Bone Histology of the Stegosaur Kentrosaurus aethiopicus (Ornithischia: Thyreophora) from the Upper Jurassic of Tanzania

Authors


Correspondence to: Ragna Redelstorff, Biological Sciences Department, University of Cape Town, Private Bag X3, Rhodes Gift, 7701 Cape Town, South Africa. Fax: +27-21-650–4007. E-mail: ragna.redelstorff@uct.ac.za

ABSTRACT

Using bone histology, a slow growth rate, uncommon for most dinosaurs, has been interpreted for the highly derived stegosaur Stegosaurus (Ornithischia: Thyreophora) and the basal thyreophoran Scutellosaurus. In this study, we examine whether this slow growth rate also occurs in the more basal stegosaur Kentrosaurus from the Tendaguru beds of Tanzania. The bone histology of six femora of Kentrosaurus representing an ontogenetic series from subadult to adult was studied, as well as one scapula. The primary bone is mainly highly vascularized fibro-lamellar bone with some reticular organization of the vascular canals. In addition to LAGs and annuli, distinctive shifts in the pattern of vascularization occur, which have been interpreted as potential growth marks. The variation in the development of growth marks may reflect annual climatic fluctuations. The overall bone depositional rate, and hence growth rate in Kentrosaurus appears to be higher than in Stegosaurus and Scutellosaurus. Considering that Stegosaurus is the larger-sized of the two stegosaurs, this would be contrary to an earlier supposition that small-bodied dinosaurs have slower growth rates than larger ones. Our finding of rapid rates of bone deposition in Kentrosaurus suggests that slow growth rates previously reported in Scutellosaurus and Stegosaurus are not a phylogenetic characteristic of the Thyreophora. Thus, slow growth rates are not plesiomorphic for the Thyreophora. We propose that the slow growth rates documented in the highly derived Stegosaurus could have been secondarily derived or alternatively that Kentrosaurus is the exception having increased growth rates. Anat Rec, 296:933–952, 2013. © 2013 Wiley Periodicals, Inc.

Bone histology of extinct animals provides a host of information about their biology (e.g., Erickson and Brochu, 1999; Botha and Chinsamy, 2004; Botha-Brink and Smith, 2011; Hugi et al., 2011; Chinsamy-Turan, 2012; Prondvai et al., 2012), and such studies have been widely applied to the Dinosauria (e.g., de Ricqlès, 1980; Reid, 1990; Erickson and Tumanova, 2000; Horner et al., 2000; Tütken et al., 2004; Erickson, 2005, Chinsamy-Turan, 2005; Cerda and Chinsamy, 2012; Chinsamy et al., 2012). However, there is a distinct scarcity of studies of stegosaur bone histology, which may be directly related to the rare occurrence of stegosaurs in the fossil record (Maidment, 2010). Apart from cursory mentions as part of larger studies (e.g., Seitz, 1907; Reid, 1990), research on stegosaurs has focused almost exclusively on types of osteoderms in the genus Stegosaurus and their function (e.g., Hayashi et al., 2012, and references therein). In recent years there have been two studies on the long bone microstructure of Stegosaurus (Hayashi et al., 2009; Redelstorff and Sander, 2009), which have permitted a comparison of their bone histology with that of other dinosaur groups.

According to Redelstorff and Sander (2009), the highly derived stegosaur Stegosaurus shows a rather unusual bone histology and inferred growth rates as compared with other dinosaurs. Fibro-lamellar bone (FLB) with a mixture of woven and parallel-fibered bone (PFB) and low vascularization indicate slower growth rates than would be expected for its size (Redelstorff and Sander, 2009). The authors compared Stegosaurus bone histology with that of various dinosaurs, including similar-sized ones such as Maiasaura, which consists of faster growing FLB only and usually high vascularization (Horner et al., 2000). Therefore, following Amprino's rule, Stegosaurus must have grown more slowly due to the unusual amount of PFB. This slow growth rate may be a result of the possession of defensive osteoderms that have reduced the necessity of outgrowing predators (Cooper et al., 2008). Other osteohistological studies on thyreophoran long bones are limited to the basal taxon Scutellosaurus lawleri (Padian et al., 2004). Long bones of Scutellosaurus possess a low degree of vascularization in a FLB matrix in the femur, indicating relatively slow growth rates, and PFB in other limb bone elements, indicating even slower growth rates.

Kentrosaurus aethiopicus Hennig 1915 is a basal stegosaurid (Mateus et al., 2009; Maidment, 2010) from the Middle and Upper Dinosaur Members (Late Jurassic) of the Tendaguru Formation (Janensch, 1914; Hennig, 1915; Aberhan et al., 2002; Bussert et al., 2009). Up to 30 localities, sometimes consisting of monodominant bone beds, yielded mainly isolated bones and some rare partially associated skeletons (Hennig, 1915, 1916, 1924, 1936; Janensch, 1925; Galton, 1982, 1988; Mallison, 2011). The majority of the Kentrosaurus material, including the type specimen (e.g., Janensch, 1914; Maier, 2003; Mallison, 2011) and the femora studied here originate from a monodominant bone bed in the quarry ‘St’ from the Middle Dinosaur Member. Mainly isolated bones belonging to almost 40 individuals of Kentrosaurus accumulated in this quarry, however, small and delicate elements, such as skull bones and phalanges, are scarce or even absent (Hennig, 1924:200). Despite dire losses of Kentrosaurus material during World War II (Maier, 2003; Mallison, 2011), a large number of femora of different sizes, including the sampled ones from the quarry ‘St’, are stored at the Museum of Natural History (Museum für Naturkunde) in Berlin, Germany. Janensch (1914) proposed a mass mortality event for this extensive accumulation of mainly isolated to rarely associated bones (Hennig, 1924:200). At least one other mass accumulation of Kentrosaurus occurs in the stratigraphically younger quarry ‘X’ in the Upper Dinosaur Member and is dominated by manual and pedal elements. Another locality (‘Ki’) in the Middle Dinosaur Member produced a complete, articulated left lower hind limb (Janensch, 1914; Hennig, 1924:220–222).

Given that Kentrosaurus aethiopicus is intermediate in body size and phylogenetic position between Stegosaurus and Scutellosaurus, it is generally assumed to have had a similarly slow growth rate as the latter two dinosaurs. This study represents an analysis of Kentrosaurus bone histology and permits a comparison with previous osteohistology studies on thyreophoran dinosaurs, thereby enabling an assessment of the growth dynamics of the taxon, as well as phylogenetic trends of growth within this clade of ornithischian dinosaurs.

MATERIAL AND METHODS

Detailed morphometric measurements of all Kentrosaurus bones in the Museum of Natural History Berlin (including the specimens sampled for this study) were taken. These included the following measurements: bone length, proximal and distal width, minimum and maximum diameter and smallest circumference (Table 1). Missing data were deduced by utilizing the ratios obtained from the complete bones. The six Kentrosaurus femora sampled for histological analysis (MB.R.3605, MB.R.3572, MB.R.3571, MB.R.3583, St 72, MB.R.3593) range in length from 345 to 689 mm (Table 1), and reflect almost the entire range of lengths of all preserved femora (Fig. 1). One scapula (Ig436) was also sampled.

