Dinosaur paleohistology: review, trends and new avenues of investigation

Dinosaur growth and physiology

The pioneering studies of Enlow & Brown, (1957), De Ricqlès (1969), and Enlow (1969) led to a plethora of histological studies designed to infer growth and maturity in fossil archosaurs. Such studies, beginning in earnest in the 1970s (De Ricqlès, 1972, 1975, 1980; Chinsamy, 1993; Varricchio, 1993; Reid, 1996; Horner, De Ricqlès & Padian, 1999, 2000; Sander, 2000a; Sander et al., 2004, 2006; Chinsamy & Elzanowski, 2001; De Ricqlès, Padian & Horner, 2001; Padian, De Ricqlès & Horner, 2001; Horner & Padian, 2004; Bybee, Lee & Lamm, 2006; Klein & Sander, 2007), continue to the present (Stein et al., 2010; Sander et al., 2011; Woodward, Horner & Farlow, 2011; Farke et al., 2013; Griebeler, Klein & Sander, 2013; Dumont et al., 2014; Vanderven, Burns & Currie, 2014; Rogers et al., 2016; Mitchell, Sander & Stein, 2017; Woodward, 2019).

It was not until the 1990s that major attempts to reconstruct dinosaurian growth rates and growth curves began. Chinsamy (1993) was the first to attempt reconstruction of a dinosaur growth curve with her study of the sauropodomorph Massospondylus. Around the same time, collaborations between de Ricqlès, Horner, and Padian produced significant advances in our current understanding of archosaur growth (Horner & Currie, 1994; Horner, De Ricqlès & Padian, 1999, 2000; Horner & Padian, 2004; Padian, De Ricqlès & Horner, 2001, Padian, Horner & De Ricqlès, 2004; De Ricqlès et al., 2001, 2008) through pivotal studies, in which they sampled the limb bones (both diaphyseal cross-sections and epiphyseal longitudinal sections) of embryos and hatchlings of extant crocodiles, turtles, and birds and compared them to similarly prepared material from dinosaur embryos, including Troodon, Maiasaura, and Hypacrosaurus (Horner, Padian & De Ricqlès, 2001; Horner & Currie, 1994; also see the more recent study by Reisz et al., 2013 on sauropodomorph embryos). These very young dinosaurs showed strong similarities in both diaphyseal vascularization and epiphyseal micromorphologies (i.e., a thickened zone of hypertrophic chondrocytes; also found by Barreto et al., 1993) intermediate between non-avian dinosaurs and birds during early ontogenetic stages, reflecting in part their close phylogenetic affinities. This was followed by examination of many other species and specimens, including ontogenetically older specimens of Hypacrosaurus and Maiasaura (Horner, De Ricqlès & Padian, 1999, 2000), the iconic Tyrannosaurus rex (Erickson et al., 2004; Horner & Padian, 2004) and hypsilophodonts (Horner et al., 2009).

Other investigators contributed to the growing literature on paleohistology, broadening the scope of these investigations. Most notably, considerable work has been done to understand the histology and growth strategies of sauropodomorphs, including sauropod dinosaurs, the largest terrestrial animals that ever lived (Sander, 2000a; Sander et al., 2004, 2006, 2011; Rogers & Erickson, 2005; Klein & Sander, 2007, 2008; Klein et al., 2012; Stein et al., 2010; Dumont et al., 2011; Griebeler, Klein & Sander, 2013; Hofmann, Stein & Sander, 2014; Mitchell & Sander, 2014; Mitchell, Sander & Stein, 2017). Other works on archosaurs included histological studies of pterosaurs (the sister group to dinosaurs; e.g., De Ricqlès et al., 2000; Padian, Horner & De Ricqlès, 2004) basal archosaurs, and even more basal Archosauriformes including key species such as Erythrosuchus and Euparkeria (De Ricqlès et al., 2008).

