“We’ve not found fossils before that showed this, and so finding this now was a huge (but really fantastic) surprise,” according to Dr. Russell Garwood of the University of Manchester. His language describes the scientific shock of the discovery of an entombed creature, such as the minuscule arachnoid, Chimerarachne yingi, that is 100 million years old, and whose anatomy represents a profound evolutionary gap between old tailed arachnids and today spiders.
The fossil is found in the rainforests of northern Myanmar, mostly undiscovered by scientists, and it shows a combination of characteristics that are not found in any current species. Similar to present day spiders, it has spider-like spinerets, or silk-producing organs, with highly complex joints and microscopic spigots that are able to produce protein-based fibres. However, with it, hanging off its abdomen, is a kind of segmented tail, or flagellum, like that of the scorpions, and of the now extinct uraraneids. The combination of these features places the status of either of the last proto-spiders or the first true spiders.
This revelation was made possible by the special preservation properties of amber. The mid-Cretaceous Burmese amber is made of fossilized tree resin and it is famous due to its clearness and the capability to preserve minute structures exterior and interior. Viewing them under a high magnification, the researchers could see not only the articulation of the spinerets, but also hairs and how the tail is divided. According to Dr. Ricardo Perez-De-La Fuente of the Oxford Museum of Natural History, “fills the gap between Palaeozoic arachnids with tails known from rocks (uraraneids) and true spiders,” with preservation that allowed “an unmatched detail of study.”
The technicality of such fidelity is remarkable as the process itself. The amber is prepared by grinding and polishing the amber to a flat surface with an optical perfection, occasionally by immersion in refractive index-matched oils in order to remove distortions. The digital dissection of an inclusion into internal morphology can be done using advanced imaging, such as high-resolution X-ray computed tomography, which eliminates the physical destruction of interior structures. This involved, in the case of *Chimerarachne* the confirmation of the existence and structure of spinerets, which was the determinant of its evolutionary position.
Their spinerets, a distinctive innovation of spiders, develop out of limb-like appendages of segments of the abdomen. In each spineret are spigots, attached to purpose-specific silk glands, that generate fibres with specific mechanical properties, such as dragline silk, which has tensile strength more than mild steel, or elastic capture silk, studded with adhesive droplets. In Chimerarachne, the well-developed spinerets suggest that the manufacture of silk was already developed 100 million years ago and the manufacture of the web was developed later, after the loss of the tail.
The fossil is enhanced through evolutionary context. Arachnids appeared more than 400,000,000 years ago, and uraraneids (relatives with tails and producing silk), existed between the Devonian and Permian. Africhan These lineages lived at least 200 million years, and survived even the end-Permian mass extinction, as the discovery of uraraneid-like forms continued into the Cretaceous. This long overlap obliges a reevaluation of the speed at which spider-characterizing features, including aerial brush building, became promoted.
The tail of Chimerarachne, approximately 3 millimetres long, was probably a sensory organ, and read vibrations or a chemical signal in the surroundings. Its ultimate loss in the evolution of spiders could be linked to a change in the mode of either active pursuit or burrow-lining to sit-and-wait predation in aerial webs, where the web itself was the major sense field.
The fact that this beast has been able to survive without being noticed until its accidental preservation speaks to the biodiversity at Myanmar untapped yet. These forests are not so well understood, as noticed by Dr. Paul Selden of the University of Kansas, who observed that some of them are only a tiny creature. Although certainly extinct, the question of whether there are related, tailed silk-spinners present in remote locations is also open, and tantalizing, though nearly certainly unprovable.
This fossil is not just a curiosity to paleontologists and evolutionary biologists, but a physical datapoint very unusual in a lineage in which soft-bodied anatomy is rarely preserved. It shows how the intersection of geological chance, chemistry of resins and modern imaging can not only tell us what ancient creatures had the appearance of, but also how they lived, hunted, and how they felt about the world around them.