Table 1. Measurements of femora of Kentrosaurus specimens studied
LabelLengthProximal widthDistal widthϕ maxϕ minSmallest circumferenceGrowth cycles
  1. Measurements are given in mm. Number of growth marks represents the minimum number. Sampled specimens are indicated in bold.

MB.R.3606310 74  110 
MB.R. 3605345809937331182
MB.R.35584451301355441160 
MB.R.357248213513558471706
MB.R.35924881311365648169 
MB.R.35595151341385948168 
MB.R.357151915814564501856
MB.R.35705231381416543180 
MB.R.35905291681406749193 
MB.R.35955821661697252211 
MB.R.35795931451477156200 
MB.R.35815941771768460232 
MB.R.35846061721859061259 
MB.R.35786081701707856221 
MB.R.358360917116578572207
MB.R.35856101801758156228 
MB.R.35886121811717658220 
MB.R.35776151801748160226 
MB.R.35806152051897657213 
MB.R.35766201751828260227 
MB.R.35866281611657558214 
MB.R.35746341741737159211 
MB.R.35736381661628255232 
MB.R.35756421731737161215 
St 7264419318586602418
MB.R.35916471801647656220 
SAM-PK 89986501801658560246 
MB.R.35826521751768462231 
MB.R.35946711901808258230 
MB.R. 359368920419291642532
MB.R.359875024522011670315 
Figure 1.

Sampled specimens, indicated by red circles, are well distributed within the size range of femora lengths of preserved Kentrosaurus specimens from 310 to 750 mm. Data are given in Table 1.

Body mass estimates are derived from the only known Kentrosaurus individual (Ng, now lost), of which both humeri and a femur were preserved (‘the small individual from quarry Ng’, see Hennig 1925, p. 200, 212 and 244). The average circumference of both humeri and the calculated circumference of the femur of Ng (see Table 2 for values and equations) were used to calculate the body mass using the equation of Anderson et al. (1985; modified by Alexander, 1989) for quadrupeds. The humeral circumference of the largest known femur MB.R.3598 (bb1) and the largest sampled femur MB.R.3593 were then estimated via relation equations based on calculations of specimen Ng. However, the conventional back-transformation model of Anderson et al. (1985) seems to overestimate body masses of large animals, so that alternative body mass estimates with the nonlinear regression model of Packard et al. (2009; see also Cawley and Janacek, 2010) were also calculated. Developmental Mass Extrapolation after Erickson and Tumanova (2000) and stylopodial scaling equations after Campione and Evans (2012) were finally applied to get additional body mass estimates for Ng and MB.R.3593 for comparison.

Table 2. Distance values and derived body mass values of Kentrosaurus aethiopicus
SpecimenElementMin diameter (mm)Circumference (mm)Body massd(kg)Body masse(kg)Body massf(kg)Body massg(kg)Body masshg(kg)Body massig(kg)
  1. a

    The circumference value is derived from the average minimum diameter of both humeri of Ng (5.35mm).

  2. b

    The circumference value is calculated using the second approximation of Ramanujan (www.paulbourke.net/geometry/ellipsecirc/) with the equation: pi * (a + b)*[1 + 3 h/(10 + (4 − 3 h)1/2)] with h = (a − b)2/(a + b)2.

  3. c

    The circumference of the humeri of MB.R.3593 and MB.R.3598 was calculated with the relation equation: x = CNgH*CMBR F/CNg F, in which CMBR F is the measured circumference of the respective femora of the MB.R specimens.

  4. d

    The body mass is calculated with the equation: 0.000084*(CH+CF)2.73 of Anderson et al. (1985) for quadrupeds and modified by Alexander (1989).

  5. e

    The body mass is calculated with the equation: 0.003352 *(CH+CF)2.125of Packard et al. (2009: Fig. 1d).

  6. f

    and gThe body mass of the two smaller individuals is calculated by using the Developmental Mass Extrapolation of Erickson and Tumanova (2000: p.555). The two body mass values obtained for MB.R3598 beforehand (d and e) represent the respective adult values. The length of the femur Ng is 487mm according to Hennig (1925) and the lengths of the other two femora can be found in Table 1.

  7. g

    h and iThe body mass is calculated using the equations by Campione and Evans (2012); i) is the phylogenetic generalized equation.

  8. The diameter values of Ng humeri and femur are taken from Hennig (1925, p.200, 212).

Ng4Humerus53168a6557761022824685717
Ng5Humerus54
Ng1Femur42166.6b
MB R 3593(Humerus)255c204818852889232821592264
MB.R.3593Femur64253
MB.R.3598 (bb1)(Humerus)318c373330083733300839544150
MB.R.3598 (bb1)Femur69.5315

Three complete cross sections from the mid-shaft region of specimen St 72 were prepared in the paleobiology thin section laboratory of the Biological Sciences Department at the University of Cape Town (UCT) following similar procedures as described by Chinsamy and Raath (1992). Complete mid-shaft cross sections were also taken from specimens MB.R.3571 and MB.R.3572 and prepared in the technical lab of the Museum für Naturkunde Berlin, Germany, and the thin section lab of the Steinmann Institute of Geology, Mineralogy and Palaeontology, University of Bonn, Germany, respectively. Core samples of specimens MB.R.3583, MB.R.3593, MB.R.3605 and Ig436 were taken from the anterior side (usually convex) of the midshaft region of the femur using a diamond drill bit of 10mm diameter following the methodology outlined by Stein and Sander (2009; see also Sander et al., 2011). Drill cores were longitudinally cut in half along the cross-sectional plane of the bone before being mounted on a slide, and thin sections of these were prepared in the thin section lab of the Steinmann Institute of Geology, Mineralogy and Palaeontology, University of Bonn, Germany. Histological sections were analyzed and photographed using a Leica DMLP microscope at the University of Bonn and a Nikon Eclipse E200 Microscope at UCT. The different Kentrosaurus samples exhibit distinctive variation in overall coloration, which suggest that they might have different chemical constituents. We tested this assumption by x-ray diffraction analysis of each of the sections using a non-destructive handheld Olympus INNOX X-ray diffraction (XRF) instrument.

RESULTS

Measurements of preserved Kentrosaurus femora show that sampled bones reflect a range of sizes, with medium-sized animals dominating while smaller and larger ones are infrequent. We considered the different size ranges to reflect varying ontogenetic age/developmental stages. The size distribution of the femora sampled for histological analysis is a good representation of the stegosaur femora from the Tendaguru beds (Fig. 1). Apart from the extensive disarticulation and sorting of the bones from the quarry “St” (mainly isolated and comprising of large bones), no signs of long transport (abrasion) and/or preburial weathering occur. Possible scavenger activity (bite marks of vertebrate or invertebrate scavengers) was noted on a few bones. Signs of recent weathering include chipping on the bone surface, development of several small, unfilled cracks in the bone surface, brittle articular ends with eroded surfaces, overgrowth with crusts of calcite and the extensive development of dendrites (the latter have been interpreted as traces of roots growing in direct contact with the bone surfaces; Keller and Frederickson, 1952; Knoll and James, 1987; Griffith et al., 1994; Hinsinger et al., 2001).

XRF Analysis

Unsurprisingly, the cortex in all bones consists of calcium phosphate (Table 3), which reflects the original composition of the tissue. Minor amounts of Cl, K, Mn, Fe, and Sr were also detected within the cortex and peripheral edges of the bone compacta. The medullary cavity of all bones is filled with calcite, probably as a result of diagenesis (Table 3).

