Introduction
The study of paleopathology offers an effective tool for understanding some
aspects of vertebrate paleobiology, including lifestyle and behavior (e.g.,
Hanna, 2002; Martinelli et al., 2015), as well as reaction of the skeletal
tissues to diseases and injuries (traumas) through the time (Rothschild and
Tanke 1991; Rothschild and Martin, 2006; Waldron, 2009). Despite numerous
paleopathological studies of different tetrapods (e.g., dinosaurs,
crocodiles), paleopathologies among fossil members of modern groups of
amphibians (i.e., lissamphibians: frogs,
salamanders and caecilians) are poorly documented (Rothschild, 2012;
Rothschild and Laub, 2013). Reported skeletal anomalies among fossil
salamanders are limited to polydactyly, abnormal phalangeal counts and
supernumerary hind limbs in the Jurassic cryptobranchid Chunerpeton tianyiensis from China (Wang et al., 2016). Other pathologies in fossil
salamanders have not been described and analyzed for etiology (Rothschild et
al., 2012).
List of specimens used for this study.
Specimen
Skeletal element
Size class
External pathological description
Details of internal structure
Etiology
Figure in
this paper
ZIN PH 1/243
prearticular
medium
focal bony protrusion (i.e., exostosis);eroded bone surface
necrotic cavities connectedwith vascular canals
traumatic-infectious(osteomyelitis?)
Fig. 1
ZIN PH 2/243
fragmentary dentary
medium
coarse callus
porous tissue of callus due tothe presence of radial canals
traumatic (healed fracture)
Fig. 2
ZIN PH 3/243
atlantal centrum
small
enlarged transverse process
no specific internal details
unknown
Fig. 3
ZIN PH 1830/242
atlantal centrum
medium
asymmetry (in anterior and posterior views) andpresence of well-developed transverse processes
no specific internal details
unknown
Fig. 4
ZIN PH 13/243
anterior trunk vertebra
large
enlarged (both) and elongated (right)transverse processes
porous transverse processeswith large irregular cavities
traumatic-infectious?(osteomyelitis?)
Fig. 5
ZIN PH 1829/242
co-ossified atlas andhemivertebra
large
fusion of vertebrae; asymmetrical atlas with the posterior cotyle oriented posterolaterally; narrow and wedge-like hemivertebral centrum with a large transverse process
difference in the densities ofatlantal and the hemivertebral centra; enlarged transverseprocess of hemivertebra withlarge cavity
congenital
Fig. 6
ZIN PH 1832/242
fused atlas and the first trunk vertebra
large
fusion of vertebrae; enlargements on the lateralsurfaces of the centra and the hypapophyses
highly porous boneenlargements
traumatic-infectious? (osteomyelitis?)
Fig. 7
ZIN PH 1831/242
fused atlas and the first trunk vertebra
large
fusion of vertebrae; no bone enlargements;shortening of the centra
asymmetry of the centra
congenital
Fig. 8
ZIN PH 4/243
fused two anterior trunk vertebrae
large
fusion of vertebrae; enlargements on the lateralsurfaces of the centra and the hypapophyses
displacement of vertebra;porous structure ofenlargements
traumatic-infectious(osteomyelitis?)
Fig. 9
ZIN PH 6/243
fused two anterior trunk vertebrae
large
fusion of vertebrae; no bone enlargements;shortening of the centra, asymmetric arrangement of the transverse processes
asymmetry of the centra
congenital
Fig. 10
ZIN PH 7/243
fused two posteriortrunk vertebrae
large
fusion of vertebrae; no bone enlargements;shortening of the centra, and asymmetricarrangement of the transverse processes
asymmetry of the centra
congenital
Fig. 11
ZIN PH 5/243
fused three anteriortrunk vertebrae
large
fusion of vertebrae; no bone enlargements;shortening of the centra
no specific internal details
congenital
Fig. 12
ZIN PH 8/243
fused neural arches oftwo trunk vertebrae
large
fusion of vertebrae
no specific internal details
congenital
Fig. 13
ZIN PH 9/243
distal part of the femur
medium
large smooth focal bone enlargement
porous structure of boneenlargement due to the presence of numerous cavities;enlargement is composed ofprimary bone
traumatic (hematoma)
Fig. 14a–h
ZIN PH 10/243
distal part of the femur
medium
smooth focal bone enlargement (callus);displacement of the distal end
porous structure of callus
traumatic (healed fracture)
Fig. 14i–o
ZIN PH 11/243
distal part of the femur
medium
smooth focal bone enlargement (callus);displacement of the distal end
porous structure of callus
traumatic (healed fracture)
Fig. 15a–f
ZIN PH 12/243
distal part of the femur
large
smooth focal bone enlargement (callus);displacement of the distal end
porous structure of callus due to the presence of numerouslongitudinal and obliquely oriented vascular canals
traumatic (healed fracture)
Fig. 15g–p
Expeditions to the Kyzylkum Desert (Uzbekistan) in 1977–1994 by Lev A. Nesov
and in 1997–2006 by the international
Uzbek/Russian/British/American/Canadian Joint Paleontological Expeditions
(URBAC) yielded several thousand bones of the fossil cryptobranchid
salamander Eoscapherpeton asiaticum from the Upper Cretaceous
(Turonian) Bissekty
Formation (Nesov 1981, 1997; Skutschas, 2009, 2013). This abundant material
contains several abnormal specimens of Eoscapherpeton asiaticum,
including fused vertebrae, fractured bones and some other osseous
pathologies. The main aims of this study are to describe osseous anomalies in
Eoscapherpeton asiaticum (using gross morphological description,
microCT, and histologic analysis) and to discuss their possible etiology.
