Osteohistology of Indian fossil vertebrates
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Growth patterns of fossil vertebrates as deduced from bone
microstructure: case studies from India
S RAY1,*, D MUKHERJEE1 and S BANDYOPADHYAY2
1
Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721 302, India
2
Geological Studies Unit, Indian Statistical Institute, 203 B T Road, Kolkata 700 108, India
*Corresponding author (Email, [email protected])
Bone microstructure is affected by ontogeny, phylogeny, biomechanics and environments. These aspects of life
history of an extinct animal, especially its growth patterns, may be assessed as fossil bone generally maintains its
histological integrity. Recent studies on the bone histology of fossil vertebrates from India encompass different types
of temnospondyls and dicynodonts from different Permian and Triassic horizons. The examined taxa show that they
had distinct bone histology and varied growth patterns. The Early Triassic trematosaurids had an overall fast growth,
which contrasts with that of the Middle and Late Triassic temnospondyl taxa examined. The dicynodonts on the
other hand, were characterized by an overall fast growth with periodic interruptions, variable growth rates dependent
on ontogeny and indeterminate growth strategy. A comparative study encompassing several neotherapsid genera
including the dicynodonts shows significant evolutionary trends towards determinate growth strategy and reduced
developmental plasticity.
[Ray S, Mukherjee D and S Bandyopadhyay S 2009 Growth patterns of fossil vertebrates as deduced from bone microstructure: case studies from
India; J. Biosci. 34 661–672] DOI 10.1007/s12038-009-0055-x
1.
Introduction
Bone is a mineralized connective tissue, produced by
the deposition of hydroxyapatite [Ca10(PO4)6(OH)2]
microcrystals on a framework of collagen fibres (Chinsamy
and Dodson 1995). Within this structure the bone cells
(osteocytes), blood and lymph vessels, and numerous
small canals are present. These components are almost
constant in all vertebrates, starting from the earliest fishes
to the present day reptiles, birds and mammals. When an
animal dies the organic components including the cells
and blood vessels decompose, whereas the inorganic
component becomes fossilized and retains the overall bone
microstructure.
Bone microstructure provides substantial information
about the palaeobiology of the fossil animal. Bone tissue
reflects ontogeny (juvenile to adult stages), age, life style
adaptations and biomechanical function as well as various
other events that punctuate the life history of an individual
Keywords.
(Chinsamy 1997). In general, the rate of growth changes
during ontogeny and the principal difference between
organisms lies in whether growth eventually ceases.
Mammals and birds have a determinate growth where a
mature stage is reached in which structural growth stops
though the organism continues to live (Chinsamy and
Dodson 1995; Foote and Miller 2007). However, in many
forms the growth is indeterminate where the organism
continues to grow throughout life.
There are numerous works dealing with the bone
histology and growth patterns of fossil vertebrates,
especially the archosaurs such as the avian and non-avian
dinosaurs (Chinsamy 1990, 1995; Chinsamy et al. 1998;
Curry 1999; Horner et al. 1999, 2000, 2001; Erickson and
Tumanova 2000; Sander 2000; Chinsamy-Turan 2005). For
example, Chinsamy and Elzanowski (2001) deduced the
evolutionary path of the growth patterns in avian dinosaurs
and suggested that these have evolved through a stage of
slowed postnatal growth and precocious flight. Another
Bone microstructure; dicynodonts; India; osteohistology; temnospondyls
http://www.ias.ac.in/jbiosci
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study by Padian et al. (2004) has revealed that moderate to
high growth is a plesiomorphic (ancestral) feature for the
archosaurs with secondary reversals to lower rates in some
clades of small size. The large body size, especially in case
of the dinosaurs is made possible by the high rate of growth,
which is comparable to those of non-reptilian amniotes than
to those of living reptilian amniotes. The function of the high
frontoparietal domes of the pachycephalosaurid dinosaurs
has been long under debate. High-resolution images of the
histological thin sections of an ontogenetic series of these
domes were used to test the hypothesis that these Cretaceous
dinosaurs had head-butting behaviour similar to that of the
modern day bighorn sheeps (Goodwin and Horner 2004).
The study revealed that the bone microstructure of the
domes was not suitable for head butting and such high
domes were probably used as cranial display in support of
species recognition and communication.
Although there are considerable works on the bone
histology of the archosaurs, relatively fewer work
encompass the synapsids (i.e. the reptilian clade on line
to the mammals). Previous work such as those by Gross
(1934), Enlow (1963), Ricqlès (1976), were mostly based
on isolated or unidentified bones, which frequently could not
be identified to the genus level. It may be mentioned here
that an ideal dataset comprises multiple skeletal elements
of several individuals in order to assess histovariation.