Table 3. The perimeter, cortical bone and medullary cavity of five samples, MB.R. 3583, 436, 3605, 3593 and St 72, have been analyzed using XRF
ElementLabelPerimeterCorticalMedullary
  1. Elements P and Ca are abundant in all samples and only traces of Cl, K, Mn, Fe and Sr were detected.

 MB.R.358338.4310
 Ig 43620.717.17.2
PMB.R. 360535.431.80
 MB.R.359329.60280
 St 7233.936.30
 MB.R.35830.400
 Ig 4360.0000
ClMB.R. 36050.800.4
 MB.R. 35931.103.3
 St 720.200
 MB.R. 35830.10.21
 Ig 4360.10.20.6
KMB.R. 3605000
 MB.R 35930.30.11.6
 St 720.100
 MB.R.358360.168.598.2
 Ig 43678.582.291.5
CaMB.R. 360561.667.699.1
 MB.R. 359367.370.893.9
 St 7264.462.299.5
 MB.R. 35830.10.10.3
 Ig 4360.20.20.2
MnMR.R. 36050.10.10.3
 MB.R. 35930.10.10.5
 St 720.10.10.3
 MB.R. 35830.40.10.4
 Ig 4360.20.20.4
FeMB.R. 36051.30.20.1
 MB.R. 35931.10.60.5
 St 721.11.10.1
 MB.R. 35830.20.10
 Ig 4360.10.10.1
SrMB.R. 36050.40.20
 MB.R. 35930.30.10.1
 St 720.10.20

Bone Histology

The term “growth mark” used herein refers to periodic slow-down (annuli) or cessation of growth (lines of arrested growth = LAGs). We use the terminology sensu Francillon-Viellot et al. (1990) and Chinsamy-Turan (2005), defining an annulus as consisting of poorly vascularised lamellar bone and a LAG as showing a distinct line that represents an arrest in growth. Cancellous bone is herein defined as the part between the medullary cavity and that area of the cortex in which a clear increase in number of erosion cavities occurs. The femoral histology of the samples is described from smallest to largest, starting with MB.R.3605 (Fig. 2A), then MB.R.3572 (Fig. 2B), MB.R.3571 (Fig. 2C), MB.R.3583 (Fig. 2D), St 72 (Fig. 2E), MB.R.3593 (Fig. 2F), and finally scapula sample Ig436 (Fig. 2G). The bone shafts of all sampled femora consist of a vacant medullary cavity surrounded by trabecular struts of cancellous bone (Fig. 2A–F).

Figure 2.

Studied Kentrosaurus samples consist of drilled cores (A, D, F–G) and cross sections (B,C, E). A: Overview of the cortex and a large, open medullary cavity of femur MB.R.3605. The frame indicates the regions shown in Fig. 3. B: Cross section of femur MB.R.3572 shows varying cortical thicknesses and an enlarged medullary cavity. Frames indicate regions shown in Fig. 4A–C. C: Cross section of femur MB.R. 3571. The thick cortex on the postero-medial side contains radial vascularization. Frames indicate regions shown in Fig. 5A–C. D: Femur MB.R.3583 shows color variation and large erosion cavities in the perimedullary region. The frame indicates the region shown in Fig. 7B. E: A partial cross section of femur St 72 shows abundant erosion cavities in the perimedullary region. Frames indicate regions shown in Fig. 8A,B. F: The primary bone in femur MB.R.3593 is remodeled up to the outermost cortex. Frame indicates the region shown in Fig. 9A. G: The scapula Ig436 has a juvenile appearance. The frame indicates the region shown in Fig. 9C. a, anterior; l, lateral; m, medial; p, posterior.

The bone tissue in sample MB.R.3605 (Fig. 2A) consists almost exclusively of primary FLB. The tissue is predominantly vascularized by a reticular organization of the canals within the FLB (Fig. 3A,B). In a limited area in the deep cortex, sparsely distributed, longitudinally and circumferentially oriented primary osteons occur (white arrow in in Fig. 3A). The latter tissue ends in an annulus (arrow in Fig. 3C) and when growth resumed, the tissue changed to the reticular pattern. In the outermost cortex, there is a similar change from reticular to longitudinal vascularization (Fig. 3B) accompanied by a distinct change from woven to PFB (Fig. 3D), suggesting a decrease in the bone depositional rate. The peripheral edge of the bone is uneven and vascular canals open to the bone surface indicating active growth at time of death. Large erosion cavities and a few secondary osteons surround the medullary cavity (Fig. 3A).

Figure 3.

Bone histology of femur MB.R.3605. A: In the deep to mid cortex, primary tissue contains mainly reticular vascular canals in a woven bone matrix. A vascularization shift in the perimedullary region is indicated by an arrow. B: Towards the outer cortex, reticular vascular canals change into longitudinal ones. C: Vascularization shift is indicated by a temporary change from reticular to longitudinal vascularization and ends in an annulus, indicated by an arrow. D: A change from FLB to PFB, indicated by blue fibers, probably indicates another growth mark in the outermost cortex. Image in polarized light.

Thickness of the compact bone varies significantly in the femur sample MB.R.3572 (Fig. 2B; Table 4): notably, the oval shape of the medullary cavity and the thickness of the medial compact bone result in the larger medio-lateral diameter (61.1 mm) compared with the antero-posterior diameter (44.7 mm) of the entire bone. Primary bone is FLB and highly vascularized with large reticular vascular canals on the posterior and anterior side (Fig. 4A,B, respectively), and radial canals on the postero-medial side (Fig. 4C). Vascular canals, both reticular and radial, decrease in number and size in the outer cortex all around the cross section and more longitudinal canals occur (Fig. 4A–F). Vascular canals open to the bone surface indicating active growth at time of death. Up to six growth marks are located in the outer half of the cortex (arrows in Fig. 4A–C), of which the innermost one is present only on the postero-medial side of the bone (Fig. 4C,G) as it is remodeled on the other sides. The growth marks evident in the compacta are mainly LAGs although the fourth is an annulus, and the fifth is a LAG accompanied by an annulus (Fig. 4D–F). The sixth growth mark is deposited close to the bone surface. Sharpey's fibers are frequent in the radially vascularized region. Primary bone is remodeled to form secondary osteons scattered in the deep and mid-cortex. The outer cortex is almost void of any secondary remodeling.

Table 4. Thicknesses of the cortical bone, cancellous bone and medullary cavity were measured for the sampled specimens
LabelLocationCompact boneCancellous boneMedullary cavity
  1. Measurements (in mm) were taken around the entire cross section (MB.R.3571, MB.R.3572, St 72) or on the anterior side only in case of drilled samples (MB.R.3605, MB.R.3583, MB.R.3593).

MB.R. 3605anterior6.42.6 
MB.R. 3571anterior10.23.818.1
 posterior6.39.2
 medial15.2  
MB.R. 3572anterior9.42.319.6
 posterior8.63.2
 medial18.23.3126.5
 lateral10.42
St 72anterior87.223.9
 posterior7.86.1
 medial5.613 
MB.R. 3583anterior11.45 
MB.R. 3593anterior   
Figure 4.