Growth strategies in extinct dinosaurs are not simply understood by analyzing collagen fiber orientations or the degree of vascularization, but also by quantitatively analyzing preserved annual growth marks (i.e., LAGs, see below). Growth marks are used to establish individual growth curves and to statistically test the best-fitting growth model (based on models successfully applied to extant species; e.g., see Erickson & Tumanova, 2000; Erickson, Rogers & Yerby, 2001; Lehman & Woodward, 2008; Griebeler, Klein & Sander, 2013). Growth model fitting is a recent quantitative approach in paleohistology to the study of dinosaur life history (Erickson & Tumanova, 2000; Erickson, Rogers & Yerby, 2001; Lehman & Woodward, 2008; Griebeler, Klein & Sander, 2013). The larger the sample size is, the more accurate the model is. For example, Woodward et al. (2015) used 50 tibiae to reconstruct the growth curve of the hadrosaur Maiasaura, utilizing more data than any previous growth study. This study represents the most accurate growth dynamic reconstruction of a dinosaur—or in fact, of any fossil vertebrate—to date (but see Bybee, Lee & Lamm, 2006 for another relevant study with a large sample size). Woodward et al. (2015) determined that Maiasaura would have reached sexual maturity in 3 years and skeletal maturity after 8 years, contributing to growing data supporting the idea that dinosaurs grew much faster than living reptiles.

Beyond species level growth reconstructions, paleohistology can be utilized at a much larger scale, possibly representing the best available tools with which to elucidate major evolutionary transitions in growth rates and physiology (Padian, De Ricqlès & Horner, 2001; De Ricqlès et al., 2008). Since the pioneering studies of De Ricqlès (1980, 1981) using histology to propose the endothermy of dinosaurs, histology has clarified many aspects of the transition from ectothermy to endothermy, and new methods have been developed (Cubo et al., 2012; Legendre et al., 2016; Myhrvold, 2016). For example, Cubo et al. (2012) conducted the first quantitative study using a paleobiological model to estimate bone growth rates (i.e., local cortical bone apposition rates) in fossils. This study showed that during the evolution of archosaurs, bone growth rates increased from the last common ancestor of Ornithodira to extant birds, but decreased from the last common ancestor of Crurotarsi to extant crocodiles (Cubo et al., 2012). Based on the equations of Montes et al. (2007), Legendre et al. (2016) hypothesized that archosaurs have a high ancestral metabolic rate and that the ectothermic physiology seen in extant crocodiles is actually a derived condition (also see Grigg & Kirshner, 2015).

Note, however, that the dichotomy of ectothermic vs endothermic dinosaurs is overly simplistic (Grady et al., 2014; Werner & Griebeler, 2014; Legendre et al., 2016). Currently, the precise type and term for the physiology of extinct dinosaurs is still debated, as it has been recently proposed that they could have been mesotherms (Grady et al., 2014), or fast-growing ectotherms (Werner & Griebeler, 2014). Certainly a range of physiologies were present given the great diversity of this clade.

The evolution of growth strategies in dinosaurs during the “dinosaur-bird” transition has also been heavily studied (Padian, De Ricqlès & Horner, 2001; Erickson, Rogers & Yerby, 2001), which we will elaborate on in the following section.

The evolution of avian growth

Not all dinosaur clades exhibit the same growth strategy, and considerable differences exist in both extinct and extant groups. These differences are partially linked to the extreme variation in size and terrestrial lifestyles observed in different groups of non-avian dinosaurs. However, in early birds, changes are undoubtedly related to the evolution of flight. Histological data suggest that Mesozoic dinosaurs took several years to reach adult size (e.g., 8 years for Maiasaura; Woodward et al., 2015; up to 40 years for sauropods, Lehman & Woodward, 2008; Rogers & Erickson, 2005; Griebeler, Klein & Sander, 2013; Waskow & Sander, 2014). All data thus far indicates non-ornithuromorph dinosaurs reached sexual maturity prior to skeletal maturity (Sander, 2000a; Erickson et al., 2009; O’Connor et al., 2014; Woodward et al., 2015) in a pattern similar to extant crocodilians (Chabreck & Joanen, 1979; Woodward, Horner & Farlow, 2011).

This type of growth is opposite to the condition in extant birds, which nearly all reach skeletal maturity rapidly (within a year) and prior to sexual maturity, that we can refer to as “avian-style” growth (Ricklefs, 1968; Castanet et al., 2000; Ponton et al., 2004). The only exception are the flightless paleognaths, which can take between 3 and 9 years to reach full size (Lee & Werning, 2008; Bourdon et al., 2009). Histology indicates that the dinosaurs most closely related to Aves were more similar to birds in terms of growth strategies than previously acknowledged, and in fact Mesozoic theropod dinosaurs exhibited metabolic rates very close to those found in modern birds (Legendre et al., 2016).