However, the destructive nature of the analysis does not
allow sectioning of an ideal data set. More recent work
on non-mammalian synapsids based on multiple skeletal
elements, especially on the dicynodonts and cynodonts
(e.g. Botha and Chinsamy 2000, 2004; Botha 2003; Ray
and Chinsamy 2004; Ray et al. 2004) has shown that
the non-mammalian synapsids possess a wide range of
histological characteristics, which can be used to decipher
growth patterns, life habits and evolution. For example,
the non-mammalian cynodonts showed markedly differing
growth patterns with the early form Procynosuchus showing
rapid cyclical growth followed by considerable decrease in
growth once adulthood was reached (Ray et al. 2004). Two
more derived but contemporary cynodonts Cynognathus
and Diademodon showed markedly differing growth
patterns (Botha and Chinsamy 2000). Diademodon, an
omnivore/herbivore experienced cyclical growth, possibly
related to environmental fluctuation whereas Cynognathus,
a carnivore had rapid sustained growth. Such differences in
growth strategy between two contemporaneous taxa have
been attributed to either contrasting food requirements or to
inherent physiological differences.
In contrast to the advances made in examining the bone
microstructure of fossil vertebrates throughout the world
and deduction of their palaeobiology, especially their
growth patterns and or rates, work related with that of the
Indian vertebrates is few. This paper attempts to summarize
J. Biosci. 34(5), November 2009
some of the recent works on the osteohistology of the Indian
vertebrates such as the Mesozoic temnospondyls, a basal
tetrapod group and the dicynodonts, an extinct group of
Permo-Triassic neotherapsids after giving a preliminary
introduction on bone microstructure.
2.
Bone microstructure
2.1 Osteohistological features
Bones grow by a process of appositional surface deposition
(Chinsamy-Turan 2005) on various surfaces such as
periosteal (from periosteum), endosteal (from endosteum),
inner surfaces of osteons, surfaces within resorption spaces,
and so on. However, bones do not simply grow by addition
of new bones on the surfaces. A typical long bone grows
in two general directions (figure 1), in length (longitudinal)
and in diameter (transverse) (Francillion-Vieillot et al.
1990; Ricqlès et al. 1991). Longitudinal growth takes place
by endochondral ossification where bone deposits over
cartilaginous precursors and gradual resorption by cartilage
– absorbing cells called chondroclasts. The transverse
growth involves both the periosteal and endosteal surfaces;
the appositional increase of bone on either periosteal or
endosteal surface involves corresponding resorption from
the contralateral surface.
In a transverse section of a bone there is usually a dense/
compact outer region (compacta or cortex) that surrounds a
central medullary cavity (or spongiosa). In addition there are
small tubular passageways for blood vessels and lymphatic
vessels that are termed as vascular channels (figure 2). Many
bones show structural discontinuities or growth rings. These
include lines of arrested growth (LAG) suggesting cessation
of bone deposition, annuli suggesting slow growth, and
reversal lines where resorption and redeposition take place
in opposite direction.
During periosteal ossification/osteogenesis, there may
be primary or secondary bone tissues. There are different
types of primary bone tissues whereas the secondary bone
is formed by the internal reconstruction of the pre-existing
bone, a process termed secondary reconstruction or Haversian
remodeling. The sequence of this formation involves first
the resorptive enlargement of the primary canals or the
primary osteons. This is followed by subsequent centripetal
deposition of concentric lamellar bone within these eroded
surfaces. These are called secondary osteons (figure 2), in
which the cementing (reversal) lines surround the deposited
concentric lamellated bone and show the extent of resorption
of the primary osteon. On the other hand, the primary bone
tissues are as follows:
(a) Fibrolamellar bone tissue (FLB) – This tissue type
results from woven fibred bone matrix and primary osteons.
Presence of fibrolamellar bone tissue in fossil vertebrates
Osteohistology of Indian fossil vertebrates
suggests rapid osteogenesis and fast overall growth. This
tissue may be further subdivided based on the arrangement of
the primary osteons: laminar (circumferential arrangement),
plexiform, radial and reticular (figure 3).
(b) Lamellar zonal or lamellar bone tissue (LZ) – LZ has
an ordered arrangement of collagen fibres and osteocytes (or
in case of fossils, osteocyte lacunae), which are flattened
and oriented in a particular direction. LZ represents period
of slowest bone deposition.
Figure 1. Schematic representation of a long bone showing
growth in length (longitudinal growth) and in diameter (transverse
growth). Modified from Chinsamy-Turan (2005). Black solid and
gray dashed lines show the size of the bones before and after
growth respectively.
