Dorsal Plate

Dorsal plates of stegosaurs are highly modified osteoderms—bony elements that develop within the dermis independently of the endoskeleton. Unlike the compact, shield-like scutes of ankylosaurs, stegosaur dorsal plates are mediolaterally thin and dorsoventrally tall, giving them a kite-shaped or subtriangular profile when viewed laterally. The internal architecture of these plates has been studied extensively through X-ray computed tomography (CT) and histological thin-sectioning. According to Farlow, Hayashi, and Tattersall (2010), the base of each plate contains multiple large openings (nutrient foramina) that lead into a linear, mesiodistally oriented vestibule. From this vestibule, multiply branching 'pipes' extend apically through the plate interior. These pipes are best developed in the basal half of the plate and communicate with extensive cancellous regions throughout the interior. The cortex surrounding this cancellous core is remarkably thin—often no more than about 10 mm—making the plates structurally unsuitable as effective armor against direct predator attacks. This thinness was one of the earliest observations that falsified the original armor hypothesis.

The bone tissue of dorsal plates differs notably from the axial and appendicular skeleton of the same individual. Hayashi et al. (2009, 2012) demonstrated that plate osteoderms show delayed growth onset relative to the rest of the skeleton, and they retain the capacity for continued growth later in life, as evidenced by ongoing remodeling and vascular supply. In older adults, the plates exhibit features such as lines of arrested growth (LAGs), high concentrations of secondary osteons, and in some individuals an external fundamental system (EFS) indicating complete cessation of growth. The rich vascular supply to the plates, both internal and external, has been a central piece of evidence in debates over plate function.

Direct fossil evidence, particularly well-preserved integumentary impressions from Hesperosaurus mjosi specimens (Christiansen & Tschopp, 2010), confirms that stegosaur plates were covered by keratinous sheaths in life. These impressions show a smooth surface with long, parallel, shallow grooves, indicating the keratin covering was continuous over the bony core. The presence of keratin sheaths means that the plates in living animals would have been somewhat larger than their preserved bony cores, and the edges may have been sharper than the bone alone suggests. This has implications for both the defense and display hypotheses: keratin-covered plates with potentially sharp edges could have served as more effective deterrents to predators attempting to bite from above, while the keratinous surface could have been brightly colored or patterned, enhancing visual signaling.

The arrangement of dorsal plates on the back of Stegosaurus has been debated since Othniel Charles Marsh first described the genus in 1877. Marsh initially interpreted the plates as lying flat over the back like roof tiles—hence the name Stegosaurus, meaning 'roof lizard.' By 1891, Marsh revised his view to depict a single row of plates standing upright along the midline. Frederick Lucas in 1901 proposed a paired, bilaterally symmetrical arrangement, which was depicted in Charles R. Knight's famous life restoration. However, Richard Swann Lull's 1910 mount at the Peabody Museum with paired plates was later changed in 1924 after Charles Gilmore (1914) demonstrated, based on multiple articulated specimens of S. stenops, that the plates were arranged in two alternating (staggered) rows near the back's midline. This alternating arrangement is now the widely accepted standard.

The total number of plates per individual is approximately 17 to 18. The holotype and several referred specimens of S. stenops preserve 17 plates, while the exceptionally complete specimen NHMUK PV R36730 ('Sophie'), a Stegosaurus housed at the Natural History Museum in London, preserves 18 plates—more than any other known stegosaur specimen. No two plates on a single individual are exactly the same size and shape, and the plates are manifestly chiral (asymmetric), meaning that left-side and right-side plates can be distinguished. Peter Galton (2010) suggested that the specific pattern of plates as viewed in profile may have varied between species and could have been important for species recognition.

