Probrachylophosaurus

The generic name is derived from the Latin Pro- , which means “in front of” and the Greek brachylophosaurus , which means “lizard with a short crest” and refers to the fact that in the stratigraphic plan the remains of the prochrachilophosaurus lie lower than the remains of the brachilophosaurus . The species name is named after Sam Berge, co-owner of the land where dinosaur remains were found, as well as a friend and relative of many members of the village of Rudyard , Montana , who supported paleontological research for many decades ^[1] .

The holotype MOR 2919 and the paratype MOR 1097 were found in the Judith River formation layer, dated to the Campanian , 79.5–79.8 Ma ago, north of Rudyard, Montana, USA .

The holotype includes most of the skull and skeleton; the skeleton is not articulated. Cranial material includes fragmentary right maxillary bone, both jaw bones, left zygomatic bone, partial right lacrimal bone, posterior part of the left nasal bone, partial middle part of the right nasal bone, articulated cerebral box (frontal, parietal, postorbital and occipital bones), both scaly bones, both square, the predental bone, both dental bones and the right arched bone. The postcranial skeleton includes atlant fragments and at least 10 other cervical vertebrae, 11 spinal and 29 caudal vertebrae, 19 chevrons, approximately 19 ribs, both ilium, both pubic and ischium, both tibia and fibula, both talus, right second and fourth metatarsal bones. The forelimbs and sacrum are absent, although individual spinous processes may belong to the sacral vertebrae.

The paratype consists of the fragmented cranial material of a young individual and includes the right posterior part of the nasal crest, the right zygomatic bone, coronoid process of the left dentition, dentinal dentition, maxillary dentition, partial left prefrontal bone, and dorsal and ventral condyles of the right square bone ^[1] .

Prochrachilophosaurus is a hadrosaurin hadrosaurid diagnosed by the following criteria: in an adult, a solid crest composed entirely of the nasal bone, which hangs less than 2 cm above the supra-temporal openings; the nasal crest is dorsoventrally strongly thickened in the middle part, as a result of which the paired nasal bones, when viewed from the rear, form a triangular frontal plane of the cross section with a dorsal angle of less than 130 degrees. These autapomorphies together with the following features form a unique combination of characters: the lacrimal bone is mediolaterally dilated posteriorly, as in Acristavus , but not like in the brachilophosaurus, the caudoventral apex of the rostral process of the zygomatic bone is located behind the caudodorsal apex, in the same way as in Acristavus , but not like the bony brachilosa medially in contact with each other, as in Acristavus , but not like in the brachilophosaurus, the rear-oriented continuous nasal crest in the same way as in the brachilophosaurus, but not like in Acristavus ^[1] .

Prochrachilophosaurus had a length of 9 meters and a weight of 5 tons ^[2] .

According to the results of phylogenetic analysis based on the 2010 Prieto-Marquez matrix, the four most economical trees were obtained. A strict consensus distinguishes the brachilophosaurus and mayasaurus as sister taxa, while the prochrachilophosaurus and Acristavus form a basal polytomy in Brachylophosaurini . The majority consensus in 55% of cladograms supports the Prochrachilophosaurus - Brachilophosaurus - Mayasaurus treasure with Acristavus as a basal member of Brachylophosaurini, and 64% supports the Brachilophosaurus and Mayasaurus as sister taxa. After modifying the matrix (recoding and eliminating some features), the two most economical trees were obtained. The strict consensus of these trees distinguishes Acristavus as a basal member of Brachylophosaurini, with the brachilophosaurus and mayasaur as sister taxa and the prochrachilophosaurus, which occupies an intermediate position. Compared to the previous result, the consensus support for the majority of the clade pro-brachilophosaurus – brachilophosaurus – mayasaurus with Acristavus as a basal member of Brachylophosaurini increased from 55 to 66%, and the support for the clades of the brachilophosaurus and mayasaurs decreased from 64 to 56%.