The timing of the origin of this “avian-style” growth is still under investigation (Fig. 2), but it evidently was absent in most stem birds. Paleohistological studies on the Jurassic Archaeopteryx (Erickson et al., 2009) and non-ornithuromorph birds from the Early Cretaceous indicate that, like non-avian dinosaurs, they also required several years to reach adult size (Chinsamy, Chiappe & Dodson, 1995; Erickson et al., 2007; O’Connor et al., 2014). Numerous paravians have been sectioned, sampling most of the phylogeny across the dinosaur-bird transition, but until recently, avian-style growth has only been reported in the derived ornithurines Hesperornis and Ichthyornis (Chinsamy, Martin & Dobson, 1998). Examination of the non-ornithurine ornithuromorph Iteravis (Fig. 2; O’Connor et al., 2015) suggests that the growth strategy observed in extant birds evolved independently in the Early Cretaceous outside the Ornithurae (O’Connor et al., 2015) but still in the crownward lineage that includes all extant birds. Similar results were also found in the non-ornithurine ornithuromorph Yanornis (Wang et al., 2019). Several studies also indicate that most stem birds, like their non-avian dinosaurian relatives, achieved reproductive maturity prior to skeletal maturity (O’Connor et al., 2014, 2018).

Much of what is known about the transition from dinosaur to avian-style growth comes from specimens found either in the middle to Upper Jurassic Yanliao Biota or the Lower Cretaceous Jehol Biota, both known from northeastern China (Zhou, Barrett & Hilton, 2003). The oviraptorosaur Caudipteryx (Erickson et al., 2009), paravians including Anchiornis (Zheng et al., 2014; Prondvai et al., 2018), basal birds including Sapeornis (Gao et al., 2012), Jeholornis (Erickson et al., 2009; O’Connor et al., 2014; Prondvai et al., 2018), Confuciusornis (Zhang, Hou & Ouyang, 1998; De Ricqlès et al., 2003; Chinsamy et al., 2013), the ornithuromophs Archaeorhynchus (Wang & Zhou, 2017) and Iteravis (O’Connor et al., 2015), and several enantiornithines (O’Connor et al., 2014, 2018) together with Late Cretaceous ornithothoracines from Argentina (Chinsamy, Chiappe & Dodson, 1995) have contributed greatly to our overall understanding of the evolution of avian growth strategies. Despite this work, the study of stem avian histology is still at a relatively early stage compared to that of non-avian dinosaurs. Increased sampling across groups, within groups and species (e.g., from multiple specimens of Archaeopteryx, and Jehol taxa from which at least partial growth series can be obtained), and between various skeletal elements will continue to clarify our understanding of dinosaurian and avian growth (O’Connor et al., 2014).

Lines of arrested growth and skeletal maturity

Histological examinations allow the assessment of individual age, and skeletal and sexual maturity in fossil specimens. Age is assessed by counting annually deposited LAGs (Fig. 1) (Horner, De Ricqlès & Padian, 1999; Erickson & Tumanova, 2000; Woodward et al., 2015). In living animals, these reflect a seasonal slow-down or cessation of growth that occurs during the harshest season of the year (see the detailed description of skeletochronology and retrocalculation methods in Woodward, Padian & Lee, 2013). Early on LAGs were correlated to ectothermy such that the presence of LAGs in a specimen was once inferred to indicate an ectothermic metabolism, whereas the absence of LAGs would suggest endothermy (e.g., see Chinsamy, 2005). However, it has recently been shown that even some extant endotherms (i.e., some mammals) arrest their growth cyclically, resulting in the formation of LAGs, as part of a plesiomorphic thermometabolic strategy to conserve energy (Köhler et al., 2012; also see Sander & Andrassy, 2006 for another study on mammals). These data confirm the annual cyclicity of LAGs in extant endothermic mammals and indicate that their presence cannot be used as an argument for ectothermy in fossils (Köhler et al., 2012).