Figure 2.
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(c) Parallel fibred bone tissue (PFB) – This is characterized by a matrix consisting of mutually parallel collagen
fibres. It is an intermediate condition between the woven
fibred and lamellar zonal bone tissues.
Endosteal ossification is generally applied to ossification
occurring after the resorption of a pre-existing bone
tissue, and is separated from primary bone tissue by a
Figure 3. Schematic representation showing different
arrangement of the peimary osteons within the fibrolamellar bone
tissue. (A) laminar; (B) plexiform; (C) radial and (D) reticular
patterns (modified from Francillon-Vieillot et al. 1990).
Schematic representation of a transverse section of a long bone microstructure showing its various components.
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S Ray, D Mukherjee and S Bandyopadhyay
cementing line. It is characterized by centripetal mode of
bone deposition. There are usually two common types of
endosteal tissues:
(a) Compacted coarse cancellous bone – In some situation
compact bone is formed from originally cancellous bone by
the thickening of trabeculae (or bony struts) until the intertrabecular spaces are reduced to narrow vascular channels.
This commonly occurs in response to external remodeling,
surrounding the medullary cavity where bone resorption
would otherwise expose cancellous bone.
(b) Inner circumferential lamellae – This is used for
bone tissues deposited on the walls of the medullary cavity
(figure 2). Medullary cavities first appear in the diaphysis
of the developing bone and then expand longitudinally
and radially (transversely) as growth proceeds. When
resorption ceases, new endosteal bone may be laid down
on the internal surface it produces. It is often deposited
as a uniform lining around the medullary cavity and is
known as the inner circumferential lamellae. Presence of
inner circumferential lamellae indicates the cessation of
medullary expansion.
2.2 Terminology frequently used in osteohistological description (after Francillon-Vieillot et al. 1990; Curry 1999)
Annuli: Growth rings in bone corresponding to periods of
slow growth.
Avascular bone: Compact bone, which is completely
devoid of an intrinsic vascular network.
Bone matrix: the collagenous and non-collagenous
organic constituents of the bone.
Bone remodeling: The normal and more or less continuous
reshaping of a bone by resorption and redeposition, mainly
related to morphogenesis during early life, but also related
to mechanical constraints and physiological demands later
in life.
Cementing lines: Thin discontinuity layers in bone tissues
representing resorption lines and resting lines.
Compact bone (=compacta): A macroscopic category
of bone architecture in which the volume of bone tissue is
higher than the volume of pore space.
Cortex: the outer peripheral region of a long bone, often
considered synonymous with compacta.
Diaphysis: The more or less cylindrical, central part of a
long bone, which contains the marrow cavity.
Endochondral ossification: It is the substitution of
preformed cartilaginous element with bony tissue by the
concomitant destruction of the cartilage model.
Endosteal ossification: Ossification on the inner surface
of the cavities of the bone.
Epiphyses: Are the extremities of long bones, which are
capped by articular cartilages and consists of a spongiosa of
cancellous bone derived from endochondral ossification.
J. Biosci. 34(4), October 2009
External circumferential lamellae: Are multiple lines of
arrested growth at the periphery that suggests cessation of
overall growth.
Fibrolamellar bone: A complex of woven or fibrous,
cancellous matrix of periosteal origin, and a lamellar matrix,
centripetally deposited, forming the primary osteons which
ultimately fill the space originally empty in the woven
periosteal matrix.
Internal circumferential lamellae: Are multiple lines of
arrested growth at the periphery of the medulla suggesting
that expansion of the medulla is complete.
Lamellar bone: Bone tissue of periosteal or dermal
origin, which is mainly parallel-fibered or truly lamellar,
with vascularization generally scattered but sometimes
absent or dense, and commonly with extensive evidence of
cyclic growth.
Lines of arrested growth (LAG): Lines in bones
corresponding to momentary but complete cessation of
growth, as opposed to periods of slow growth (annuli).
Medulla: The inner or deep central part of a long bone.
Metaphysis: Transitional region between the epiphysis
and diaphysis. In this region, the cortex contains spongy
bone of endochondral origin which is later converted into
compact, coarse, cancellous bone.
Osteocyte lacunae: Lacunae left behind by the bone cells
(osteocytes) after decomposition of the organic component
of the bone.
Parallel-fibered bone: Bone tissue characterized by a
matrix consisting primarily of mutually parallel collagenous
fibres.
Periosteal ossification: Ossification resulting from the
activity of the periosteum at the outer surface of the bone.
