Hypacrosaurus

H. stebingeri holotype skull

H. altispinus, the type species, is known from 5 to 10 articulated skulls with some associated skeletal remains, from juvenile to adult individuals found in the Horseshoe Canyon Formation. H. stebingeri is known from an unknown but substantial number of individuals, with an age range of embryos to adults.[9] The hypothesis that H. altispinus and H. stebingeri form a natural group excluding other known hadrosaur species may be incorrect, as noted in Suzuki et al.'s 2004 redescription of Nipponosaurus; their phylogenetic analysis found that Nipponosaurus was more closely related to H. altispinus than H. stebingeri was to H. altispinus.[11] This was rejected by Evans and Reisz (2007), though.[12]

The new species Hypacrosaurus stebingeri was named for a variety of remains, including hatchlings with associated eggs and nests, found near the top of the late Campanian (Upper Cretaceous) Two Medicine Formation in Glacier County, Montana, and across the border in Alberta. These represent "the largest collection of baby skeletal material of any single species of hadrosaur known".[10]

H. altispinus skull, AMNH

The hollow crest of Hypacrosaurus most likely had social functions, such as a visual signal allowing individuals to identify sex or species, and providing a resonating chamber for making noises.[9] The crest and its associated nasal passages have also figured in the debate about dinosaur endothermy, specifically in discussions about nasal turbinates.

Turbinates are thin bones or cartilages that come in two types, with two functions. Nasal olfactory turbinates are found in all living tetrapods and function in smell. Respiratory turbinates function to prevent water loss through evaporation and are found only in birds and mammals, modern endotherms (warm-blooded animals) who could lose a great deal of water while breathing because they breathe more often than comparably sized ectotherms (cold-blooded animals) to support their higher metabolism.[18] Ruben and others in 1996 concluded that respiratory turbinates were probably not present in Nanotyrannus, Ornithomimus or Hypacrosaurus based on CT scanning, thus there was no evidence that those animals were warm-blooded.[19]

Examining the oxygen-isotope ratio from the bones from different parts of an extinct animal's body should indicate which thermoregulation mode an animal used during its lifetime. An endothermic (warm-blooded) animal should maintain a very similar body temperature throughout its entire body (which is called homeothermy) and therefore there should be little variation in the oxygen-isotope ratio when measured in different bones. Alternatively, the oxygen-isotope ratio differs considerably when measured throughout the body of an organism with an ectothermic (cold-blooded) physiology.[20] Oxygen-isotope ratios calculated for Hypacrosaurus suggesting that the ratios varied little, indicating that Hypacrosaurus was a homeotherm, and likely was endothermic.[21] This is in contrast to the Ruben et al. (1996) finding that Hypacrosaurus was not warm-blooded, which was based on the absence of nasal turbinates (see Crest functions subsection, above).

H. stebingeri nest

Hypacrosaurus stebingeri laid roughly spherical eggs of 20 by 18.5 cm (7.9 by 7.3 in), with embryos 60 cm (24 in) long. Hatchlings were around 1.7 m (5.6 ft) long. Studies of lines of growth (i.e. lines of von Ebner) in the teeth of embryonic H. stebingeri suggests plesiomorphically long incubation times, with a minimum incubation time of 171.4 days for H. stebingeri.[22] Young and embryonic individuals had deep skulls with only slight expansion in the bones that would one day form the crest.[10] Growth was faster than that of an alligator and comparable to ratite growth, for several years, based on the amount of bone growth seen between lines of arrested growth (analogous to growth rings in trees).[23] Research by Lisa Cooper and colleagues on H. stebingeri indicates that this animal may have reached reproductive maturity at the age of 2 to 3 years, and reached full size at about 10 to 12 years old. The circumference of the thigh bone at postulated reproductive maturity was about 40% that of its circumference at full size. The postulated growth rate of H. stebingeri outpaces those of tyrannosaurids (predators of hypacrosaurs) such as Albertosaurus and Tyrannosaurus; rapidly growing hypacrosaurs would have had a better chance to reach a size large enough to be of defensive value, and beginning reproduction at an early age would also have been advantageous to a prey animal.[24] Secondary cartilage has been found in the skull of a hatchling specimen of H. stebingeri.[25]