Currently, LAGs are the most accurate indicators of ontogenetic age in extinct vertebrate organisms, the best records to infer growth models (Erickson & Tumanova, 2000; Erickson, Rogers & Yerby, 2001; Lehman & Woodward, 2008; Griebeler, Klein & Sander, 2013) and the best indicators of skeletal maturity. Indeed, as an individual approaches maturity, the space between the LAGs in limb bones gradually diminishes as they approach the periosteal surface (as seen in Fig. 1) eventually becoming organized into an external fundamental system (EFS; Cormack, 1987) or outer circumferential layer (OCL; Ponton et al., 2004). The presence of an EFS/OCL reflects the slowing or cessation of growth, which in turn indicates the attainment of skeletal maturity. EFS have been reported in birds (Ponton et al., 2004) and more recently, in American alligators (Woodward, Horner & Farlow, 2011). EFSs have been described in a few dinosaur specimens (e.g., in some sauropodomorphs, ankylosaurs, stegosaurs, and hadrosaurs; Sander & Klein, 2005; Redelstorff et al., 2013; Stein, Hayashi & Sander, 2013; Woodward et al., 2015). This absence has often been interpreted as a marker of “indeterminate growth” (growth continuous throughout the animal’s life, e.g., Chinsamy, 1993). However, in a recent examination of a large sample of American alligators (animals often considered to possess “indeterminate growth”), Woodward, Horner & Farlow (2011) found EFSs in the oldest specimens. Indeterminate growth is no longer considered a valid term, and presumably in all reptiles (including dinosaurs) growth is determinate (e.g., see discussions in Sander, 2000a; Redelstorff & Sander, 2009; Sander et al., 2011; Redelstorff et al., 2013; Stein, Hayashi & Sander, 2013; Padian & Lamm, 2013). Woodward, Horner & Farlow (2011) and Woodward et al. (2015) demonstrated that the absence of an EFS in specimens of non-avian dinosaurs simply indicates that they died before reaching skeletal maturity.

Woodward (2019) also recently described a new type of cortical growth mark in Maiasaura that only occurs prior to the deposition of the first LAG under normal conditions. Referred to as localized vascular changes, these are unlike LAGs in that they are non-annual and are hypothesized to represent times of temporary but repeated stress (Woodward, 2019). They cannot be used to directly assess maturity and age assessment, as can LAGs, but they do hold potential to interpret other aspects of extinct vertebrate biology if they are further investigated in extant archosaurs and other dinosaur species.

Medullary bone and sexual maturity

Arguably, the most accurate way to assess sexual maturity in fossil dinosaurs and birds is to find skeletons in direct association with a clutch, or with unlaid eggs preserved internally (Sato et al., 2005; Erickson et al., 2007; Bailleul et al., 2019). However, medullary bone (MB), a female specific skeletal tissue formed during egg production, can also be used as a marker of reproductive activity (and thus reproductive maturity) if identified correctly. Today MB is only found in extant, reproductively active female birds (Schweitzer et al., 2007). This ephemeral skeletal tissue acts as a calcium reservoir, helping birds meet the high calcium demands imposed by eggshell formation (Bloom, Bloom & McLean, 1941; Dacke et al., 1993). Derived from the endosteum, MB can line the medullary cavities and cancellous spaces of the entire avian skeleton in some taxa (Canoville, Schweitzer & Zanno, 2019). In living birds, MB is estrogen dependent, forms extremely fast (although this might not have always been the case in stem taxa; see O’Connor et al., 2018; Bailleul et al., 2019), is high in calcium relative to other types of bone tissue, and is highly vascular, all which facilitate its function as an easily mobilized calcium reservoir (Bloom, Bloom & McLean, 1941; Taylor & Moore, 1953; Yamamoto et al., 2001).

Two recent studies have demonstrated the widespread presence of MB in Neornithes (Werning, 2018; Canoville, Schweitzer & Zanno, 2019). Canoville, Schweitzer & Zanno (2019) report that the skeletal distribution of MB is directly related to the distribution of red bone marrow, and inversely correlated to the combined skeletal distributions of pneumaticity and yellow bone marrow. They also confirmed that avian MB can be deposited within virtually all skeletal elements, including those within the skull.

Although the biology and chemistry of this tissue are now relatively well understood in living birds, the timing of the evolution of this tissue is still subject to debate. MB was first reported outside Aves in the femur (Fig. 3) and tibiae of a specimen of T. rex from Montana (Schweitzer, Wittmeyer & Horner, 2005). This discovery suggested: first, that it may be possible to assess gender and sexual maturity in fossil dinosaurs and birds using histology to demonstrate the presence of MB; second, that this reproductive tissue, previously limited to birds, may have evolved prior to the origin of birds, in a more inclusive clade of theropod dinosaurs; third, that the presence of this tissue sheds light on reproductive age in extinct theropods; and fourth, that theropod dinosaurs including birds may have shared at least some elements of reproductive strategies to the exclusion of other archosaurs. Because extant crocodilians cannot form MB (Elsey & Wink, 1986; Schweitzer et al., 2007), the presence of this tissue in a non-avian theropod dinosaur suggested its origin in this lineage prior to the divergence of Aves.

