Brachylophosaurus

Size comparison

Brachylophosaurus is notable for its bony crest, which forms a horizontally flat, paddle-like plate over the top of the rear skull.[12] Some, depending on their age, had crests that covered nearly the entire skull roof, while others had shorter, narrower crests.[5] Some researchers suggest it was used for pushing contests,[13][14] but it may not have been strong enough for this. Other notable features are a relatively small head, the unusually long lower arms and the beak of the upper jaw being wider than other hadrosaurs of that time.[12]

Apart from the above, Brachylophosaurus was a typical hadrosaur which reached an adult length of at least 9 metres (30 ft).[12] In 2010, Gregory S. Paul estimated maximum length at 11 metres (36 ft) resulting in weight of 7 metric tons (7.7 short tons).[15] Like other hadrosaurs, Brachylophosaurus had features like cheeks to keep fodder in the mouth and dental batteries with hundreds of stacked teeth.[12] These teeth could be used to chew efficiently,[12] a feature rare among reptiles, but common among some cerapodan ornithischian dinosaurs like Brachylophosaurus.

Restoration

In 2015, Jack Horner established some distinguishing traits. Two of these are autapomorphies, unique derived characters. The crest formed by the nasal bones is flat and paddle-shaped in adult individuals and largely or totally overhangs the supratemporal fenestrae. The rear edge of the prefrontal bone overgrowths the frontal bone and more to the rear is oriented inwards and downwards to support the base of the crest and contribute to the edge of the supratemporal fenestra. Additionally, there is one trait that is not unique in itself but forms a unique combination with the two autapomorphies: the front branch of the lacrimal bone is extremely elongated and with its tip only touches the maxillary bone.[2]

Partial tail with tendons

The head of Brachylophosaurus is elongated. It is wide at the rear and very narrow along most of the length of the snout. The upper beak however, abruptly widens at its rear edge, forming a broad bone core for a horn sheath. The nostrils are extremely large and between them the nasal bones form a narrow tall bone wall on top of much of the snout. More to behind the nasal bones stretch out horizontally, creating a flat tongue-shaped skull crest that overgrowths and ultimately overhangs, most of the skull roof. The crest is not hollow but consists of massive bone. The crest has a low longitudinal ridge on the midline.

The maxilla, the tooth-bearing upper jaw bone, is rather elongated in front. Its tooth positions increase during the lifetime of the animal, ranging from thirty-three in younger individuals to forty-eight in the holotype specimen. The teeth are stacked in a tooth battery, with up to three teeth per position. The battery forms a sharp cutting edge, bending inwards, with one or sometimes two teeth per position contributing to the attrition surface. More to behind, the lower jugal bones and quadrate bones flare out sidewards, so that the skull is much wider at its rear lower edges than at the top surface, resulting in a trapezium-shaped profile in posterior view.

Underside of "Leonardo"'s head and neck, showing skin impressions

Several so-called "mummies" provide information about the soft tissues of Brachylophosaurus. These "mummies" actually consist of natural casts formed in moulds in the stone matrix surrounding the skeleton, preserving the outline of the body and showing skin imprints. The best studied "mummy" has been "Leonardo", a specimen 90% of the cast surface of which is covered by imprints. Generally, the surface is close to the bones, which could be caused by desiccation before burial or the compressive action of the covering sediment. An exception is the region around the right shoulder, which shows the profile of about six centimetres thick muscles. "Leonardo" also indicates that the base of the neck was heavily muscled and that the soft tissue upper neck profile was placed in an elevated position, running much higher than was usually reconstructed in drawings which tended to follow the curvature of the vertebral column, and filling much of the bend between the front back and the head.[7]

On the snout, the remains of a broad keratinous beak are visible. The skin impressions show many folds and a structure of small polygonal scales. On the back a midline frill formed by triangular or hatchet-shaped projections is present. These seem to be individually separated and are placed as extensions of each neural spine of the vertebral column. The second, third and fourth finger of the hand are contained in a shared soft tissue "mitten".[7]

Examination of the stomach of "Leonardo" also reveals that the dinosaur was parasitized by small, needle-like worms covered in fine bristles. The discovery indicates that other dinosaur species might have been hosts of similar parasites.[16]

A 2008 study conducted on the famous dinosaur mummy Leonardo found that Brachylophosaurus had a diet that consisted of leaves, conifers, ferns, and flowering plants like magnolias. The study also found that Brachylophosaurus was a generalist herbivore; being both a browser and a grazer, but it did more of the former rather than the latter due to the contents found in its stomach.[19][20][21][22][23][24][25]