Materials and methods
A total of 17 specimens representing isolated elements from the Dzharakuduk
locality of the Turonian Bissekty Formation (Redman and Leighton, 2009) that
belong to the cryptobranchid salamander Eoscapherpeton asiaticum
were examined. All these specimens are stored at the Paleoherpetological
collection (ZIN PH) of the Zoological Institute of the Russian Academy of
Sciences, Saint Petersburg, Russia. The abnormal specimens include two bones
of the lower jaw (prearticular ZIN PH 1/243, fragmentary dentary ZIN
PH 2/243), two atlases (ZIN PH 3/243, ZIN PH 1830/242), trunk vertebra (ZIN
PH 13/243), fused vertebrae (ZIN PH 1829/242, ZIN PH 1831/242, ZIN
PH 1832/242, ZIN PH 4/243, ZIN PH 5/243, ZIN PH 6/243, ZIN PH 7/243, ZIN
PH 8/243), and distal fragments of femora (ZIN PH 9/243, ZIN PH 10/243, ZIN
PH 11/243, ZIN PH 12/243) (Table 1). All specimens were CT-scanned (at 100 kV
and 0.1 mA, generating a resolution of 2.9 µm of pixel size and an
output of 4000 × 4000 pixels per slice) at the Saint Petersburg
State University Research Centre for X-ray Diffraction Studies (Saint
Petersburg, Russia) using a Skyscan 1172 CT scanner. CT scan data were imported to
the software Amira 6.3.0 (FEI-VSG Company), where the model was reconstructed
and segmented.
The prearticular ZIN PH 1/243 of Eoscapherpeton asiaticum.
(a) Medial view, (b) lateral view, (c) medial view
(digital restoration) with the locations of the microCT digital sections,
(d) digital transverse section, and (e–f) digital longitudinal
sections. Abbreviations: E – exostosis; NC – necrotic cavity.
The CT data are deposited in the Department of Vertebrate Zoology of the
Saint Petersburg State University, Saint Petersburg, Russia, and can be made
available by the present authors for the purpose of scientific study.
For the histologic analysis, standard petrographic thin sections (transverse)
of two abnormal femora (distal fragments ZIN PH 9/243, ZIN PH 12/243) were
prepared. The sections were observed under polarized light using an optical
microscope (Leica 4500, Leica Microsystems, Wetzlar, Germany) in the Saint
Petersburg State University Research Centre for X-ray Diffraction Studies,
Russia. Histological terminology follows Francillon-Vieillot et al. (1990).
The thin sections used in study are housed in the histological collection of
the Department of Vertebrate Zoology, Saint Petersburg State University,
Saint Petersburg, Russia.
The dentary ZIN PH 2/243 of Eoscapherpeton asiaticum.
(a) Medial view, (b) lateral view, (c) ventral
view (digital restoration) with the location of the transverse microCT
digital section, (d) dorsal view (digital restoration) with the
location of the longitudinal microCT digital section, (e) digital
transverse section, and (f) digital longitudinal section. Abbreviations:
C – callus; RC – radial canals.
Results
Pathologies of bones of the lower jaw
Osseous anomalies were found in the medium-sized prearticular ZIN PH 1/243
and the fragmentary dentary ZIN PH 2/243 (Table 1). The prearticular ZIN
PH 1/243 (Fig. 1) exhibits a focal bony protrusion (i.e., exostosis) on the
medial surface. The bone surface around the exostosis is eroded due to the
presence of relatively deep notches of irregular shape. Internal examination
reveals the presence of large necrotic cavities which were connected with
vascular canals (Fig. 1d–f) and with erosion notches on the bone surface.