Thermoregulation Hypothesis

The thermoregulation hypothesis was most famously advanced by Farlow, Thompson, and Rosner in a 1976 paper in Science (vol. 192, pp. 1123–1125), which proposed that the plates functioned as forced convection heat loss fins. Their experiments and calculations showed that the alternating arrangement of plates along the arched back of Stegosaurus was well-suited for shedding excess body heat to wind currents via forced convection. De Buffrénil, Farlow, and de Ricqlès (1986) provided histological support, documenting that the extensive internal vascularity of the plates was consistent with a thermoregulatory role. Farlow et al. (2010) further confirmed the elaborate internal vascular distributary system using CT scanning, drawing comparisons with the vascular osteoderms of extant crocodylians, where infrared thermographic imaging of basking caimans demonstrated differential blood flow to osteoderms during thermoregulation.

However, the thermoregulation-as-primary-function hypothesis has faced substantial criticism. Growing evidence suggests that non-avian dinosaurs, including ornithischians, likely had metabolic rates closer to the endothermic range than to ectothermic reptiles. If so, they would have possessed more advanced physiological thermoregulatory mechanisms than simple external fins. Furthermore, other stegosaurs (e.g., Kentrosaurus, Tuojiangosaurus, Wuerhosaurus) have much smaller plates or plates with lower surface-area-to-volume ratios, yet there is no evidence that these taxa suffered thermoregulatory disadvantages. Closely related ankylosaurs with extensive osteoderms show no comparable 'thermal fin' structures. Thus, while the plates almost certainly had some thermodynamic impact on the animal's physiology, thermoregulation is now more commonly regarded as a secondary or incidental function rather than the primary selective driver for plate evolution.

Defense Hypothesis

The idea that dorsal plates served as body armor was the earliest functional interpretation, stemming from Marsh's original (1877) conception and Gilmore's (1914) description. However, as more complete specimens revealed the plates' thin cross-sections and cancellous internal structure, the armor hypothesis lost support. The plates were simply too thin and fragile to withstand direct bites from large theropods such as Allosaurus. That said, the defense hypothesis has not been entirely abandoned in a more nuanced form. Carpenter (1998) proposed that plates could have functioned as a visual threat display, making the stegosaur appear larger to approaching predators. When covered in keratin, the tightly packed dorsal plates projecting high above the flesh, especially over the vulnerable neck region, may have deterred theropod bites from above. The 'life-dinner principle' (Dawkins & Krebs, 1979) suggests that predators are risk-averse and may avoid prey that appears difficult or dangerous to attack, even if the 'armor' is not structurally impervious.

Sociosexual Display Hypothesis

The hypothesis that dorsal plates functioned primarily for sociosexual display has gained significant traction in recent decades. Saitta (2015), in a study published in PLOS ONE, presented evidence for sexual dimorphism in the plates of Hesperosaurus mjosi. This study identified two distinct plate morphs: one with wide, oval plates reaching surface areas approximately 45% larger than those of the other morph, which possessed taller, narrower, subtriangular plates. Through principal component analysis, taphonomic investigation of a multi-individual quarry (JRDI 5ES Quarry, Montana), and histological analysis, Saitta tested and rejected alternative hypotheses including non-sex-related individual variation, ontogenetic change, and interspecific differences. The study concluded that the dimorphism most likely represented sexual dimorphism, suggesting the wide-morph plates (hypothesized males) served as billboard-like display surfaces for female mate choice, while the tall-morph plates (hypothesized females) functioned as more prickly predator deterrents under natural selection.

A subsequent 2025 bioRxiv preprint by Saitta et al. expanded on these findings with additional statistical analyses, outline morphometrics, and examination of pathologies on caudal vertebrae and tail spikes. This study proposed a comprehensive evolutionary model in which osteoderms originally adapted for defense in basal thyreophorans became exapted for sexual display (plates) and intra-sex combat (tail spikes in stegosaurs, clubs in ankylosaurs), with sexual selection potentially driving the unique alternating plate arrangement seen in Stegosaurus and Hesperosaurus.

Dorsal plates evolved from the low-keeled osteoderms characteristic of basal thyreophorans. The phylogenetic transition from small, shield-like scutes to the tall, flattened plates of derived stegosaurids represents one of the most dramatic morphological transformations in ornithischian dinosaur evolution. Basal thyreophorans such as Scutellosaurus and Scelidosaurus possessed relatively simple, flattened osteoderms across much of their dorsal surface. In early stegosaurians such as Huayangosaurus (Middle Jurassic, China), dorsal osteoderms were already differentiated into recognizable plates and spikes, though the plates were smaller and more spike-like than in derived forms. By the Late Jurassic, derived stegosaurids such as Stegosaurus and Hesperosaurus had evolved the characteristically large, thin, kite-shaped plates that define the clade in the popular imagination.

