Although Deinosuchus was far larger than any modern crocodile or alligator, with the largest adults measuring almost 11 m (36 ft) in total length, its overall appearance was fairly similar to its smaller relatives. It had large, robust teeth built for crushing, and its back was covered with thick hemispherical osteoderms. One study indicated Deinosuchus may have lived for up to 50 years, growing at a rate similar to that of modern crocodilians, but maintaining this growth over a much longer time.
Deinosuchus fossils have been found in 10 US states, including Texas, Montana, and many along the East Coast. Fossils have also been found in northern Mexico. It lived on both sides of the Western Interior Seaway, and was an opportunistic apex predator in the coastal regions of eastern North America. Deinosuchus reached its largest size in its western habitat, but the eastern populations were far more abundant. Opinion remains divided as to whether these two populations represent separate species. Deinosuchus was probably capable of killing and eating large dinosaurs. It may have also fed upon sea turtles, fish, and other aquatic and terrestrial prey.
Despite its large size, the overall appearance of Deinosuchus was not considerably different from that of modern crocodilians. Deinosuchus had an alligator-like, broad snout, with a slightly bulbous tip. Each premaxilla contained four teeth, with the pair nearest to the tip of the snout being significantly smaller than the other two. Each maxilla (the main tooth-bearing bone in the upper jaw) contained 21 or 22 teeth. The tooth count for each dentary (tooth-bearing bone in the lower jaw) was at least 22. All the teeth were very thick and robust; those close to the rear of the jaws were short, rounded, and blunt. They appear to have been adapted for crushing, rather than piercing. When the mouth was closed, only the fourth tooth of the lower jaw would have been visible.
Modern saltwater crocodiles, with the strongest bite of any living animal, have a maximum force of 16,460 N (3,700 lbf). The bite force of Deinosuchus has been estimated to exceed 18,000 N (4,000 lbf). Even the largest and strongest theropod dinosaurs, such as Tyrannosaurus, probably had a bite force inferior to that of Deinosuchus.
Deinosuchus had a secondary bony palate, which would have permitted it to breathe through its nostrils while the rest of the head remained submerged underwater. The vertebrae were articulated in a procoelous manner, meaning they had a concave hollow on the front end and a convex bulge on the rear; these would have fit together to produce a ball and socket joint. The secondary palate and procoelous vertebrae are advanced features also found in modern eusuchian crocodilians.
The osteoderms (scutes) covering the back of Deinosuchus were unusually large, heavy, and deeply pitted; some were of a roughly hemispherical shape. Deep pits and grooves on these osteoderms served as attachment points for connective tissue. Together, the osteoderms and connective tissue would have served as load-bearing reinforcement to support the massive body of Deinosuchus out of water. Consequently, despite its bulk, Deinosuchus was probably almost as agile on land as its modern relatives.
Despite the fragmentary nature of the fossils assigned to Deinosuchus, the giant size of its remains has always been recognized, but estimates of how big it really was have varied over the years. The original estimates from the 1950s for the type specimen of the then named "Phobosuchus riograndensis" were a skull 1.5 m (5 ft) long, reconstructed based on the Cuban crocodile and a total length of 15 m (50 ft), this reconstruction, however, is currently considered inaccurate and too big for that specimen. Based on sufficiently complete remains, it was estimated in 1999 that the size attained by specimens of Deinosuchus varied between 8 to 10 m (26 to 33 ft) and weighted between 2.5 to 5 t (3 to 6 short tons), this was corroborated later, when it was noted that most known specimens of D. rugosus usually had skulls about 1 m (3.3 ft) with estimated total lengths of 8 m (26 ft) and weights of 2.3 t (3 short tons) however, the biggest fragmentary remains of D. riograndensis were 1.5 times bigger than those of the average D. rugosus and it was determined that the largest individuals of this species could have been up to 12 m (39 ft) in length and perhaps weighed 8.5 t (9.4 short tons).