Since this first identification, MB has also been reported in several other non-avian dinosaurs, including taxa outside the theropod lineage (Lee & Werning, 2008; Hübner, 2012), as well as pterosaurs (Prondvai & Stein, 2014; Prondvai, 2017). It has also been reported in two Mesozoic bird lineages recovered from the Jehol biota: in Confuciusornis (Chinsamy et al., 2013) and in two enantiornithines (an enantiornithine indet., O’Connor et al., 2018 and in Avimaia schweitzerae, Bailleul et al., 2019). Definitive evidence in support of this identification in Avimaia is provided through the preservation of an intra-abdominal egg, indicating this adaptation evolved outside the crown clade (Bailleul et al., 2019). However, because some pathologies can produce MB-like tissues in extant birds (Canoville, Schweitzer & Zanno, 2019), it has been noted that unambiguous identification of fossil tissues as reproductive MB requires more than just morphological similarity, particularly with increasing phylogenetic distance from the avian lineage, and/or when it appears in patterns different from those seen in extant birds (O’Connor et al., 2018). Although chemical and antibody staining of MB tissues in T. rex support the original diagnosis, given that the fossil tissues reacted to these methods in a manner similar to birds, later studies also shown that at least one pathology is also chemically consistent with MB (Canoville et al., 2018). We propose that multiple lines of evidence taken together are required to support the identification of MB in fossil taxa, including (but not limited to) skeleton-wide distribution (i.e., more than one element), absence of indicators of pathology, and histological consistency.

In summary, the evolutionary origin of MB is still the subject of debate: it is either considered to have evolved prior to the origin of Coelurosauria (Schweitzer, Wittmeyer & Horner, 2005; 2016; Canoville, Schweitzer & Zanno, 2019) or earlier, outside of Dinosauria (i.e., also hypothesized to be present in pterosaurs; Prondvai & Stein, 2014; Prondvai, 2017). A reanalysis of the potential MB reported in the skull of pterosaurs must be made, as it does not appear to meet any criteria of consistency with extant MB; i.e., it was reported in juveniles as well as adult specimens; it was reported in the mandibular symphysis but not elsewhere in the body (a pattern not observed in any living birds; Canoville, Schweitzer & Zanno, 2019); and the endosteal origin of the purported MB is unclear.

Further histochemical studies on purported fossil MB may clarify this issue in the future (O’Connor et al., 2018, based on the chemical results of Canoville et al., 2018). It is clear that robust conclusions regarding the presence of MB in fossils can no longer rely solely on standard paleohistology and must involve other methods to rule out pathology and or identify a chemical signal unique to MB (e.g., see the immunological test that ruled out osteopetrosis; Fig. 5 in Schweitzer et al., 2016).

Histological correlates of cranial muscle attachments

Histological correlates of muscle insertions have been described in the heads of extant birds (Hieronymus, 2006), as well as facial epidermal structures in ceratopsids (Hieronymus et al., 2009; Tumarkin-Deratzian et al., 2010), in the attempt to reconstruct dinosaurian cranial soft-tissues. Even though reconstructing soft-tissues in dinosaurs using histological correlates has limitations (notably because not all soft-tissue insertions leave marks directly on the cranial bones (Hems & Tillmann, 2000; Hieronymus, 2006, also see Discussion in Petermann & Sander, 2013 and Lambertz, Bertozzo & Sander, 2018) further investigation in combination with other methods will surely improve reconstructions of dinosaurian cranial anatomy.

