The armored or tyreophoran dinosaurs are among the most popular, and it is not for less. They are very characteristic for their armor (made up of bone plates that we call osteoerms) and for their specialized weapons at the end of their tails: pairs of spikes in stegosaurus (that complex called a thagomizer , or thagomizer ) and large maces in ankylosaurs . Thyreophoran fossils have been found on all continents, mainly stegosaurs in the Jurassic, and ankylosaurs in the Cretaceous. And within the big picture of their evolution as a group, the southern Gondwanan thyreophorans are rare and enigmatic, and most likely represent the earliest, most primitive forms of Ankylosauria.
In a new article recently published in Nature , headed by the Chilean paleontologist Sergio Soto-Acuña , a partially articulated skeleton (that is, with part of its bones in the position they occupied in life) and quite complete of a small dinosaur has been described. battleship about 2 m in length. This new thyreophore has been found in Upper Cretaceous rocks of the Magallanes region, in the extreme south of Chile. This new dinosaur has been called Stegouros elengassen and its most outstanding characteristic is that it developed a caudal weapon different from all those known until now in any dinosaur: a flat structure, formed by seven pairs of laterally directed plates or osteoderms enclosing the end of the tail. .
In addition, the characteristics of the anatomy of Stegouros are very interesting, since it shows a mixture of primitive characters, very similar to those of stegosaurus, and clearly ankylosaurian characters, more derived. For example, it shows a typically ankylosaurian skull, but a very ancient postcranial skeleton (which is what we call the rest of the skeleton excluding the skull), with some very primitive characters, similar to those shown by stegosaurus.
A Primitive Ankylosaurus, Not a Missing Link
What is the meaning of these primitive characters similar to those of stegosaurus? If ankylosaurs were predominantly Cretaceous, and stegosaurs Jurassic, could we be looking at a missing link between stegosaurus and ankylosaurs? Is this evidence that ankylosaurs are descended from stegosaurus? Short answer: no.
Let’s go for the long answer. Although evolution has traditionally been seen as a linear process, in which intermediate or transitional forms were seen as links in a chain, long agowe have discarded this interpretation. There is neither a line nor there are links. The vision of the evolutionary process that is being considered today is that of a process that generates diversity and variability in the entire possible spectrum, not in a line. For example: imagine a primitive armored thyreophoran dinosaur (a shape similar toScelidosaurus, for example). This dinosaur manages to be successful, and its population grows, and its distribution expands. Imagine this happening for thousands of years. And that the populations of this little dinosaur, in different habitats and subjected to different selection pressures, are differentiating into different species. As they evolve, not all these populations change the same things, nor do they maintain the same primitive characters. A population that maintains primitive features in its skeleton and developstriangular osteodermsand thorns gives rise tostegosaurus. Another population, which initially also maintains primitive features in its skeleton, develops the plates that form thearmorof theankylosaurs. And the ankylosaurs, over time, and possibly because of such extensive armor, end up modifying the vertebrae of their spine, ceasing to be so primitive, ceasing to resemble those of the first ankylosaurs. And ceasing to resemble those of the stegosaurus.Stegourosit would be a primitive form of the ankylosaur radiation. And for this reason, it maintains primitive characteristics that the stegosaurus also maintained. Furthermore, the oldest fossils of ankylosaurs have been described in theUpper Jurassic, living with stegosaurus.
Furthermore, numbers rule: analyzes of their kinship relations placed Stegouros within the Ankylosauria group. Specifically, it appears closely related to Kunbarrasaurus from Australia and Antarctopelta from Antarctica, forming an ankylosaur clade that appears to be unique to Gondwana, a branch that diversified before all other ankylosaurs. An ancient and primitive group that has been given the name Parankylosauria .
A unique weapon, or not so much
As we have mentioned, Stegouros possessed a unique caudal weapon, unlike any previously known in any dinosaur. This weapon was a flat structure formed by seven pairs of plates or osteoderms directed laterally, and whose bases enclosed the last vertebrae of the tail. Externally, this weapon may bear some resemblance to a macuahuitl , a typical Aztec weapon.
A study in conjunction with the rest of the caudal arms of the thyreophorans (especially in skeletons that have been found in anatomical connection, with the bones in their original position) including this new member, can shed a lot of light on how each has been formed. one of these caudal arms. And it is that, apparently, they all have the same base and are formed in a similar way. Stegosaurus thagomizers are made up of paired spines that also laterally enclose or encapsulate the last vertebrae of the tail. And the clubs of the most derived ankylosaurs are formed by the fusion of the last osteoderms that enclose the last vertebrae of the tail.
In addition, thanks to this new ankylosaur, we can make inferences for its closest relatives, of which it was unknown if they had a caudal weapon. This is the case of Antarctopelta . Their osteoderms are very similar, as is the shape of their tail vertebrae. All this seems to suggest that it could have had a caudal weapon similar to that of Stegouros .
This is one of many examples of how a new species, which can be considered a mere curiosity, is much more. And it is that it provides new data that allow us to understand the evolution of an entire group. New pieces of a puzzle that, little by little and with each discovery, is taking the form of an extinct world .
References :
Soto-Acuña, S. 2021. Bizarre tail weaponry in a transitional ankylosaur from subantarctic Chile. Nature, 600 (7888): 259–263.
Brown, B. 1908. The Ankylosauridae, a new family of armored dinosaurs from the Upper Cretaceous. American Museum of Natural History Bulletin, 24: 187—201.
Norman, D.B. 2021. Scelidosaurus harrisonii (Dinosauria: Ornithischia) from the Early Jurassic of Dorset, England: biology and phylogenetic relationships. Zoological Journal of the Linnean Society, 191 (1): 1–86.
Norman, D. B. et al. 2004. Basal Thyreophora. In: The Dinosauria. Berkeley: University of California Press.