The femur MB.R.3572 demonstrates variation of growth mark types. A: Five growth marks, indicated by arrows, occur in the outer cortex of the posterior side of the femur. B: Five growth marks occur in the outer cortex on the anterior side of the femur as indicated by arrows. C: Six growth marks, much more widely spaced, occur on the thick postero-medial side of the cross section. The fifth growth mark, a LAG with an annulus, appears separated into two growth marks on this side of the bone, indicated by a white bracket. D: Growth marks, indicated by arrows, on the anterior side of the cross section are a LAG (growth mark 6), a LAG accompanied by annulus (growth mark 5), and an annulus (growth mark 4). E: On the posterior side of the cross section, growth marks are LAGs (indicated by arrows) with the exception of growth mark 4, which is an annulus, and growth mark 5, which is a LAG accompanied by an annulus (indicated by a bracket). Left: polarized light; right: normal light. F: The fifth growth mark, an LAG accompanied by an annulus, separates into two growth marks, indicated by a bracket, in the radially vascularized region on the postero-medial side of the cross section. Arrows indicate growth marks. Left: polarized light; right: normal light. G: Growth marks, indicated by arrows, are more faint and even wider spaced in the radially vascularized region on the postero-medial than on the anterior and posterior side of the cross section. Left: normal light; right: polarized light.

In MB.R.3571 (Fig. 2C), primary bone is visible in the entire cortex, which consists of FLB. On the posterior side of the bone (Fig. 5A), secondary remodeling is scarce and restricted to near the medullary cavity, whereas secondary osteons occur up to the outer cortex in the anterior and postero-medial sides of the bone (Fig. 5B,C, respectively). Three distinct LAGs occur in the posterior side of the cortex (arrows in Fig. 5A). The innermost LAG is deposited in the deep cortex; the other two LAGs occur closer to the bone surface, i.e., the inner one ∼2.8 mm and the outer one ∼270 µm below the bone surface (arrow in Fig. 6A indicates outermost LAG). The three LAGs can be correlated with three annuli in the postero-medial side of the bone (arrows in Fig. 5C), one in the deep (arrow in Fig. 6B) and two in the outer cortex (arrow in Fig. 6C,D). In the anterior side, only two LAGs are visible in the outer cortex (white-rimmed arrows in Figs. 5B and 6E) corresponding to the outermost LAGs on the posterior side, while the inner LAG disappears due to remodeling. In the outer cortex and below the LAGs, the anterior side also shows two annuli (black and white arrows in Figs. 5B and 6E,F). Vascularization is generally of the reticular type on the posterior (Fig. 5A) and anterior (Figs. 5B and 6E) side, and grades into radial on the postero-medial side (Figs. 5C and 6B–D). On the posterior side, three cycles of bone deposition occur in which vascularization has a circumferential arrangement, accompanied by a change to PFB (brackets in Fig. 5A; arrows in Fig. 6G). All three cycles occur between the first and the second LAG. The outermost cycle is at least twice as wide vertically as the other two cycles, which have the same approximate vertical width. In between these cycles, vascularization reverts to the reticular type. The outermost cycle is continuous around the cross section (bracket in Figs. 5B and 6E,F), with the exception of the postero-medial side, where it is interrupted by the area of radial vascularization. On the anterior side, this outer cycle is narrower than on the posterior side, and begins with an annulus (white arrow in Figs. 5B and 6E,F). A second annulus on the anterior side (black arrow in Fig. 5F) can be correlated with the second cycle of vascularization change on the posterior side; the innermost is probably obliterated due to remodeling. They are absent in the area of radial vascularization on the postero-medial side (Fig. 5C). In summary, three LAGs on the posterior side (two on the anterior due to remodeling) become three annuli on the postero-medial side; three vascularization changes on the posterior side become two annuli on the anterior side, one accompanied by a vascularization change, and the innermost is probably remodeled. Sharpey's fibers are frequent on the radially vascularized postero-medial side.

Figure 5.

Bone histology of femur MB.R.3571. A: The primary bone on the posterior side of the cross section contains three LAGs, indicated by arrows, and three vascularization shifts, indicated by brackets. B: Secondary remodeling is more extensive on the anterior than on the posterior side of the cross section. The second and third vascularization shift on the posterior side can be correlated with annuli (black arrow), and an annuli (white arrow) accompanied by a vascularization shift (bracket) on the anterior side. Two LAGs occur in the outermost cortex, indicated by white-rimmed arrows. C: Only two LAGs, indicated by arrows, occur in the primary bone on the postero-medial side of the cross section; the remaining growth marks are most likely removed by extensive remodeling.

Figure 6.

Bone histology of femur MB.R.3571. A: Vascularization changes from circumferential to longitudinal in the outer cortex of the posterior side. A LAG is indicated by an arrow. B–D: Fast growth, indicated by radial vascularization, is interrupted by three annuli (arrows) in B) the outer cortex, C) the mid cortex, and D) the deep cortex of the postero-medial side of the femur. Left: polarized light; right: normal light. E: Varying types of growth marks include LAGs, indicated by black arrows, and a vascularization shift with an annulus, indicated by white arrow and bracket. F: A shift in vascularization, indicated by a bracket, is accompanied by a change of woven to PFB and an annulus (white arrow). Another annulus is indicated by a black arrow. Image in polarized light. G: Vascularization shows a clear temporary shift from reticular to circumferential vascular canals in the deep cortex on the posterior side of the cross section, indicated by arrows. Top: normal light; bottom: polarized light.

The medullary cavity of sample MB.R.3583 is surrounded by a thick region of cancellous bone with frequent, large erosion cavities (Fig. 2D; Table 4). The primary bone consists of FLB with longitudinally oriented vascular canals. In the outermost cortex, longitudinal vascular canals become scarce, almost absent, and the subparallel alignment of osteocytes indicates the presence of lamellar bone, representing an external fundamental system (EFS; white bracket in Fig. 7A). The deep cortex is highly remodeled by at least two generations of secondary osteons. The amount of secondary osteons decreases gradually towards the outer cortex (Fig. 7B); only the outermost cortex lacks them (Fig. 7A). Despite the high remodeling rate, up to seven growth marks can be seen throughout almost the entire cortex (arrows in Fig. 7B); four of them narrowly spaced in the outer cortex (black arrows in Fig. 7A), which form the EFS, and three deeper in the cortex. Except for the outermost growth mark, which may in fact be an annulus, all of them are LAGs. Growth marks 5 and 6 consist of two closely spaced LAGs each and therefore appear to be double LAGs (black brackets in Fig. 7A). Sharpey's fibers occur throughout the primary bone but are exceptionally abundant in the outermost cortex (e.g., white arrows in Fig. 7A).

Figure 7.

Bone histology of femur MB.R.3583. A: Growth marks five to seven in the outer cortex are annuli and probably represent an EFS. Sharpey's fibers, indicated by white arrows, are frequent in the outer cortex. B: Seven LAGs, indicated by arrows, occur in the primary bone of femur MB.R.3583 despite extensive but not very dense secondary reconstruction. Image in polarized light.