The possible diagnosis for this pathology is osteomyelitis – inflammation of
bone and bone marrow (usually posttraumatic) caused by bacteria (more rarely
by fungi), resulting in the periosteal reaction and irregular bone surface
(Rothschild and Martin, 2006; Peterson and Vittore, 2012; Marais et al.,
2014; García et al., 2016; Ramírez-Velasco et al., 2016 ).
The atlantal centrum ZIN PH 3/243 of Eoscapherpeton asiaticum. (a) Ventral view, (b) dorsal view, (c)
right lateral view, (d) ventral view (digital restoration),
(e) right lateral view (digital restoration) with the locations of
the microCT digital sections, (f) digital transverse section, and
(g) digital longitudinal section (horizontal plane) in ventral view.
Abbreviations: TP – transverse process.
The medium-sized fragmentary dentary ZIN PH 2/243 (Fig. 2) displays a focal
bone enlargement (i.e., callus). Internally, the tissue of the callus is
porous (in comparison to that of normal tissue of the bone) and contains
several radial canals (Fig. 2e–f). The callus was formed during the healing
of traumatic fracture of the dentary (traumatic nature of pathology)
(Rothschild and Martin, 2006; Pardo-Pérez et al., 2017).
The atlantal centrum ZIN PH 1830/242 of Eoscapherpeton asiaticum. (a) Anterior view, (b) posterior view,
(c) anterior view (digital restoration), (d) posterior view
(digital restoration), (e) right lateral view (digital restoration)
with the locations of the microCT digital sections, (f) left lateral
view (digital restoration), (g) digital longitudinal section
(horizontal plane) in dorsal view, and (h) digital transverse section in
posterior view. Abbreviations: TP – transverse process.
Atlantal pathologies
Two atlantal centra in the sample show the osseous anomalies: the small
centrum ZIN PH 3/243 and one medium-sized centrum ZIN PH 1830/242 (Table 1).
The small atlas ZIN PH 3/243 (Fig. 3) is characterized by an enlarged
transverse process (i.e., rib bearer) on the right lateral surface. The inner
structure of the enlarged transverse process demonstrates the presence of a
large cavity (i.e channel) that in life was likely filled by cartilage
(Fig. 3f–g). There are no differences in porosity in the different parts of
the cortical regions and no clear evidence of fractures or inflammation
process. The origin of the pathology is unknown and it could be
traumatic-infectious, developmental or neoplasm (Rothschild and Martin, 2006;
Waldron, 2009).
The anterior trunk vertebra ZIN PH 13/243 of Eoscapherpeton asiaticum. (a) Anterior view, (b) posterior view,
(c) dorsal view, (d) left lateral view, (e)
anterior view (digital restoration), (f) posterior view (digital
restoration), (g) dorsal view (digital restoration), (h)
left lateral view (digital restoration) with the locations of the microCT
digital sections, (i) digital transverse section in anterior view,
and
(k) digital longitudinal section (horizontal plane) in dorsal view.
Abbreviations: TP – transverse process; R? – rib?.
The centrum of the larger atlas ZIN PH 1830/242 (Fig. 4) is asymmetric (in
anterior and posterior views) and bears well-developed transverse processes
on the both lateral surfaces. The inner structure of the enlarged transverse
processes is similar to that in ZIN PH 3/243 (the presence of the large
cavity; Fig. 4g). Like in ZIN PH 3/243, the origin of the pathology is
unclear.
The co-ossified atlas and hemivertebra ZIN PH 1829/242 of
Eoscapherpeton asiaticum. (a) Anterior view, (b)
posterior view, (c) ventral view, (d) dorsal view,
(e) dorsal view (digital restoration), (f) ventral view
(digital restoration), (g) left lateral view (digital restoration)
with the locations of the microCT digital sections, and (h–i) digital
longitudinal sections (horizontal plane) in ventral view. Broken lines and an
arrow show the border between atlas and hemivertebra. Abbreviations: HV –
hemivertebra; PC – posterior atlantal cotyle.
Abnormal trunk vertebra
The large anterior trunk vertebra ZIN PH 13/243 (Fig. 5) exhibits an abnormal
structure of the transverse processes on both lateral surfaces of the
centrum: right transverse processes are slightly enlarged, while left
transverse processes are significantly enlarged and elongated (probably due
to the fusion with a rib). The internal structure of the enlarged transverse
processes is highly porous and contains large irregular cavities. Reactive
bone growth is likely the result of complication of healing after trauma and the
subsequent enlargement of transverse processes could be caused by
osteomyelitis (traumatic-infectious origin of pathology) (Rothschild and
Martin, 2006; Waldron, 2009; García et al., 2016).