Primary bone: Bone deposited where antecedent bone
tissue did not exist.
Primary osteon: A vascular canal surrounded by
concentric bone lamellae which are centripetally deposited
without resorption at the previous periphery of the canal.
Secondary osteon: A vascular canal surrounded by
concentric bone lamellae which are centripetally deposited
after resorption at the previous periphery of the canal.
Sharpey’s fibres: Closely packed collagen fibres which
insert perpendicularly into the anchoring bone, that serve
as attachment processe for soft tissues such as muscles,
tendons and ligaments.
Trabecular bone: A type of cancellous bone in which
trabeculae show a precise three- dimensional relationship
which reflects mechanical forces acting on the bone, with
some trabeculae oriented according to compressional and
some oriented according to tensional forces. Trabecular
bone is mechanically more porous than the compact bones,
and is usually found in the ends of long bones.
Vascular bone: Any bone tissue containing an intrinsic
network of blood vessels.
Osteohistology of Indian fossil vertebrates
Woven fibered bone matrix: Coarsely and loosely packed
collagen fibres of varying sizes which are distributed without
any ordered spatial arrangement.
Zones: Growth marks in bone corresponding to periods
of high growth rate (not necessarily annual).
3.
3.1
Osteohistological description
Case study I: Triassic temnospondyl
The osteohistological analysis includes examination of
several skeletal elements including the limb bones (humeri,
femora and tibiae), ribs and intercentra of three Triassic
temnospondyl taxa from India (Mukherjee et al. 2008).
These include the Trematosauridae from the Early Triassic
Panchet Formation of the Damodar basin (Huxley 1865;
Tripathi 1969), Paracyclotosauridae from the Middle Triassic
Denwa Formation of Satpura basin (Mukherjee and Sengupta
1998) and Chigutisauridae from the Late Triassic Maleri
Formation, Pranhita-Godavari basin (Sengupta 1995).
The transverse sections of all the bones examined,
irrespective of the skeletal elements, are composed of a
well-differentiated central medullary region surrounded
by a relatively compact cortex (figure 4). Most of the
‘vascular channels’ (sensu Ray and Chinsamy 2004) are
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isolated, discrete and longitudinally oriented, often varying
in shape and orientation, and have radial and circumferential
anastomosis (figure 4A–B). At the regions of muscle
insertions such as the deltopectoral crest, cnemial crest, and
anterior and posterior ends of the ribs, vascular channels
are radially oriented, often running from the inner to outer
cortex. The diaphyseal sections of the bones also reveal
extensive secondary reconstruction in the form of enlarged
resorption cavities (figure 4C) and secondary osteons in the
perimedullary region. Margin of the medulla is often uneven
and resorptive in nature, whereas medullary region is filled
with a dense network of bony trabeculae of the cancellous
bone.
Apart from these generalized features, the three families
of Indian temnospondyls examined show distinctly
different primary tissues (figure 4A-C) that are preserved
in the cortical region (Mukherjee et al. in press). The Early
Triassic trematosaurids are characterized by a thick zone
of avascular lamellar bone along the periosteal periphery
and fibrolamellar bone tissue in the inner cortex (figure
4A). Growth rings (annuls and LAG) are absent in most
of the elements examined except for one humerus (ISIA
181/1) [Mukherjee et al. in press]. In addition, the ribs of
other indeterminate Early Triassic temnospondyls examined
document woven fibred bone matrix.
Figure 4. Transverse sections of (A) Trematosauridae. ISIA181/2, femur showing fibrolamellar bone tissue (flb). (B) Paracyclotosauridae.
ISIA181/7, humerus showing a thick cortex (cor) surrounding a large medullary spongiosa (ms). (C) Chigutisauridae. ISIA41, humerus
showing a narrow cortex containing LAGs and large resorption cavities (rc). Note the LAGs are shown by arrows. Scale bars equal
300 μm.
S Ray, D Mukherjee and S Bandyopadhyay
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The Middle Triassic paracyclotosaurids examined
were characterized by lamellar bone in the cortex. In the
humerus, the vascular channels are longitudinally oriented
and with circumferential anastomoses resulting in a densely
parallel network (figure 4B). The femur and tibia have a
low vascularity, and in the latter the osteocyte lacunae are
arranged in a parallel network. All the ribs examined in this
group exhibit woven fibred bone matrices. Growth rings are
absent in most of these bones except the three LAGs found
in one intercentrum (ISIA 181/11). Abundant Sharpey’s
fibers are also found present in the intercentra.
