Tyrannosaurus Osborn 1905
Creators: name: Erickson, Gregory M.
Publisher: name: Zenodo
Year: 2005
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Reptilia
Dinosauria
Tyrannosauridae
Tyrannosaurus

description: One of the oldest questions in paleontology is how dinosaurs attained giant size. Increased phylogenetic resolution has enabled Carrano to conclude that dinosaurian lineages attained enormous proportions (3+ tons) on at least seven or eight occasions [48]. How did these events occur? Evolutionary theory offers three possible solutions: (i) acceleration, whereby growth rates increased from those present in their ancestors; (ii) delay in the onset of maturity; or (iii) through a combination of both processes [49]. A growth-curve study published by my research group focused on the evolution of the enormous Tyrannosaurus within Tyrannosauria [12] and showed that acceleration in growth rates of fourfold or more was the key to the great stature of this taxon (Box 3). A subsequent study by Sander and colleagues looked at the same phenomenon within the Sauropodomorpha and interestingly revealed the same pattern [13].   
These findings stand in contrast to three earlier osteohistological studies on gigantism in the dinosaurian outgroups Lepidosauria [30], Crocodyliformes [50] and Crocodylia [51], which all revealed retention of ancestral growth rates and delays in the onset of maturity. It will be interesting to see in the future if all cases of dinosaurian gigantism involved acceleration.  
Of course, not all dinosaurs were large. By which heterochronic mechanism(s) did dinosaurs become smaller? There have been several explorations of this phenomenon using osteohistology. One of the most interesting relates to the idea that relatively small dinosaurs found in Eastern Europe (e.g. hadrosaurs and sauropods) are island dwarfs [52]. Sander and colleagues tested this hypothesis using diminutive sauropod specimens from Germany [15,16]. They believe that they have found evidence that individuals just 8–9 years of age show histological attributes (tightly packed growth lines called an external fundamental system or EFS that suggest growth was plateauing) indicating full adult size. The same EFS structures occurred at ages of 15 years or later in their giant relatives [15]. As in cases of dwarfism in proboscidians (elephants) on islands [53], it is believed that selection for smaller sizes enabled these animals to maintain viable population sizes with limited resources.  
 Along the same lines, a heated debate surrounds whether Nanotyrannus, a purported dwarf species of tyrannosaur, is in fact just a juvenile of Tyrannosaurus [54,55]. Anatomical studies have revealed juvenile attributes in support of the latter hypothesis [55,56]. However, if the dwarfing event simply involved early sexual maturation, immature features might still be expected. I recently aged one of these specimens (Burpee Museum of Natural History, Rockford, BMR P2002.4.1) to see if it has adult histological features or falls outside the confidence interval for   
 Tyrannosaurus development [12].   
The 11-year-old specimen plots on the growth curve. This lends support to it being a juvenile of the larger taxon (unless of course maturity occurred somewhat later in development) and suggests there was only one large carnivorous taxon in the Latest Maastrichtian of North America.  
 The rediscovery that birds are theropod dinosaurs [57,58] has led to interest in how early birds, such as Archaeopteryx, attained small size [10,11,43]. A survey of dinosaurian and avian osteohistological types and formative rates using Amprino’s rule led to the conclusion that the diminutive size of the first birds was brought about by a decrease in the length development compared with that of their dinosaurian ancestors [11]. It was posited that selection favored reduced body size because it enabled decreases in wing loading and improved powerto-weight ratios. However, the latest discoveries of Archaeopteryx -sized theropods, such as Microraptor and Sinosauropteryx have led by Xu, Hwang and colleagues to conclude that this miniaturization event actually occurred before the cladogenesis of birds and was not driven by selection related to flight [59 – 61]. How then did these dinosaurs become small? The aforementioned dinosaur growth-rate regression (Box 4) and associated theropod growth curves suggest that absolute decreases in growth rates and truncated development facilitated dwarfing [10].   Late developmental patterns  
In addition to revealing aspects of evolutionary changes in growth rates, osteohistology has utility for analyzing late developmental patterns in dinosaurs. Some of the first dinosaur growth studies reported the absence of EFS structuring that would indicate the specimens were full adult size [21,26,62]. It was theorized that these dinosaurs had an indeterminate growth strategy [19,21,26,62] (i.e. the capacity to grow appreciably throughout life; this is not to be confused with the more common use of this term in ecology, where it refers to sexual maturation before the attainment of maximal body size [63,64]). Subsequent osteohistological analyses have since revealed that EFS structuring is in fact commonplace and it appears that all dinosaurs had determinant growth strategies [10,12,19,21,22,24,31,65]. Growth curves graphically reveal these size plateaus and show that some species of sauropods and tyrannosaurs spent as much as 30% of their total lifespan as full-grown adults [10,12,23]. These growth curve studies also point to an interesting taphonomic conclusion: most specimens in museums are not full-sized adults. Perhaps this is to be expected given that for every specimen that reached late adulthood, many younger individuals perished and are more likely to be represented in the fossil record.  
 When did the dinosaurs reach somatic maturity (i.e. adult body size)? Growth curves derived from various laboratories reveal that this occurred at an age of 2.5–3.0 years in tiny theropods such as Shuvuuia [10], at ~4–12 years in small- to moderate-sized dinosaurs such as Syntarsus and Massospondylus [21], at ~16.0–18.5 years in large dinosaurs such as Albertosaurus and Tyrannosaurus [12,24], and at ~20–26 years in giant sauropods, such as Lapparentosaurus [62] and Janeschia [23].   
It has been speculated that precipitous rate changes in growth curves [23] (Box 3) or EFS bone structuring [19,26,63] (Box 1) could reflect the onset of sexual maturity, whereby energy allocation shifted from growth to reproduction. However, pending evidence for definitive correlations with sexual reproduction, this deduction is tenuous. Sexual maturity in most animals, including living reptiles, occurs well before full adult size is reached [66]. In birds (which are avian dinosaurs), however, it occurs once growth has come nearly to a standstill [67]. Furthermore, in most vertebrates, there are multiple pulses in growth rates [18]. Discerning which pulse, if any, reflects the onset of sexual maturity remains unclear.
description: Published as part of Erickson, Gregory M., 2005, Assessing dinosaur growth patterns: a microscopic revolution, pp. 677-684 in Trends in Ecology and Evolution 20 (12) on pages 680-682, DOI: 10.1016/j.tree.2005.08.012, http://zenodo.org/record/3733050