Figure 1.
Hypsilophodontid ontogenetic bone microstructure.
The bone microstructures observed in hypsilophodontids from Victoria (left column) suggest ontogenetic status, and strongly resemble microstructures observed in hypsilophodontids from lower latitudes (right column; Museum of the Rockies- MOR; Brigham Young University- BYU). Arrows indicate LAGs. Scale bars, 500 µm. A) Femur (NMV 216768) diaphyseal section from a skeletally immature polar hypsilophodontid. Growth marks are absent and tissue consists of rapidly deposited, disorganized woven fibers. This resembles the femoral diaphyseal section in B), from an Orodromeus (MOR 407). C) Diaphyseal section of a femur (NMV 208495) showing one LAG within fast growing woven tissue. LAGs within woven tissue are also observed in D), a femur from a Dryosaurus (BYU 13312). E) Diaphyseal section of a tibia (NMV 228434) consisting of three well-defined LAGs within loosely parallel-fibered cortex, representing a slowly growing individual approaching skeletal maturity. A similar microstructural pattern is observed in F), an Orodromeus tibia (MOR 973) possessing LAGs within a parallel-fibered matrix.
Figure 2.
Bone microstructure of polar hypsilophodontid approaching skeletal maturity.
Arrows indicate annuli. Scale bars, 500 µm. A) Diaphyseal section of femur NMV 177935 consisting of parallel-fibered tissue with two growth marks within the cortex. B) Detail of inner growth mark showing a distinctive annulus terminating with a bright line.
Table 1.
Data for Australian polar dinosaur fossils included in this study.
Figure 3.
Diaphyseal sections of high-latitude and lower-latitude theropods. Scale bars, 500 µm. A) Theropod femur diaphyseal section from Victoria (NMV 186323). Note the well-vascularized cortex consisting of rapidly deposited tissue. B) Troodon tibia (MOR 563) from Montana, USA, showing a similarly well-vascularized cortex and rapidly deposited tissue.