Similar examination of the Late Triassic chigutisaurid
shows that the predominant primary tissue of the cortex is
lamellar. However, the woven fibred matrix is seen in the
cortex of the rib. Although growth rings are absent in the
rib and intercentrum three LAGs are present in the humerus
(figure 4C). Other features of the humerus include extensive
secondary reconstruction. Almost 90% of the cortex is
composed of erosionally enlarged vascular channels
resulting in a high cortical porosity (figure 4C). Few welldeveloped secondary osteons are seen in the perimedullary
region of the humerus, whereas the medullary region is
partially filled with a network of bony trabeculae of the
cancellous bone.
3.2
Case study II: Permo-Triassic therapsids
(‘Dicynodontia’)
Three Indian ‘dicynodonts’ were examined to deduce their
growth patterns and rate of growth. These include the Late
Permian taxon, Endothiodon mahalanobisi collected from
the Kundaram Formation of the Pranhita-Godavari basin
(Ray 2000), Lystrosaurus murrayi from the Early Triassic
Panchet Formation of the Damodar basin (Tripathi and
Satsangi 1963) and Middle Triassic Wadiasaurus indicus
from the Yerapalli Formation of the Pranhita-Godavari
basin (Bandyopadhyay 1988). In addition, these taxa were
compared with other dicynodonts and neotherapsids from
other parts of the world, especially those collected from
South Africa to elucidate their evolutionary trends.
3.2.1 Endothiodon: Several skeletal elements including a
humerus, a tibia and a fibula of Endothiodon mahalanobisi
were examined. Bone microstructure shows that there is
a distinct and small medullary cavity surrounding a thick
cortex (Ray et al. 2009). The cortex contains moderately
vascularized fibrolamellar bone tissue with the primary
osteons arranged in a reticular to plexiform pattern (figure
5A). In comparison to the humerus, the fibula and tibia
are relatively poorly preserved because of diagenetic
alteration. However, the tibia and fibula also have thick
cortices similar to the humerus. The fibula is characterized
by a low vascularity, primary vascular channels and discrete
and sparse globular osteocyte lacunae. The tibial cortex
contains fibrolamellar bone tissue. A LAG is present in
the mid-cortical region of the humerus, whereas growth
rings are absent in the tibia and fibula. All the limb bones
show large, irregular resorption cavities (figure 5B). Few
secondary osteons are present in the perimedullary region
of the tibia.
3.2.2 Lystrosaurus: Several disarticulated skeletal
elements of Lystrosaurus murrayi, collected from a
monospecific bone bed of the Early Triassic Panchet
Formation, Damodar Basin, India were examined.
Specimens of the present study include five humeri (60.87–
79.72 mm), six femora (36.77–90.3 mm), a tibia (88.4 mm),
fibula (85.39 mm), a clavicle and a rib fragment. Diaphyseal
sections of the humeri examined reveal that the cortex is
predominantly composed of the fibrolamellar bone (Ray
et al. 2005). The primary osteons are arranged in a laminar
pattern, though this may vary from one humerus to the next,
or even locally within the same humerus. Growth rings are
absent in the humeri. In ISIR751, the perimedullary region
to the midcortex contains erosionally enlarged channels that
Figure 5. Endothiodon mahalanobisi. A-B, ISIR771/1, transverse sections of a humerus showing (A) highly vascularized fibrolamellar
bone tissue (flb) in reticular pattern. Arrow indicates LAG. (B) Large erosionally enlarged resorption cavities (arrows) shown at a higher
magnification. Scale bars equal 300 μm.
J. Biosci. 34(5), November 2009
Osteohistology of Indian fossil vertebrates
resulted in a high cortical porosity. The medullary region of
the humeri contains bony trabeculae of the cancellous bone.
Examination of the femora on the other hand shows a
narrow cortex containing fibrolamellar bone tissue, high
cortical porosity and a large medullary region. The primary
osteons show reticular-subplexiform arrangement (figure
6A, B). A LAG is present in ISIR758 (figure 6B). In ISIR
757 the cortical porosity is about 12%. A noteworthy feature
of the tibia is the very narrow compact cortex (figure 6C)
in comparison to that of the other elements examined.
Other features include extensive secondary reconstruction,
which resulted in erosionally enlarged vascular channels
in the inner and mid-cortex and an irregular periosteal
periphery (figure 6D). Only the metaphyseal region of the
fibula (ISIR760) was available for study and its transverse
section shows that the primary osteons are in a subreticular
arrangement. Growth rings are absent. There are a few
secondary osteons.
