cartilage in the fossilized remains of any prior specimens.

Their discovery that this species is also remodeling it lower back to conform to the more modern delineation between upper and lower vertebrae present in modern mammals added to the growing "missing link" reputation of this specimen. They also theorize that on a genetic level this is an important discovery as well. A cluster of genetic material called the Homeobox gene governs the creation of the current mammalian structure for the vertebra in the spine. It had been theorized that this set evolved from a set of genes called the Hox gene cluster, which evolved during the Paleozoic era.