In the femur sample of St 72 (Fig. 2E), the cancellous region is almost as thick as the compact bone on the anterior and posterior side of the sample but is significantly extended in the lateral region of the bone sample (Table 4). Primary bone is FLB and the internal half of the cortex is remodeled by two generations of secondary osteons. Erosion cavities are frequent in the internal cortex (Fig. 2E). Eight growth marks occur in the un-remodeled outer half of the cortex on the lateral side of the bone (arrows in Fig. 8A). On the anterior side, the innermost growth mark is obscured by remodeling (Fig. 8B). Growth marks 2, 4, 6 and 8 are LAGs and growth marks 1, 3, 5 and 7 are annuli. Growth marks 3 and 6 differ slightly as in growth mark 3, two annuli that are separate on the anterior side (bracket in Fig. 8B) merge to a single one on the lateral side (Fig. 8A), and in growth mark 6, two LAGs that are separate on the lateral side (bracket in Fig. 8A,C), merge into one in the anterior side (Fig. 8B,D), which is why these are counted as a single annulus and LAG, respectively. Vascularization changes from being predominantly reticular to circumferential after deposition of the fifth growth mark and to longitudinally oriented vascular canals after the seventh growth mark (Fig. 8A–D). Open vascular canals on the bone surface suggest active periosteal growth at time of death (Fig. 8C). On the posterior side, the primary bone is remodeled discontinuously up to the bone surface, and Haversian systems are dense enough to obscure growth marks. Sharpey's fibers occur throughout the primary bone.

Figure 8.

Bone histology of femur St 72. A: Eight growth marks, LAGs (black arrows) as well as annuli (white arrows) occur in the cortex of the lateral side of the femur. The sixth growth mark, a LAG, separates into two on this side of the cross section, indicated by a bracket. B: On the anterior side of the cross section, the third growth mark, an annulus, separates into two, indicated by a bracket. C,D: On both the lateral (C) and anterior (D) side of the cross section, circumferential vascularization occurs in the primary bone between the sixth and seventh growth mark and is accompanied by PFB. Separation of the LAG into two is indicated by a bracket in C). Image in (C) in polarized light; in (D) left: normal light; right: polarized light.

In MB.R.3593 (Fig. 2F), only the outermost cortex contains a narrow region of primary bone due to the extensive remodeling throughout the cortex (Fig. 9A,B). It is of lamellar nature with scarce, small, longitudinal primary osteons and includes two annuli (arrows in Fig. 9A,B), representing an external fundamental system (EFS). In the periphery of the medullary cavity, two generations of secondary osteons with a little primary bone in between and thus, not yet dense Haversian bone, replace the primary tissue (Fig. 9A). Secondary osteons are almost completely filled with secondarily formed lamellar bone.

Figure 9.

Bone histology of femur MB.R.3593 (A,B) and scapula sample Ig436 (C,D). A: As a result of high remodeling, only two growth marks occur in the cortex, indicated by arrows. B: PFB in an almost avascular cortex represents an external fundamental system. Annuli are indicated by arrows. Left: normal light; right: polarized light. C: The scapula sample reveals abundant trabeculae in the medullary cavity and no growth marks. D: Primary bone consists of the FLB type with large longitudinal canals that give the scapula a juvenile appearance. Left: in polarized light; right: normal light.

The scapula sampled, Ig436 (Fig. 2G), shows a thick cancellous region (9.3 mm) compared with the rather thin compact region (3.2 mm). Large erosion cavities reaching cancellous dimensions developed in the deeper cortex (Fig. 9C) but remnants of primary bone are still recognizable in between. The elongate medullary cavity is filled with trabecular bone and is 8.1 mm in diameter at the sampling location. On the opposite side, the cancellous region is distinctly thinner (1.9 mm). The outer region of the compact bone is not preserved on this side. Secondary osteons and growth marks are absent. The primary bone consists of FLB with frequent longitudinal primary osteons, which are still in the process of being formed (Fig. 9C,D). Vascular canals open to the periosteal bone surface.

DISCUSSION

Taphonomy

During the Late Jurassic/Early Cretaceous, the Tendaguru area was located at a paleolatitude of 30° south of the equator (Scotese, 2001). Using sedimentary and fossil floral and invertebrate evidence, Aberhan et al. (2002) concluded that the Middle and Upper Saurian Beds, for which the new terms “Middle Dinosaur Member” and “Upper Dinosaur Member” were suggested by Bussert et al. (2009), represent deposits on siliciclastic tidal flats of a lagoonal system, intercalated with small tidal channels. In the upper part of the Middle Saurian Bed, occasional sabkha (salt flat)-like coastal plains with brackish lakes and ponds formed (Aberhan et al., 2002). These environments tend to be poorly vegetated. However, fossil floral elements such as Cheirolepidiaceae, Podocarpaceae, Araucariaceae and conifers, suggestive of a forest, and shrub-like elements such as pteridophytes and pteridosperms (Aberhan et al., 2002; Rees et al., 2004) most likely originate from the vegetated hinterland. Cheirolepidiaceae are widely considered to be a dominant element in forests bordering the sea in the Late Jurassic and Early Cretaceous (Alvin, 1982; Gee, 2011). The paleoenvironment of the hinterland can be best visualized as a savannah, covered by low shrubs, with higher vegetated areas around water courses (see Heinrich, 1999 for references; Rees et al., 2004; Heinrich et al., 2011). Dinosaurs that used the vegetated hinterland as feeding grounds may have died there and, after decomposition, their bones were transported by water courses, together with the plant remains, to the coastal plains where they were deposited and finally buried. Alternatively, dinosaurs, in search of water and food during dry seasons, may have died on the coastal plain itself being trapped in the mud and/or due to weakness as a result of water and food scarcity. The accumulation of mainly isolated large bones belonging to almost 40 individuals of Kentrosaurus in the ‘St’ quarry suggests some transport or reworking of the carcasses and/or sorting of the bones by tides or floods, during which smaller and, thus, lighter bones are carried farther by the water than large, heavy ones. The lack of abrasion indicates short to medium transport distances or reworking, during which disarticulation of the skeletal remains especially that of the peripheral body parts such as the skulls and most distal limb elements occurred. Trample marks on the bones and signs of scavenging may have been caused prior to or after transport/reworking and final deposition. The latter are rare and this observation as well as the lack of signs of pre-burial weathering indicate relatively rapid transport/little reworking after death and rapid burial after deposition of the isolated bones or partial skeletons. The stratigraphically younger quarry (‘X’ in the Upper Dinosaur Member) contains mass accumulations of manual and pedal elements of Kentrosaurus. This observation suggests two explanations: (a) Bones may have been sorted according to their size and weight. (b) Members of a Kentrosaurus herd might have died trapped in the mud and the decomposing carcasses were then transported by tides often leaving the stuck feet behind (Janensch 1914). The occurrence of manual and pedal elements only but lack of other similar-sized skeletal elements in the “X” quarry support the latter hypothesis.

Bone Histology and Ontogenetic Variation

The primary bone in all samples consists predominantly of FLB, which is mostly well vascularized. For comparative purposes, since we studied both cores and cross sections, histological characteristics and ontogenetic variation were assessed for the anterior regions of the femoral shaft for all samples (which is the standard sampling location for drill-core samples).