3.2.3 Wadiasaurus: Various limb bones (humerus, femur,
radius, ulna, fibula) along with several dorsal ribs and
scapulae were used for the study. The transverse sections of
all the bones examined, irrespective of the skeletal element,
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show a well differentiated centrally located medullary cavity/
spongiosa surrounded by an outer relatively compact cortex
(Ray et al. in press). Based on the gross morphology, relative
size and bone microstructure the examined specimens of
Wadiasaurus constitute three size classes (Ray et al. 2009)
showing distinctly different bone microstructure.
In the first size class (<30% adult size, Group A), the
specimens show well vascularized fibrolamellar bone tissue
in the cortex, with the primary osetons arranged in a reticular
pattern. Other characteristic features in this size class
include a large medullary spongiosa, irregular subperiosteal
periphery, absence of growth rings and a high cortical
porosity. Although all the specimens of the second size class
(30%-60% adult size, group B) show a preponderance of
the fibrolamellar bone tissue (figure 7), the feature varies
depending on the bone examined. The ulna is characterized
by a high prevalence of radially oriented vascular channels
at the medial and lateral edges and presence of three LAGs
(figure 7A) whereas the radius does not contain any growth
rings. The femora examined show presence of parallelfibred bone in the outer cortex and fibrolamellar bone in
the inner cortex (figure 7B). Bone microstructure of all the
Figure 6. Lystrosaurus murrayi. Transverse sections of (A) ISIR752, femur showing highly vascularized fibrolamellar bone tissue in the
cortex; (B) ISIR758, femur showing extensive erosionally enlarged resorption cavities surrounding a medullary spongiosa. Arrow indicates
a LAG; C-D, ISIR759, tibia showing (C) a narrow compact cortex and resorption cavities extending from the perimedullary region to
mid-cortex. Note the extensive secondary reconstruction; (D) Fibrolamellar bone tissue shown at higher magnification. Note the irregular
periosteal periphery (arrow). Scale bars equal 300 μm.
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Figure 7. Wadiasaurus indicus. Transverse sections of the different limb bones of group B. (A) ISIR175/75, ulna showing zonal
fibrolamellar bone tissue separated by multiple LAGs (arrows). Note the presence of a prominent annulus (an). (B) ISIR175/68, femur
showing peripheral parallel fibered bone (pfb) and fibrolamellar bone (flb) in the deeper cortex. An arrow indicates an annulus. Scale bars
equal 100 μm.
femora examined reveals multiple growth rings (LAGs and
annuli). The perimedullary region in all the bones show
erosionally enlarged resorption cavities and few secondary
osteons. In the third size class (>60% adult size, group C) the
specimens contain a peripheral zone of parallel fibred bone
and fibrolamellar bone tissue in the inner cortex. Primary
osteons are mainly arranged in a laminar pattern, whereas
inner circumferential lamellae are seen in the ribs.
4. Discussion
4.1
Case study I: Temnospondyli
Growth of the trematosaurids involved a fast initial growth
as is evidenced by the presence of fibrolamellar bone tissue
(Amprino 1947; Margerie et al. 2002). This fast initial
growth is followed by a slowing down of growth later
in ontogeny as is evidenced by the presence of lamellar
bone in the outer cortex. In contrast, the Middle Triassic
paracyclotosaurids and the Late Triassic chigutisaurids had
an overall slow growth as evidenced by the predominance
of lamellar bone (Mukherjee et al. 2008; Mukherjee et al. in
press). However, vascularity decreases towards the cortical
periphery in all the skeletal elements.
An interesting feature noted in the ribs is its continuous
fast growth in comparison to the other skeletal elements
irrespective of the Triassic temnospondyl families examined.
This fast growth of the ribs is in marked contrast with the
slow growth of the Middle and Late Triassic temnospondyls.
This may be attributed to the usual blood cell synthesizing
function of the ribs, which makes them regions of high
metabolic activity and hence growth (Ten Cate 1994).
Similar feature has also been noted for the Middle Triassic
dicynodont Wadiasaurus (Ray et al. 2009).
Another feature noted in the trematosaurids and the
Middle Triassic paracyclotosaurids examined here is the
J. Biosci. 34(5), November 2009
absence of LAGs in most of the skeletal elements examined,
implying a continuous growth pattern for both the families.
This is in contrast to that seen in the extant lissamphibians
where distinct zonations of the cortex by multiple LAGs
are evident (Laurin et al. 2004). In the humerus of the
Late Triassic chigutisaurid examined, the bone microstructure is characterized by multiple LAGs in the outer
cortex similar to that observed in the femur of Dutuitosaurus
(Steyer et al. 2004) implying that growth was intermittent.
