Armadillosuchus

Description

The fossil specimen of Armadillosuchus is not complete, but thanks to the protective armour plating much of it has been reasonably well preserved. The skull is robust and slightly flatted, tho maintaining the overall oreinirostral form typical for Notosuchians. While the snout is relatively longer and wider compared to Adamantinasuchus and Sphagesaurus, it is still rather short. Like in other sphagesaurids, the teeth show a wide variety of morphologies, different from the uniform homodont dentition of most crocodyliforms. These tooth morphologies include curved caniniforms, protruding front teeth that resembled incisors and conical molariform teeth with shearing edges filling the remainder of its mouth. Each premaxilla only contained two to three teeth, with the second pair being the aforementioned caniniforms that greatly exceed the first pair in size. Further back on the upper jaw the teeth are distinctly asymmetrical, with the outer edge being relatively smooth while the side facing inward is marked by a ridge covered in a series of tubercles. The lower jaw is notably narrower than the upper and the first pair of dentary teeth is directed forward like in Adamantinasuchus and Mariliasuchus. The fourth tooth in the lower jaw shows flattened sides and a keel directed towards the front, while further back in the jaw the dentition more closely matches that of the posterior upper jaw. However, a key difference here is that the tubercle-lined keel faces outwards rather than inwards.

The ribs of Armadillosuchus show signs of having undergone pachyostosis, meaning that they are thickened. The limbs were elongated like in other Notosuchians, but still relatively short compared with other sphagesaurids, with the humerus being much more robust than in Caipirasuchus. In Armadillosuchus, the widest point of the humerus measures about half of the bones maximum length. The shoulder blade was also stout with its height and width being roughly equal to one another.

Among sphagesaurids, Armadillosuchus is classed among the large-bodied forms with a skull length around 300 mm (12 in), nearly twice that of the smaller members of the family (such as Yacarerani and Adamantinasuchus). Armadillosuchus may have reached a total body length of up to 2 m (6 ft 7 in).

Osteoderms

Like other crocodylomorphs, Armadillosuchus had prominent body armor formed by small bony plates known as osteoderms. Directly behind the animal's skull, protecting the neck, the osteoderms are fused together to create a rigid shield of hexagonal plates, known as the cervical shield or nuchal shield, which is ornamented by parallel horizontal ridges. Although this element itself was rigid, the way it connected to the body meant it could be articulated independently from the head, rather than locking it in place. This is then loosely linked to seven bands of osteoderms that form the mobile thoracic armor, which superficially resembles that of modern armadillos. The central portion of this armor is formed by a series of overlapping osteoderms, which are rectangular in shape and ornamented with circular pits similar to those seen in other neosuchians. The first five bands of Armadillosuchus' thoracic armor are composed of four parallel osteoderms, while the sixth and seventh show only two. Due to the resemblance of those final two bands to the posterior osteoderms of modern crocodiles, Marinho and Carvalho suggest that the armor behind the preserved section may have lacked the mobile bands and been much simpler, consisting of only a single double-row. However the armor of the back of the body is only known from isolated material and thus difficult to reconstruct. Based on fragmentary remains described by Cunha and colleagues, the osteoderms of the tail ranged from being rectangular in shape to being subtriangular, but generally wider than they were long like those of the torso. A variety of accessory osteoderms are also known.

While Armadillosuchus, alongside Simosuchus, is among the most heavily armored Notosuchian, a 2022 study suggests that the osteoderms may not have been visible in life in the same way they appear in modern crocodilians. The paper specifically covers Sharpey's fibers, a collagen structure anchoring the osteoderms to the underlying layer of the skin, known as the stratum compactum. While in modern crocodilians these fibers help keep the bottom of the osteoderms connected to the skin, the study shows that in many Notosuchians including Armadillosuchus, Sharpey's fibers also run perpendicular to their outer surface. A similar condition can be seen in today's leatherback and softshell turtles, leading the authors to suggest that the osteoderms of Armadillosuchus were deeply embedded into the skin of the animal and covered in a leathery dermis. This may have had the advantage of increased flexibility.