Cladogram based on strict consensus of two frugal trees built on the basis of a modified Prieto-Marquez matrix, 2010:

After the inclusion of the prochrachilophosaurus in the modified matrix of Gates et al. In 2011, two of the most economical trees were obtained. A strict consensus highlights the prochrachilophosaurus and brachilophosaurus as sister taxa with Acristavus as the most basic member of Brachylophosaurini. The majority consensus in 50% of cladograms was supported by the prochrachilophosaurus - brachilophosaurus.

Cladogram based on the strict consensus of two frugal trees built on the basis of a modified matrix by Gates et al., 2011:

The holotype left tibia includes 14 growth retardation lines . Since growth retardation lines represent annual pauses in bone growth, this indicates that MOR 2919 was 14 years old at the time of death. The lines farthest from the center (11-14) are located close to each other, but still retained blood supply. This indicates that the tibia of MOR 2919 was still growing in breadth, and therefore, at the time of death, the length of the dinosaur and its mass did not reach a maximum.

The level of bone apposition in hadrosaurids is usually higher on the posteromedial side of the bone cortex than on the anterolateral. In MOR 2919, this is represented by thick zones between the early growth retardation lines on the posteromedial side. Later, layering continued on the posteromedial side, while it stopped on the lateral side, as a result of which the greatest number of growth retardation lines (14) are present on the posteromedial side. The number of growth retardation lines decreases along the posterolateral (from 12 to 8) and anteromedial (from 13 to 11) sides of the cortex; their smallest number is concentrated on the lateral side (8). Lines 1-12 are clearly distinguishable and visible in tenfold magnification. Lines 13, 14 are visible at a magnification of forty. Line 13 is observed as a dark strip on which at times one or more lines can be distinguished. Thus, line 13 may actually not be a growth retardation line, just at some point, growth slowed down, but did not stop.

As a rule, the thickness of the zones (the interval between the lines) decreased every year, which indicates a rapid growth rate in the first few years of life with its subsequent slowdown. However, lines 5, 6, 7 are located relatively close to each other, with growth zones narrower than between subsequent lines. This may indicate that two growth periods between lines 5 and 6 and lines 6 and 7 were difficult for MOR 2919 due to low access to resources, so the growth rate slowed down.

Line 8, in fact, is a zone with closely spaced lines (1 to 5). Similar to the localization of the extreme growth retardation lines, zone 8 contains the largest number of lines (5) on the posteromedial side of the tibia, which gradually decrease along the circumference to a single line along the lateral side. This zone is interpreted as the result of a moderately unfavorable season, possibly a mild winter, during which small increments of bone layers occurred between several growth pauses, and not after one large delay for the entire period of adverse conditions.

The inflection points of the growth curve in MOR 2919 occur later than in the mayasaurs, which indicates later puberty and puberty. While the mayasaur hypothetically reaches puberty between 2 and 3 years old, similar inflection points of the growth curve in MOR 2919 are observed at 5 years old. This suggests that prochrachilophosaurus has a delay in puberty in relation to the mayasaur.

The second inflection point on the Mysaurus growth curve indicates that skeletal maturity has been reached. The growth rate of MOR 2919 is reduced at the second inflection point, but the external fundamental system (the microstructure of the bone that appears on the outer layer of the cortex and marks the complete development of the skeleton) did not develop, so MOR 2919 did not reach true skeletal maturity. The second decrease in growth rate occurs in MOR 2919 later (at 10 years) than at mayazavra (8 years). All this suggests that the prochrachilophosaurus reached the age of skeletal maturity later than the mayasaurs. However, the age of skeletal maturity in mayasaurs is an individual variable, therefore, it is necessary to increase the sample size of the prochrachilophosaurus in order to determine whether there really is a delay in maturation with respect to the mayasaurus or is it simply a reflection of individual differences in growth rates. In both cases, the histology of MOR 2919 suggests that the prochrachilophosaurus is a larger individual than the mayasaur at all stages of growth ^[1] .