The spacing of growth marks changes from wide, as in MB.R.3605, to more narrow, as seen e.g., in MB.R.3572 (Fig. 4B) and St 72 (Fig. 8A), and finally to very narrow spacing as seen e.g., in MB.R.3583 (Fig. 7B) and MB.R.3593 (Fig. 9A). The first decrease in spacing, i.e., slow-down in growth, is probably linked to attainment of sexual maturity due to resource allocation from growth to sexual reproductivity (Andrews, 1982; Shine and Charnov, 1992; Sander, 2000). The second decrease occurs when the animal is somatically mature (Sander, 2000). Non-avian dinosaurs reach sexual maturity at a fraction of their asymptotic size (somatic maturity; Sander, 2000; Erickson et al., 2008; Lee and Werning, 2008). Thus, only specimen MB.R.3605 (345mm femur length) is interpreted as a sexually immature individual; specimens MB.R.3572, St 72 and possibly also MB.R.3571 (482mm, 644mm and 519mm femur lengths, respectively) are sexually mature, and specimens MB.R.3583 and MB.R.3593 (609mm and 689mm femur lengths, respectively) appear to be somatically mature. Growth marks are absent in the scapula Ig436 and it is thus considered to be sexually immature, which is supported by the juvenile appearance of the overall tissue including large, almost unfilled vascular canals. Secondary remodeling is almost absent in this scapula sample as well as in the sexually immature specimen MB.R.3605. In the sexually mature, somatically immature specimens, secondary osteons replace primary tissue up to the outer cortex (up to the mid cortex in St 72), and up to the outermost cortex in somatically mature specimens. Secondary reconstruction in the deep and mid cortex in specimen MB.R.3571 (Fig. 5B) supports our interpretation that it is a sexually mature though somatically immature specimen. The prevalent reticular vascularization changes into a more longitudinal orientation of the vascular canals in the outermost cortex in sexually mature, somatically immature specimens. The outermost cortex in somatically mature specimens is almost avascular and consists of lamellar bone, which is indicative of an EFS.

Ontogenetic changes are well correlated to increasing lengths of sampled femora (Table 1, Fig. 1), with the exception of specimen MB.R.3583 and St 72. The latter specimen is sexually mature but lacks an EFS, i.e., it is somatically immature, although its femur length of 644 mm exceeds the length of the somatically mature specimen MB.R.3583 (609 mm). Either, specimen St 72 is a particularly large specimen or MB.R.3583 a particularly small one. Specimen St 72 appears even younger than the next smaller, somatically immature specimen MB.R.3571 as secondary remodeling is less extensive and the outermost cortex is higher vascularized than in the latter. We therefore assume specimen St 72 to be an exception. This variation in size in relation to growth rate and timing may be an indication of either strong developmental plasticity in the species (Sander and Klein, 2005; Sander et al., 2011) or perhaps of sexual dimorphism.

Growth Variation

Growth marks are common but surprisingly variable in the Kentrosaurus specimens sampled. (1) LAGs, annuli and shifts in vascularization patterns (see discussion below) can occur in a single bone, e.g. in a single cross section of specimen MB.R.3571; (2) a growth mark can change from an annulus to a LAG or can be reflected as a vascularization shift on different sides of a single cross section of the bone, e.g., in specimen MB.R.3571; and (3) a single growth mark can separate into two growth marks on just one side of the cross section as in e.g., sample St 72. All these variations influence the count of the number of growth marks. Canal organization, which is used herein as a proxy for vascularization (even though vascular canals contain blood vessels, connective tissue etc.; Starck and Chinsamy, 2002; Hurum and Chinsamy-Turan, 2012), varies within each sample. Vascularization shifts occur in two specimens only, the smallest and youngest individual, MB.R.3605, and the third smallest, sexually mature, somatically immature specimen MB.R.3571. In MB.R.3605, the two shifts in vascularization patterns from reticular to longitudinal are the only growth marks present. On the posterior side of sample MB.R.3571, the mainly reticular vascularization pattern changes three times to mainly circumferential vascular orientations between LAGs 1 and 2, accompanied by switches from woven to PFB. Two of the vascularization shifts change laterally into two annuli on the anterior side, one still accompanied by a vascularization shift. The innermost shift is completely remodeled anteriorly. All shifts are not evident on the postero-medial side of the bone; lateral changes also affect the other growth mark types in this specimen, e.g., the three LAGs on the anterior and posterior side become three annuli on the postero-medial side. These changes are possibly the result of localized higher relative growth rates due to a muscular attachment site in the postero-medial region, resulting in a thicker bone wall.

Growth variation in specimen MB.R.3572 includes the occurrence of LAGs and annuli, as well as a LAG accompanied by an annulus. Four annuli and four LAGs alternate in specimen St 72. Lateral changes include separation of an annulus into two annuli on the anterior side and separation of a LAG into two LAGs on the lateral side of the bone. Specimens MB.R.3583 and 3593 experienced the least growth variation, showing regular LAGs and/or annuli, which may, however, be a result of the growth record being reduced by secondary remodeling.

It is widely accepted today that bone depositional rates and hence, relative growth rates can be inferred from the organization of histological tissues in the bone (Amprino, 1947; Chinsamy-Turan, 2005, 2012; Erickson, 2005; Cubo et al. 2008; Montes et al., 2010). It is further well recognized that FLB with frequent vascular canals, found especially in birds, mammals and many dinosaurs, reflects higher growth rates than in most reptiles and all amphibians which regularly have lamellar-zonal tissue with sparse vascularization reflecting slow growth (e.g., de Ricqlès, 1980; Reid, 1987; Castanet and Smirina, 1990; Chinsamy-Turan, 2005; Erickson, 2005; Cubo et al. 2008; Montes et al., 2010). Varying vascularization patterns in the FLB indicate different bone depositional rates, sometimes even within a single bone tissue or a single individual (Castanet et al., 1996, 2000; Starck and Chinsamy, 2002; de Margerie et al., 2004; Klein and Sander, 2007; Hübner, 2012; Cubo et al., 2012). Experimental studies on penguin chicks documented that the FLB with a radial organization of canals has the fastest depositional rates, while FLB with reticular and longitudinal organization of canals have intermediate depositional rates, and FLB with circumferential (laminar) canal organization is formed with the slowest depositional rates. These growth rates are, however, relative and not absolute as they vary in different skeletal elements (de Margerie et al., 2004:876).

A vascularization change from reticular to longitudinal canals and change in tissue to lamellar bone in the outermost cortex of the somatically mature specimens, representing an EFS, suggest slow relative growth rates for the longitudinal pattern in Kentrosaurus. Shifts in vascularization from reticular to circumferential or longitudinal in MB.R.3605 and MB.R.3571, accompanied by a shift to PFB, indicate a periodic decrease in the relative bone depositional rate and hence, growth rate. A combination of a vascularization shift with an annulus in MB.R.3605 and a lateral change of a vascularization shift into an annulus in MB.R.3571 support our interpretation that the changes in vascularization reflect growth marks.

The increasing number of growth marks throughout the specimens correlates with the ontogenetic changes observed in the histology as well as with their femur lengths (Table 1). The largest specimen, MB.R.3593, shows the fewest growth marks but the overall compacta shows extensive secondary reconstruction that has obliterated evidence of earlier growth marks.