LAGs and annuli are considered as genetically controlled,
as well as strongly synchronized and reinforced by
seasonality (Esteban et al. 1999), and have been associated
with environmental stress (Hutton 1986; Klevezal 1996;
Chinsamy et al. 1998). Presence of growth rings in such
limited elements may be due to individual response to
particular unfavourable climatic conditions (Mukherjee et
al. in press).
4.2 Case study II: Dicynodontia
Examination of the bone microstructure of the three
‘dicynodonts’ taxa shows the presence of a thick cortex
surrounding a well differentiated medullary region. The
dicynodonts are characterized by the predominance of
fibrolamellar bone tissue in the cortex. Fibrolamellar bone
is considered to indicate rapid osteogenesis (Amprino 1947;
Buffrénil 1980; Reid, 1990; Chinsamy 1997; Margerie et al.
2002), which in turn suggests an overall fast growth. Ricqlès
(1972), Chinsamy and Rubidge (1993), Botha (2003), Ray
and Chinsamy (2004) and Ray et al. (2005) have reported a
similar occurrence of fibrolamellar bone in other dicynodont
taxa such as Diictodon and Oudenodon. Fibrolamellar bone
has been noted in other extinct vertebrates such as non-avian
dinosaurs, captorhinids, pterosaurs, pelycosaurs (Enlow and
Brown 1957; Enlow 1969; Ricqlès 1972, 1976; Curry 1999;
Horner et al. 1999, 2000), gorgonopsians, therocephalians
Osteohistology of Indian fossil vertebrates
and cynodonts (Botha and Chinsamy 2000, 2004, 2005;
Ray et al. 2004; Chinsamy and Hurum 2006).
The growth patterns as deduced from bone microstructure
show that the dicynodonts had varied growth strategies,
which are discussed in the following section.
4.2.1 Endothiodon: Presence of fibrolamellar bone
suggests rapid growth. The skeletal elements examined do
not show any growth rings suggesting continuous growth
except for the humerus and centrum (Ray et al. 2009).
Similarly Chinsamy and Rubidge (1993) had reported
growth rings in the humerus of Endothiodon specimens
collected from South Africa. A fast growth with periodic
interruptions in growth was suggested for E. mahalanobisi
by Ray et al. (2009).
4.2.2 Lystrosaurus: Examination of the bone
microstructure of Lystrosaurus murrayi from India and
South Africa reveals a predominance of fibrolamellar bone
tissue, which suggests rapid periosteal osteogenesis and an
overall fast growth. Ray et al. (2005) while examining the
bone microstructure of South African and Indian forms
have identified distinct ontogenetic stages based on tissue
type, organization of the primary osteons, incidence of
growth rings, secondary reconstruction and endosteal
bone deposition. Most of the Indian specimens pertain
to late juvenile (15-30% adult size), sub-adult (30-60%
adult size) and adult (>60% adult size) stages. In the late
juvenile stage, the bone microstructure is characterized
by the presence of fibrolamellar bone tissue in the cortex,
primary osteons arranged in laminar-subplexiform
pattern, absence of LAGs, presence of secondary
reconstruction in the form of erosionally enlarged vascular
channels and few secondary osteons, and presence of
cancellous bone in the medullary region. The sub-adult
stage is characterized by fibrolamellar bone tissue in the
cortex, presence of LAGs and annuli, medullary spongiosa,
extensive secondary reconstruction, and onset of endosteal
bone deposition. In the last ontogenetic stage (adult stage)
the cortex is composed of fibrolamellar bone tissue. Primary
osteons are arranged in a laminar pattern in the inner cortex
whereas mid-to outer cortex shows sub-reticular and radial
organization of vascular channels, presence of narrow
avascular peripheral zones of parallel-fibred bone tissue,
annuli and LAGs, extensive secondary reconstruction
with profuse secondary osteons in the deeper cortex
and presence of compacted coarse cancellous bone and
medullary spongiosa. Based on these bone microstructures
it was proposed by Ray et al. (2005) that Lystrosaurus
had a continuous fast growth during the early ontogenetic
stages (up to 30% adult size), after which it became periodic
with intermittent slowing down or cessation of growth.
However, growth continued and an indeterminate growth
strategy (sensu Foote and Miller 2007) was proposed for
Lystrosaurus.
