The origin of snakes continues to be one of the most hotly debated topics in vertebrate paleontology. Some of the major questions surrounding snake origins include, first and foremost, how did snakes lose their legs. However, many additional important questions include: how did snakes lose several important bones in their skulls (which contributed to the development of their highly mobile skulls)? We know snakes evolved from fully-limbed lizards, but which lizard group are snakes most closely related to in the lizard tree of life? Finally, which limbs snakes lost first (front or rear?), and how long did it take for snakes to acquire their current body plan, with extreme body elongation and no externally visible limbs?
Although various answers have been proposed to those questions for over a century, such debate got specially heated up after the discovery of the first fossil snake still possessing well-developed rear limbs in 1997 in Nature by Mike Caldwell and Mike Lee. Since then, the last couple of decades have seen a surge of interest and a series of studies trying to explain those important evolutionary transitions leading up to the modern snake body plan, using both the fossil record and molecular data.
This year, two very important publications have just came out to elucidate some of those questions. The first, which I would like to call as the most comprehensive introduction to snake origin debates ever published, came out in the form of a book by my previous PhD advisor, Mike Caldwell: “The Origin of Snakes: Morphology and the Fossil Record”. The book provides both a historical and an up-to-date perspective on the scientific advances concerning several topics on snake origins, mostly from the perspective of morphology in the fossil record, but also presenting data and information stemming from molecular and developmental research. Although I am admittedly biased, this is a “must have” piece of literature for all of those interested on lizard and snake evolution, as well as the decades of research surrounding perhaps one of the most iconic evolutionary transitions in the history of vertebrates.
Fig. 1. The new released book on the origin of snakes.
The second publication, just published today in the journal Science Advances, brings some incredible new hard evidence to answer some of those long-standing questions in early snake evolution. The paper, led by my Argentinian colleague, Fernando Garberoglio (and in which I am a contributing author), presents not just one, but a series of articulated skeletons and skulls of the early snake Najash rionegrina, which are approximately 90 million years old. Although previously known from articulated individuals, only small bits of the skull of this ancient snake species were previously known. With the new findings, now we have available several three-dimensionally preserved and articulated skulls, with the most complete one pound by Fernando himself in the La Buitrera outcrops in the Argentinan Patagonia.
Fig. 2. Fernando studying the new Najash skulls, and both of us working together during his time visiting Canada.
After the initial “hurrah!” and excitement surrounding the discovery of the skulls of this key species to understand early snake evolution, it came the time to transform a great discovery into a great piece of scientific work. To achieve that, Fernando spent some time with us in Canada (during my final PhD years at the University of Alberta, in Edmonton, Alberta). During that time, he managed to work very closely to Mike Caldwell and myself, doing some serious comparative anatomy work (and debating on the quality of Argentinian soccer and his beloved hero and second god, Maradona!). The end result was a carefully revised morphological data set of snake relationships, additional data from fossil snakes obtained from high resolution micro-CT scans, and exploring multiple evolutionary scenarios using various data sets and analytical methods.
Fig 3. CT scans from the newly discovered skulls of the legged snake Najash, and new evolutionary scenario for the acquisition of key features characteristic of the modern snake skull.
In just one piece of work, the new findings and analyses settle a long-standing debate on the sequence of bone loss on the skull of snakes during their early evolution: the postorbital bone, not the jugal bone, was the major “cheekbone” element lost early in snake evolution, and after the disappearance of the temporal bones. As a major consequence, it is therefore the jugal bone (not the postorbital) that is observed as a small remnant on some species of living snakes, revising decades of anatomical literature. Additionally, all evolutionary scenarios explored by our team (based on different evolutionary trees and divergence time estimates for snakes), indicated that snakes retained well-developed hindlimbs (and likely no forelimbs) for several million years during their early evolutionary history. This was not a quick and transient evolutionary experiment, but a body plan that lasted for at least 70 million years, and maybe more. Further, some of the key features that have been historically utilized to characterize all snakes, turn out to be characteristic only of the modern snake lineages, such as the unique enclosure of the middle year bone (enclosure termed crista circumfenestralis) in the braincase of modern snakes. [Or is it? Recent findings indicate that even well known modern snakes may actually lack this feature too!].
Perhaps the most controversial point indicated by the results of this paper, at least among the students of snake evolution, may be the strong morphological signal for the placement of blindsnakes (small, blind and well-adapted to burrowing) further up on the snake tree of life. This contradicts hypotheses supported by molecular data, in which blindsnakes represent the earliest evolving form among all modern snake lineages. Both in our results and other recent studies including fossil data, morphology tends to place fossil snake lineages (such as Najash) within the earliest branches of snake evolution, before the radiation of the modern lineages (in which blindsnakes would be the earliest modern lineages to evolve). However, in our results using updated morphological data, blindsnakes are not the earliest evolving snake lineage even among the modern species. Instead, they are placed higher up in the snake tree, as a highly modified snake lineage relative to the snake ancestral condition: a topic discussed in further detail in the new book just mentioned above! Our analyses combining morphological and molecular data provide some conciliatory results between both datatypes, indicating that blindsnakes may be the most primitive among the living forms, whereas the fossil species form a separate group of their own. This would support the idea that blindsnakes may be primitive among living forms, but not among all snake lineages that ever lived.
I believe the main take-home message is that the placement of blindsnakes is the key to understand the early patterns in snake evolution, as well as understanding their relationship to other lizards. Whereas molecular data provides consistent results (even at the genomic level), morphological data is still being constantly updated based on different approaches to the construction and analyses of morphological data, besides key new fossils, such as the new Najash skulls, which together are constantly overturning previous hypotheses.
Where do we go from here? My personal opinion is that the next chapters in snake evolution will be answered by improved approaches of combining the strong signal from the molecular data with improved approaches to incorporate morphological data from fossils (some steps which have already been taken by us and others), besides continuous exploration for substantially older well-preserved fossil snakes, closer to their time of origin at some point in the Middle Jurassic, at about 165 million years ago.
Fig. 4. Life reconstruction of Najash (art by Raúl Gomez)