Cyclic variations in the tissue, such as LAGs or annuli, are assumed to reflect seasonal/annual changes in the ambient temperatures, precipitation and/or vegetation and, thus food supply, by many authors (e.g., Peabody, 1961; Castanet et al., 1993; Sander and Andrassy, 2006). This assumption is strongly supported by a recent study of growth marks in modern wild ruminants from varying climatic environments (Köhler et al., 2012), showing that (a) growth marks occur in ruminants from all regions and (b) growth is arrested during the unfavorable season (low temperature and rainfall and thus, low food supply) for energy conservation, resumes with progressively faster rates (represented by circumferential or reticular vascularization) peaking at the height of the favorable season, and declines towards the next unfavorable season.

Beside LAGs and annuli, we observed vascularization shifts in the primary bone of two Kentrosaurus samples. These are very similar to the textural shifts documented in the polar Edmontosaurus (Chinsamy et al., 2012), in which periodic shifts from circumferential to reticular FLB are observed. Shifts are less distinct in Kentrosaurus than in the polar Edmontosaurus, but they are more distinct and frequent than in the temperate Edmontosaurus (Chinsamy et al., 2012). Spacing between the cycles varies from the anterior to posterior side of the femur according to the cortical thickness, and cross-sectional anatomical shape of the bone. The occurrence of various growth mark types in Kentrosaurus may indicate variable responses by the animal to environmental changes or other paleoecological influences, such as breeding season. We further postulate that these changes were seasonal since they tend to interrupt periods of rapid bone deposition and the modulations in growth rate appear similar to those recorded for other dinosaurs (Chinsamy et al., 2012; Hübner, 2012). Thus, a LAG forms when bone deposition is paused, while an annulus, often having lamellar bone tissue, suggests slowed-down rates of bone deposition (e.g., Chinsamy-Turan, 2005, 2012). In the case of two Kentrosaurus specimens and the polar Edmontosaurus, it appears that during the period of slow growth, growth was still reasonably fast to assure the deposition of PFB and FLB, respectively, but changes in the organization of vascularization suggests a slight change in the rate of bone deposition. In the sympatric ornithopod dinosaur Dysalotosaurus lettowvorbecki, LAGs are very rare to absent in femora (Chinsamy, 1995; Hübner, 2012) but a remarkably similar pattern of vascularization shifts as in Kentrosaurus occurs in the femur, tibia and humerus and have been interpreted as seasonal growth marks by Hübner (2012: Figs. 5C–H, 8E–H, 10). However, this type of growth marks is only evident in a restricted part of the cortex in Dysalotosaurus long bones (Hübner, 2012: Fig. 1). Interestingly, LAGs are also rare in the sympatric sauropods Brachiosaurus (Giraffatitan) brancai, Dicraeosaurus spp., Janenschia robusta and Diplodocinae indet. (=“Barosaurus africanus”) (Sander, 2000; Sander and Tückmantel, 2003; Sander et al., 2011), and when they occur, they developed in the EFS. In contrast, the so-called ‘polish lines’, which are growth lines in FLB that are visible in polished section but usually not in thin-section, are much more abundant and more regularly developed in these taxa. They were interpreted as growth marks demonstrating a strong slow down but not a complete cease of growth (Sander, 2000).

Varying growth marks may reflect varying periodic stress, either in duration or severity of the unfavorable season, and thus, the co-occurrence of vascularization shifts and/or LAGS and/or annuli in the Tendaguru dinosaurs may suggest variable responses to fluctuations in the seasonal changes of this environment. Köhler et al. (2012) documented that arrested growth is of longer duration in high latitudes (longer duration of unfavorable season) than in low latitudes and is expressed by simple LAGs but not in variations of the type of growth marks as seen in Kentrosaurus. This may suggest that Kentrosaurus reacted differently to fluctuations in seasonal duration and/or severity than modern ruminants.

Klevezal (1996) suggested that populations living in strongly seasonal environments develop regular growth marks whereas populations inhabiting less seasonal environments (even if both populations belong to the same species, e.g., wolf or Edmontosaurus) show much more variation in the development and regularity of growth marks (see also Hübner, 2012). Sedimentary evidence and the occurrence of fossil charcoal in the Tendaguru Formation support pronounced dry seasons alternating with seasonal rainfalls in a subtropical to tropical, semi-arid paleoclimate (Hallam, 1985, 1993; Valdes and Sellwood, 1992; Valdes, 1994; Aberhan et al., 2002; Rees et al., 2004). Seasonal rainfalls are indicated by the presence of Glyptostrobus- and Podocarpus-related wood (Figueiral et al., 1999), fungi infesting and degrading the wood (Alvin et al., 1981) as well as caliche nodules (Heinrich, 1999; Rees et al., 2004). Modern savannahs in similar latitudes and of similar climate experience one rainy season per year during the summer months (the 'high-sun' period from October to March; Rees et al., 2004) of the southern hemisphere with an average of 4–6 months of rainfall, which can decrease to < 4 months during drier years and increase to > 6 months during wetter years (e.g., Nicholson, 2000). Fluctuations of rainfall were remarkable throughout the African continent from the late Pleistocene to the last decades and even large yearly fluctuations were measured in the last approximately 100 years (Nicholson, 2000). Climate fluctuations of the Mesozoic cannot be directly compared to today's climate as the continental distribution differed and would have affected the circulation patterns of water currents, winds and atmospheric pressure that result in different precipitation patterns during the Mesozoic. However, as the presence of evaporites and microphyllous flora and conifers indicate generally arid to semi-arid conditions, and caliche nodules an alternating dry-wet climate (Heinrich, 1999; Rees et al., 2004), this savannah-like climate with one rainy season per year and fluctuations of rainfall in the rainy season are likely for the Mesozoic Tendaguru environment. Growth in Kentrosaurus may have been arrested, resulting in the deposition of LAGs, when vegetation was restricted to a few water courses, i.e., in a dry season (winter), and resumed at its usual rapid rate at the beginning of the following rainy season (summer) when vegetation was flourishing. Fluctuations in precipitation would presumably result in varying responses to bone depositional rate (i.e., FLB with circumferential canals indicating a slightly decreased rate of bone formation, even slower growth indicated by an annulus, or an arrest of growth reflected in LAGs). If the observed variation of growth marks in Tendaguru dinosaurs indeed reflects fluctuations in annual precipitation and food supply, it would indicate that growth of these animals was remarkably susceptible to environmental/climatic changes.

Direct correlation of the varying growth marks in Kentrosaurus to climatic fluctuations is, however, problematic since variation across a single section can lead to different interpretations. Variation in type and regularity of growth marks is influenced by a combination of extrinsic factors, such as climatic fluctuations, and intrinsic factors, such as biomechanical properties of the bone (e.g., Chinsamy and Abdala, 2008; Hübner, 2012).