669
4.2.3 Wadiasaurus: Based on bone microstructure, Ray
et al. (2009) considered that Wadiasaurus indicus had three
ontogenetic stages, which can be corroborated with the three
size classes mentioned earlier. In the juvenile stage, when
up to 30% of adult size is attained, growth was fast and
sustained as is suggested by the presence of fibrolamellar
bone in the cortex and absence of growth rings (annuli and
LAGs). During the sub-adult stage (30–60% adult size) the
growth was fast but periodic as is evident from the presence
of zonal fibrolamellar bone in the cortex and multiple annuli
and LAGs. Other features of this ontogenetic stage comprise
extensive secondary reconstruction and onset of endosteal
bone deposition. In the last ontogenetic stage, i.e, adults
(>60% adult size), the bone microstructure is characterized
by fibrolamellar bone in the deeper cortex with peripheral
parallel fibred/lamellar bone, decreasing vascularity towards
periosteal periphery and presence of LAGs suggesting that
though growth was fast and periodically interrupted earlier,
later growth slowed down considerably. However, growth
still continued and the slow growth possibly resulted with
the onset of sexual maturity. A flexible and indeterminate
growth strategy was proposed for Wadiasaurus.
4.2.4 Evolutionary trends: From examination of the
available dicynodont material, it is apparent that they
had an indeterminate growth strategy where growth
continued even though it slowed down considerably later
in ontogeny. Ray et al. (2009) mapped the histological
character-state distributions on to a cladogram showing
the inter-relationships of the available neotherapsid taxa
collected from different stratigraphic horizons around the
world (figure 8) and showed that the dominance of wellvascularized fibrolamellar bone, an overall fast growth,
and an indeterminate growth strategy were plesiomorphic
conditions for the neotherapsids. Most of the non-mammalian
therapsids including the dicynodonts show flexible growth
strategy and a high degree of developmental plasticity
(sensu Smith-Gill 1983; Ray et al. 2004). This flexibility in
growth patterns resulted because of periodic environmental
stress conditions, which caused either periodic cessation
or slowed growth whereas favorable conditions were
conducive for sustained fast growth. A considerably reduced
developmental plasticity and inflexible growth trajectories
evolved within the non-mammalian cynodonts, with the
advanced tritylodontids achieving almost a mammalian
growth trajectory.
5.
Concluding remarks
It is apparent from the foregoing discussion that examination
of the bone microstructure of extinct vertebrates helped
in the deduction of the various growth regimes of these
animals. The three Indian temnospondyl families examined
showed differing growth patterns, where the Early Triassic
J. Biosci. 34(5), November 2009
670
S Ray, D Mukherjee and S Bandyopadhyay
Figure 8. Evolution of growth patterns as deduced from bone microstructure (modified from Ray et al 2009). Asterisks indicate taxa that
had sustained growth. The synapomorphies are as follows: (A) Neotherapsids: zonal fibrolamellar bone tissue; growth rings (annuli and
LAGs), fast growth with periodic interruptions, indeterminate growth trajectory, high developmental plasticity. (B) Dicynodontia: cyclical
fast growth. (C) Gorgonopsia: zonal fibrolamellar bone tissue, growth rings, high developmental plasticity. (D) Sustained/continued growth,
reduced developmental plasticity, possibly determinate growth trajectory.
J. Biosci. 34(5), November 2009
Osteohistology of Indian fossil vertebrates
trematosaurids had a fast growth in comparison to the Middle
and Late Triassic taxa. Similarly variable growth patterns
are also documented from the dicynodont taxa examined.
In addition, examination of other non-Indian forms and a
synthesis of all the results show that the evolution towards
a reduced development plasticity and sustained growth
originated within the neotherapsids. It may be mentioned
here that the number of taxa examined is too small and more
works need to be done for precise assessment of the growth
patterns, histovariation and phylogenetic implications of
the bone microstructure of extinct vertebrates including
the Indian forms. However, the destructive nature of the
analysis makes it virtually impossible to examine an ideal
dataset, though bone histology is a useful tool for deducing
palaeobiology of extinct vertebrates such as growth,
ontogenetic variations and life habits.
Acknowledgements
One of the authors (SR) is grateful to Prof. A ChinsamyTuran, Department of Zoology, University of Cape Town,
South Africa for introducing her to the subject of bone
histology. We thank Prof. A Sahni, Panjab University, and
Prof. S Bajpai, Indian Institute of Technology, Roorkee for
encouragement. The critical comments of the reviewer Prof.
S Mishra, School of Engineering Systems and Institute of
Health and Biomedical Innovations (IHBI), Queensland
University of Technology, Australia are gratefully
acknowledged. Thanks are due to the Department of Science
and Technology, New Delhi, Indian Institute of Technology,
Kharagpur and Indian Statistical Institute, Kolkata for fund
and infrastructural facilities.
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ePublication: 20 October 2009
J. Biosci. 34(5), November 2009