Hypacrosaurus MOR 548 supraoccipital with microscopic magnification showing the preserved chondrocyte-like structures, in comparison to those of an emu

In 2020, Alida M. Bailleul and colleagues reported cartilage traces on a hatchling specimen of H. stebingeri. The team performed histological analyses on skull and limb bones of nestling individuals of the specimen MOR 548, a large nesting ground in the Two Medicine Formation attributed to H. stebingeri, and the results showed calcified cartilage within a supraoccipital bone, and upon microscopic magnification, chondrocyte-like structures were found. Several of these structures were preserved in the final stages of mitosis, with some preserving putative traces of celular nuclei. Bailleul and colleagues isolated some of these cells in order to be tested with DNA staining: stains DAPI and PI. They also exposed emu chondrocytes, and these tied up to DNA fragments. H. stebingeri cells tested positive to possible chemical markers of DNA, in a similar way to the emu cells, suggesting the potential preservation of this molecule. The team concluded that the find was not a product of fossil contamination, and DNA may last much longer than previously assumed.[26]

The discovery of tooth marks in the fibula of a Hypacrosaurus specimen inflicted by a bite from the teeth of a tyrannosaurid indicated that this, and other hadrosaurids were either preyed upon or scavenged by large theropod dinosaurs during the Late Cretaceous period.

The large, monospecific assemblage of Hypacrosaurus stebingeri in Montana was interpreted as a group of dinosaurs that was killed by a volcanic ashfall.[20] This assemblage is considered autochthonous, meaning that the remains are thought to have been buried on or near the same spot where the individuals died. The variety of ages in this group supports that this was a biocoenosis—an actual life assemblage of animals.[27] The cause of death in a volcanic ashfall is suffocation by the ash and by the gases released from volcanic eruptions. The preservation of this diverse group of dinosaurs provides researchers with a growth series, which is a sequence of growth stages from juvenile to adult.

Restoration of H. altispinus in environment

H. altispinus shared the Horseshoe Canyon Formation with fellow hadrosaurids Edmontosaurus and Saurolophus, hypsilophodont Parksosaurus, ankylosaurid Anodontosaurus, nodosaurid Edmontonia, horned dinosaurs Montanoceratops, Anchiceratops, Arrhinoceratops, and Pachyrhinosaurus, pachycephalosaurid Stegoceras, ostrich-mimics Ornithomimus and Struthiomimus, a variety of poorly known small theropods including troodontids and dromaeosaurids, and the tyrannosaurs Albertosaurus and Daspletosaurus.[28] The dinosaurs from this formation are sometimes known as Edmontonian, after a land mammal age, and are distinct from those in the formations above and below.[29] The Horseshoe Canyon Formation is interpreted as having a significant marine influence, due to an encroaching Western Interior Seaway, the shallow sea that covered the midsection of North America through much of the Cretaceous.[29] H. altispinus may have preferred to stay more landward.[9]

The slightly older Two Medicine Formation, home to H. stebingeri, was also populated by another well-known nesting hadrosaur, Maiasaura, as well as the troodontid Troodon, which is also known from nesting traces. The tyrannosaurid Daspletosaurus, caenagnathid Chirostenotes, dromaeosaurids Bambiraptor and Saurornitholestes, armored dinosaurs Edmontonia, Oohkotokia, and Scolosaurus, hypsilophodont Orodromeus, hadrosaur Prosaurolophus, and horned dinosaurs Achelousaurus, Brachyceratops, Einiosaurus, and Rubeosaurus were also present.[28] This formation was more distant from the Western Interior Seaway, higher and drier, with a more terrestrial influence.[30]