Lateral Histological Variation

A distinctive region with radial organization of vascular canals occurs in the postero-medial part of the cross sections of MB.R.3571, MB.R.3572, and St 72. Lateral changes also occur in the degree of secondary remodeling in Kentrosaurus. It decreases from the anterior side towards the posterior side with the exception of the postero-medial part of the cross section where it is exceptionally high. Beside radial vascularization and extensive remodeling, this postero-medial region also has a high abundance of Sharpey's fibers, which suggests that this was a site of muscular attachment. These variations have been observed in previous studies, e.g., as early as de Ricqlès (1969) and more recently by Chinsamy-Turan and Ray (2012), Hübner (2012), and Jasinoski and Chinsamy-Turan (2012). In a detailed osteological description of Kentrosaurus, Hennig (1924) notes that the fourth trochanter, which is positioned in the postero-medial midshaft region of a femur, is hardly developed in a series of Kentrosaurus femora up to 530 mm length. Even in larger femora, this origin for the caudofemoralis muscle is vestigial and appears as an obtuse swelling, similar to Stegosaurus (Gilmore, 2011). This “swelling” can in fact be observed in the cross sections of femora shorter than 530 mm analyzed herein (MB.R.3572 and 3571; Fig. 2B,C) and a pronounced thickening of the cortex that forms the “swelling” can clearly be assigned to the fast-growing radially vascularized tissue in this region. In response to the stresses and strains exerted on this area by the muscles, the cortex is thickest in this region and secondary remodeling is more extensively developed as well as closer to the periphery than in other parts of the bone. A similar observation, i.e., higher portion of radial canals in the postero-medial corner in femoral cross sections, including a higher amount of Sharpey's fibers, and more extensive secondary remodeling, was described for the postero-lateral side in femoral cross sections of Dysalotosaurus (Hübner, 2012). Lateral vascularization changes due to mechanical reasons, such as the radial example described herein indicating muscle attachment, clearly cause lateral changes in, and thus interpretation of, growth marks as discussed above.

Body Mass

The body masses of the largest known specimen MB.R. 3598 and largest sampled specimen MB.R.3593 were calculated, based on known measurements of the only specimen reported with both articulated humeri and femur (Ng; Hennig, 1925; Tables 1 and 2). Using these, imprecisions for estimates of growth rates, however, arise as allometric growth of humerus and femur length and circumference are unknown for Kentrosaurus due to a lack of juvenile material, articulated humeri and the general scarcity of stegosaur material. In addition, the number of growth marks varies considerably as discussed above and we do not have a complete growth record preserved (due to a lack of juvenile material or full growth mark record in a specimen with unremodelled cortical bone), which makes retro-calculation of growth marks and their comparison with body mass highly erroneous and therefore of no scientific value. Various authors (e.g., de Margerie et al., 2004) showed that growth rates vary remarkably for different tissues in different species, bones and even in a single bone and that PFB grows more slowly than woven bone.

Comparison with Other Thyreophora

Bone histological studies on long bones of thyreophoran dinosaurs (Fig. 10) include that of the basal Scutellosaurus (Padian et al., 2004) and Stegosaurus (Hayashi et al., 2009; Redelstorff and Sander, 2009). Two Scutellosaurus femora sampled by Padian et al. (2004) consist of woven bone and are poorly vascularized by mainly longitudinal canals; vascularization is slightly denser in the internal cortex, however, remains less so than in most dinosaurs. Other long bones of Scutellosaurus (radius, tibia, ulna) even exhibit PFB with scarce vascular canals (and osteocyte lacunae). Padian et al. (2004) concluded that Scutellosaurus bone histology is characterized by a very slow growth rate, more similar to the bone histology of a captive Alligator (e.g., Lee, 2004; Woodward et al., 2011). Various long bones of small (femur length 233 mm) to large-sized (femur length 915 mm) Stegosaurus specimens were analyzed by Hayashi et al. (2009). They reported FLB with predominating radial and reticular canals with strong regional variation: radial canals on the posterior side and longitudinal canals grading into reticular ones on the anterior side. The medium- and large-sized specimens show similar bone histologies with mainly longitudinal vascular canals in FLB. In the studied specimens of Stegosaurus, longitudinal primary osteons are prevalent in the FLB that consists of a mixture of woven and PFB (Redelstorff and Sander, 2009). Although vascularization patterns are similar in Scutellosaurus and Stegosaurus (both being dominantly longitudinal), vascular canals are notably more scarce in the former and located within PFB, which implies even more slowly formed bone tissue than in Stegosaurus. Our finding of well vascularized, FLB in the phylogenetically intermediate Kentrosaurus is indicative of a more rapid growth rate, and suggests that the slow growth rates previously observed in other thyreophorans are not uniformly present throughout the clade.

Figure 10.

Phylogenetic relationships of the Thyreophora and outgroups, after Butler et al. (2009), Maidment (2010), and Pol et al. (2011).

An osteohistological analysis of a range of small- to large-sized dinosaurs by Padian et al. (2004) included the basal thyreophoran ornithischian Scutellosaurus (∼1 m adult body length), the basal ornithopod Orodromeus (1.5 m body length), the theropod Coelophysis (∼1.7 m adult body length), the basal ceratopsian Psittacosaurus (∼2 m adult body length) and the hadrosaur Maiasaura (∼7 m adult body length) and shows that large dinosaurs exhibit faster growth rates than small dinosaurs. Linear regression analysis of adult body mass and maximum growth rates quantatively supports this observation (e.g., Erickson et al., 2001, 2009; Lehman and Woodward, 2008; Hübner, 2012). This hypothesis is further supported when comparing the very slow growing, small Scutellosaurus (∼1 m adult body length) with the slightly faster-growing, large Stegosaurus (approximately 7–9 m). However, intermediate-sized Kentrosaurus (∼6 m) shows higher absolute growth rates than the large, more derived Stegosaurus, and contradicts the supposition by Padian et al. (2004). It is therefore possible that the plesiomorphic condition in the thyreophorans is (a) a rapid growth strategy, but that Stegosaurus for some as yet unknown reason secondarily reduced its growth rate or (b) a slow growth strategy and that Kentrosaurus exhibits an autapomorphic increase in growth rate.

CONCLUSIONS

  1. The long bone histology of Kentrosaurus shows highly vascularized FLB indicative of the fast rates of growth described for many dinosaurs. In addition to LAGs and annuli, vascularization shifts from reticular to circumferential vascular patterns occur, similar to the ones described in the polar Edmontosaurus and reported in Dysalotosaurus (sympatric with Kentrosaurus), and most likely represent annual growth marks.
  2. Variation in the types of growth marks suggests a certain amount of growth plasticity in Kentrosaurus in response to seasonal environmental stress. Lateral variations of growth marks (e.g., vascularization shifts becoming annuli; annuli transforming into LAGs) reflect local growth conditions across the cross sectional geometry of the bone.
  3. Thyreophoran bone histology is not uniform. Their tissues vary significantly from almost Alligator-like, sparsely vascularized PFB in the small, basal Scutellosaurus to highly vascularized woven bone in Kentrosaurus, that is intermediate in both body size and phylogenetic position, to a combination of woven and PFB with also relatively sparse vascularization in the large, more derived Stegosaurus. It is possible that Stegosaurus has a secondarily reduced growth rate or that Kentrosaurus is the exception with an increased growth rate.

ACKNOWLEDGEMENTS

The authors thank O. Hampe, J. Müller, D. Schwarz-Wings and Wolf-Dieter Heinrich (Museum für Naturkunde Berlin, Germany) for access to the collections and permission to sample the specimens. O. Dülfer's (Steinmann Institute for Geology, Mineralogy and Palaeontology, University of Bonn, Germany) involvement in sample preparation is much appreciated. D.B. Thomas (Smithsonian Institution, Washington DC) is thanked for assistance with the XRF analysis.

Ancillary