Phylogeny

Recent phylogenetic analysis consistently show that Armadillosuchus is a derived member of the Sphagesauridae, a family of Notosuchians endemic to South America known for their heterodont dentition. Research has found that sphagesaurids can generally be divided into short-snouted, small-bodied and generally more gracile taxa such as Adamantinasuchus and much larger, more robust and better armored animals, which includes Armadillosuchus. Within this group Armadillosuchus appears to be most closely related to Caryonosuchus. The phylogenetic tree below shows the result of the phylogenetic analysis conducted by Cunha et al. (2020) following the discovery of the remains of an unnamed species of Armadillosuchus. Their analysis suggests a clear link between Armadillosuchus arrudai and the as of yet unnamed second species, but find the two to be placed in a polytomy alongside Caryonosuchus.

Paleobiology

Metabolism

Research conducted to determine the resting metabolic rate as well as the estimated dimensions of the red blood cells indicate that Armadillosuchus was an ectothermic animal like modern crocodilians. However, despite this Armadillosuchus and other terrestrial Notosuchians may have regulated their internal temperature in a manner more similar to modern monitor lizards. It is possible that they would have basked in sunlight to raise their body temperature and stuck to sunlit areas while foraging, but retreated into burrows should they be under threat of overheating. During basking, the osteoderms may have helped to absorb and retain heat over extended periods of time, helping the animal to maintain high levels of activity even without direct sunlight.

Diet and foraging

The teeth of Armadillosuchus give several clues to its ecology. Both their morphology and the way they are arranged give the animal powerful, scissor-like jaws. Wear facets seen on the teeth suggests that the animal performed back and forward movement with its jaws in a chewing motion similar to Sphagesaurus. The precise way the teeth occlude, with the fifth dentary tooth being located just before the third maxillary tooth, differs significantly from Sphagesaurus however and it has been suggested that this might relate to the animal foraging beneath ground. Additional evidence for such habits can be found in the robust anatomy of the shoulder girdle, the long claws on its forelimbs and the robust neck. The forelimbs in particular may have been used either to create burrows or to dig while foraging, with the former matching with the inferred thermoregulatory behavior of Notosuchians. One hypothesis on the dietary preferences of this animal proposes that Armadillosuchus was an omnivore, feeding on a variety of animals and plants including molluscs, arthropods, roots and pines. Although no remains of molluscs were found in the direct vicinity of the fossils, the fact that the Adamantina Formation featured rivers and lakes and preserves the remains of bivalves in other localities still lends support to this hypothesis. It is also possible that Armadillosuchus scavenged at least occasionally and that the tooth wear seen in the known material was caused by the animal feeding on dried up carcasses. Besides the tubercles on the keels of the molariform teeth and the ability to move its jaws back and forth in a grinding motion, Armadillosuchus may have also employed the help of gastroliths to deal with grind food.

A 2019 study more closely examined the teeth of Armadillosuchus using a method known as orientation patch count rotated (OPCR), originally designed to analyse the complexity of the tooth occlusion in rodents. This method has previously shown a clear relation between the morphology of premolar and molar teeth and the diet of various animal groups, including reptiles. Both the value of the most complex tooth and the overall average of Armadillosuchus were found to form an intermediate between a carnivorous diet (represented by Boverisuchus and Notosuchus) and durophages (represented by Allognathosuchus and Brachychampsa). While this could be indicative of both insectivorous or omnivorous diet, the latter is more likely given the large size of Armadillosuchus.

Defense

In addition to being favorable for digging, the stout and robust body of Armadillosuchus, combined with the underlying armor plating formed by the osteoderms, may have also been an effective deterrent against predators. The extensive armor of Armadillosuchus has been likened not just to armadillos but also to the osteoderms of ankylosaurids. This unifies the large bodied sphagesaurids like Armadillosuchus and Caryonosuchus and sets them apart from the more gracile early sphagesaurids like Caipirasuchus.