Timing of evolution of dinosaurian cranial tissues and “avian” secondary cartilage

Secondary cartilage, a presumably exclusively avian tissue (Hall, 1967, 1968, 2000), has been discovered in the skulls and cranial joints of some ornithischian embryos and nestlings (Fig. 5; Bailleul, Hall & Horner, 2012, 2013). The skull of vertebrates is composed of two main types of bones: membrane bones that arise directly within the mesenchyme as bone blastemas, and endochondral bones, which first arise as primary cartilage models before being (fully or partially) replaced by bone (Hall, 2005; Couly, Coltey & Le Douarin, 1993). To avoid any confusion, we will refer to these two types of “bones” as “membranous elements” and “endochondral elements.” In modern birds, secondary cartilage is found exclusively on membranous elements, either as articular cartilage (e.g., in ducks, secondary cartilage is found on the squamosal, within the socket that articulates with the quadrate; Bailleul, Witmer & Holliday, 2017) or at muscle or ligamentous insertions (e.g., on the surangular underlying the ligamentum squamosomandibulare in the chick; Hall, 1968). Note that mammals also have secondary cartilage, but based on parsimony and different mechanisms of tissue initiation, it has been determined that avian and mammalian secondary cartilages are not homologous and arose independently during evolution (i.e., in birds, mechanical stimulation is required for both the initiation and maintenance of secondary cartilage, whereas in mammals it is only required for its maintenance; Hall, 2000; also see Supplemental Material in Bailleul, Hall & Horner, 2012).

Avian secondary cartilage arises from the periosteum of membranous elements, and in comparison, the articular cartilage found on the ends of endochondral elements (e.g., the quadrate, and elements of the chondrocranium) is a type of primary cartilage ( because it originated from the primary cartilage model; Hall, 1967, 1968, 2000; Bailleul, Hall & Horner, 2012). Secondary cartilage may differ slightly microstructurally from the more common primary cartilage (i.e., it may have less extracellular matrix, at least very early in ontogeny, or it may be more fibrous later in ontogeny, e.g., see Bailleul, Hall & Horner, 2012; Bailleul, Witmer & Holliday, 2017) but the identification of this tissue is mostly based on its location (i.e., on a membranous element) and not purely on histological differences (Bailleul, Hall & Horner, 2012). The presence of this “avian” tissue in non-avian dinosaurs suggests that birds inherited this tissue from their non-avian dinosaur ancestors (Bailleul, Hall & Horner, 2012), and provides further phylogenetic evidence at the microscopic scale for the close evolutionary relationship of these groups. Together with MB (Schweitzer, Wittmeyer & Horner, 2005; Schweitzer et al., 2016), these could be considered the only two skeletal tissues (Figs. 3 and 5) shared exclusively between dinosaurs and birds (being absent in crocodilians, and other more basal archosaurs), thus supporting the dinosaurian origin of birds.

Skeletal tissues and cranial biomechanics

Skeletal tissues can shed light on cranial biomechanics and function. Although some functional aspects of the postcranium in both fossil and extant archosaurs have been investigated using histology (De Ricqlès et al., 2000; Ponton et al., 2007; Zhao et al., 2013; Cubo et al., 2015; Tsai & Holliday, 2015; Mitchell et al., 2017), in comparison, very few functional histological studies on cranial elements exist. Histological analyses of cranial skeletal tissues conducted to reconstruct skull biomechanics are still in their infancy, but they have the potential to be highly informative. Specifically, secondary cartilage, in addition to its phylogenetic importance, has a biomechanical significance. Experimental studies that induce cranial joint paralysis and inhibit embryonic motility (e.g., movements such as beak clapping), creating an immobile environment in developing birds, show that this tissue does not develop in the absence of movement, whereas primary cartilage forms in both mobile and immobile conditions (Murray & Drachman, 1969; Persson, 1983; Hall, 1972). Therefore, the presence of secondary cartilage within a fossil joint (Fig. 5) can be used to infer mobility. The hypothesis that physical movement may be responsible for secondary cartilage formation has been previously proposed for some cranial joints in both birds and mammals (Murray, 1963; Murray & Drachman, 1969; Bareggi et al., 1994), but the degree and/or direction of such movement has not been quantified. Nevertheless, cranial tissues have great potential for more accurate inferences regarding skull structure and function (e.g., cranial kinesis or akinesis) in non-avian dinosaurs and other fossil archosaurs (e.g., see Bailleul & Holliday, 2017; Lessner et al., 2019).

Future histological studies on the skulls of extant archosaurs will expand our ability to reconstruct cranial biomechanics in their extinct relatives (especially studies that would also provide quantitative data), and will, no doubt, elucidate other aspects of the evolution and function of extinct dinosaurs and birds.