More recent and thought-out estimates based on regression equations for the American alligator put the total body length attained by the largest known specimen of D. riograndensis, a mandibular fragment estimated to have come from a 1.47 m (4.8 ft) long skull, at 10.6 metres (35 ft). Deinosuchus has often been described as the largest crocodilian of all time, but some other crocodyliforms including Purussaurus, Rhamphosuchus, and Sarcosuchus may have equaled or exceeded it in size
Deinosuchus was present on both sides of the Western Interior Seaway. Specimens have been found in 10 of the modern-day United States. A Deinosuchus osteoderm from the San Carlos Formation was also reported in 2006, so the giant crocodilian's range may have included parts of northern Mexico. Deinosuchus fossils are most abundant in the Gulf Coastal Plain region of Georgia, near the Alabama border. All known specimens of Deinosuchus were found in rocks dated to the Campanian stage of the Late Cretaceous period. The oldest examples of this genus lived approximately 80 Ma, and the youngest lived around 73 Ma.
The distribution of Deinosuchus specimens indicates these giant crocodilians may have preferred estuarine environments. In the Aguja Formation of Texas, where some of the largest specimens of Deinosuchus have been found, these massive predators probably inhabited brackish-water bays. Although some specimens have also been found in marine deposits, it is not clear whether Deinosuchus ventured out into the ocean (like modern-day saltwater crocodiles); these remains might have been displaced after the animals died. Deinosuchus has been described as a "conspicuous" component of a purportedly distinct biome occupying the southern half of Late Cretaceous North America.
In 1954, Edwin H. Colbert and Roland T. Bird speculated that Deinosuchus "may very well have hunted and devoured some of the dinosaurs with which it was contemporaneous". Colbert restated this hypothesis more confidently in 1961: "Certainly this crocodile must have been a predator of dinosaurs; otherwise why would it have been so overwhelmingly gigantic? It hunted in the water where the giant theropods could not go." David R. Schwimmer proposed in 2002 that several hadrosaurid tail vertebrae found near Big Bend National Park show evidence of Deinosuchus tooth marks, strengthening the hypothesis that Deinosuchus fed on dinosaurs in at least some instances. In 2003, Christopher A. Brochu agreed that Deinosuchus "probably dined on ornithopods from time to time." Deinosuchus is generally thought to have employed hunting tactics similar to those of modern crocodilians, ambushing dinosaurs and other terrestrial animals at the water's edge and then submerging them until they drowned. A 2014 study suggested that it would had been able to perform a "death roll", like modern crocodiles.
Schwimmer and G. Dent Williams proposed in 1996 that Deinosuchus may have preyed on marine turtles. Deinosuchus would probably have used the robust, flat teeth near the back of its jaws to crush the turtle shells. The "side-necked" sea turtle Bothremys was especially common in the eastern habitat of Deinosuchus, and several of its shells have been found with bite marks that were most likely inflicted by the giant crocodilian.
Schwimmer concluded in 2002 that the feeding patterns of Deinosuchus most likely varied by geographic location; the smaller Deinosuchus specimens of eastern North America would have been opportunistic feeders in an ecological niche similar to that of the modern American alligator. They would have consumed marine turtles, large fish, and smaller dinosaurs. The bigger, but less common, Deinosuchus that lived in Texas and Montana might have been more specialized hunters, capturing and eating large dinosaurs. Schwimmer noted no theropod dinosaurs in Deinosuchus's eastern range approached its size, indicating the massive crocodilian could have been the region's apex predator.
A 1999 study by Gregory M. Erickson and Christopher A. Brochu suggested the growth rate of Deinosuchus was comparable to that of modern crocodilians, but was maintained over a far longer time. Their estimates, based on growth rings in the dorsal osteoderms of various specimens, indicated each Deinosuchus might have taken over 35 years to reach full adult size, and the oldest individuals may have lived for more than 50 years. This was a completely different growth strategy than that of large dinosaurs, which reached adult size much more quickly and had shorter lifespans. According to Erickson, a full-grown Deinosuchus "must have seen several generations of dinosaurs come and go".
Schwimmer noted in 2002 that Erickson and Brochu's assumptions about growth rates are only valid if the osteodermal rings reflect annual periods, as they do in modern crocodilians. According to Schwimmer, the growth ring patterns observed could have been affected by a variety of factors, including "migrations of their prey, wet-dry seasonal climate variations, or oceanic circulation and nutrient cycles". If the ring cycle were biannual rather than annual, this might indicate Deinosuchus grew faster than modern crocodilians, and had a similar maximum lifespan.