Main, de Ricqlès, Horner, and Padian (2005) studied the histological evolution of thyreophoran scutes and argued that stegosaur plates and spikes are primarily hypertrophied keels of primitive thyreophoran scutes, sometimes with elaboration of dermal bone from the scute base. Their histological survey demonstrated that the internal structure of plates—particularly the extensive vascularity and cancellous architecture—diverged significantly from the more compact, less vascular osteoderms of ankylosaurs, suggesting divergent functional trajectories despite shared developmental origins.

Not all stegosaurians possessed the large, flat plates characteristic of Stegosaurus. There is considerable morphological diversity across the clade. Kentrosaurus (Late Jurassic, Tanzania) had relatively small plates restricted to the anterior portion of the body, transitioning to paired spikes along the posterior back, hips, and tail. Tuojiangosaurus (Late Jurassic, China) had smaller, more pointed plates. Miragaia (Late Jurassic, Portugal) is notable for its exceptionally long neck and a series of small plates. Wuerhosaurus (Early Cretaceous, China) had plates that were lower and more rounded than those of Stegosaurus. This variation underscores that the extreme plate morphology of Stegosaurus represents one end of a morphological spectrum rather than a universal stegosaurian body plan.

Within Stegosaurus itself, Galton (2010) noted differences in plate morphology between species. S. stenops has proportionately large, broad plates that have been found articulated in the alternating arrangement. S. ungulatus appears to have had proportionately smaller, more pointed plates with wider bases and narrower tips, and at least the posterior caudal plates appear to have been paired based on mirrored specimens. These species-level differences in plate shape and arrangement may have served as visual cues for species recognition among sympatric stegosaurian taxa.

The study of stegosaur dorsal plates has a rich research history spanning nearly 150 years. Othniel Charles Marsh first described Stegosaurus (and its plates) in 1877 based on fragmentary remains from Morrison, Colorado. The name 'Stegosaurus' ('roof lizard') itself reflects Marsh's initial misinterpretation of the plates as flat, overlapping roof-tile-like armor. The discovery of the nearly complete, articulated holotype of S. stenops (USNM 4934) by Marshall Felch at Garden Park, Colorado, in 1885–1886, revolutionized understanding of plate morphology and arrangement. Charles Gilmore's 1914 monograph provided the definitive early description of the dermal armor and established the alternating plate arrangement as the standard reconstruction.

The functional debate intensified in the latter half of the twentieth century. Farlow et al.'s 1976 thermoregulation hypothesis was enormously influential and brought stegosaur plates into the broader discourse on dinosaur physiology and the 'Dinosaur Renaissance.' De Buffrénil et al. (1986) added histological depth to the thermoregulation argument. Carpenter (1998) broadened the discussion to include defense and display functions. Main et al. (2005) reframed plate evolution in the context of thyreophoran phylogeny. Saitta (2015) introduced quantitative evidence for sexual dimorphism. The discovery and description of 'Sophie' (NHMUK PV R36730), an 85% complete Stegosaurus specimen at the Natural History Museum in London, described by Maidment et al. (2015), provided an unprecedented level of anatomical detail for plate description and arrangement.

The dorsal plates of Stegosaurus are among the most recognizable features of any dinosaur and have made Stegosaurus one of the most iconic prehistoric animals in popular culture. From Charles R. Knight's early twentieth-century paintings to appearances in films such as King Kong (1933) and the Jurassic Park franchise, the distinctive silhouette of a stegosaur with its double row of plates has become a universal symbol of the Age of Dinosaurs. The plates feature prominently in museum displays worldwide, postal stamps, toys, and educational media. This cultural prominence has, in turn, driven significant public and scientific interest in understanding the biological function of these remarkable structures.