The fused atlas and the first trunk vertebra ZIN PH 1832/242 of
Eoscapherpeton asiaticum. (a) Dorsal view,
(b) ventral view, (c) anterior view, (d) dorsal
view (digital restoration), (e) ventral view (digital restoration),
(f) anterior view (digital restoration) with the location of
longitudinal the microCT digital section, (g) left lateral view
(digital restoration) with the locations of the microCT digital sections,
(h) digital longitudinal section (vertical plane), (i)
digital longitudinal section (horizontal plane) in dorsal view, and (k)
digital transverse section in posterior view. Broken lines show the border
between atlas and trunk vertebra. Arrows show bone enlargements.
Abbreviations: AT – atlas; TV – trunk vertebra.
Hemivertebra
The specimen ZIN PH 1829/242 is an asymmetrical (in dorsal and ventral view)
fragmentary large atlas that is co-ossified with a hemivertebra (Fig. 6). The
posterior cotyle of the atlantal centrum is oriented posterolaterally. The
hemivertebral centrum is narrow, wedge-like and located at the posterior left
portion of the specimen. The hemivertebral centrum bears a large transverse
process. The bases of neural arches of the atlas and hemivertebra are not
fused. Externally, the boundary between atlantal and the hemivertebral centra
is clearly traceable as a groove on the lateral surface of the hypapophysis.
Internally, the boundary is not clearly evident, but some differences in the
densities of atlantal and the hemivertebral centra are visible in some
virtual slices (Fig. 6i). The inner structure of the enlarged transverse
process demonstrates the presence of a large cavity (i.e., channel).
The fused atlas and the first trunk vertebra ZIN PH 1831/242 of
Eoscapherpeton asiaticum. (a) anterior view,
(b) posterior view, (c) dorsal view, (d) ventral
view, (e) left lateral view, (f) anterior view (digital
restoration) with the location of longitudinal the microCT digital section,
(g) ventral view (digital restoration) with the location of
longitudinal the microCT digital section, (h) right lateral view
(digital restoration), (i) left lateral view (digital restoration),
(k) digital longitudinal section (horizontal plane) in dorsal view,
and (l) digital longitudinal section (vertical plane). Broken lines show
the border between atlas and trunk vertebra. Abbreviations: AT – atlas; TV
– trunk vertebra.
Fusion of vertebrae
The sample of fused vertebrae (all vertebrae are relatively large in size)
includes two specimens of fused atlas and the first trunk vertebrae (ZIN
PH 1831-1832/242), two specimens of the fused two anterior trunk
vertebrae (ZIN PH 4/243, 6/243), one specimen of the fused three anterior
trunk vertebrae (ZIN PH 5/243), one specimen of the fused two posterior trunk
vertebrae (ZIN PH 7/243) and one specimen of the fused neural arches of two
trunk vertebrae (ZIN PH 8/243) (total number of specimens – 7; Table 1).
The fused two anterior trunk vertebrae ZIN PH 4/243 of
Eoscapherpeton asiaticum. (a) Dorsal view, (b)
ventral view, (c) anterior view, (d) right lateral view,
(e) left lateral view, (f) anterior view (digital
restoration), (g) dorsal view (digital restoration) with the
location of longitudinal the microCT digital section, (h) right
lateral view (digital restoration) with the location of longitudinal the
microCT digital section, (i) left lateral view (digital
restoration), (k) digital longitudinal section (vertical plane), and
(l) digital longitudinal section (horizontal plane) in dorsal view.
Blue lines show the original median line of the centra prior to the trauma.
Arrows show bone enlargements. Abbreviations: NC – necrotic cavity.
The fused two anterior trunk vertebrae ZIN PH 6/243 of
Eoscapherpeton asiaticum. (a) Dorsal view, (b)
ventral view, (c) anterior view, (d) left lateral view,
(e) right lateral view, (f) ventral view (digital
restoration) with the location of longitudinal the microCT digital sections,
(g) anterior view (digital restoration) with the location of
longitudinal the microCT digital section, (h) left lateral view
(digital restoration), (i) right lateral view (digital restoration),
(k–l) digital longitudinal sections (vertical plane), and
(m) digital longitudinal section (horizontal plane) in ventral view. Broken lines
show the border between trunk vertebrae. Abbreviations: NA – neural arch;
TP1 – transverse process of more anterior vertebra; TP2 – transverse
process of more posterior vertebra.
The fused two posterior trunk vertebrae ZIN PH 7/243 of
Eoscapherpeton asiaticum. (a) Ventral view,
(b) dorsal view, (c) anterior view, (d) right
lateral view, (e) left lateral view, (f) ventral view
(digital restoration) with the location of the microCT digital sections,
(g) anterior view (digital restoration) with the location of
longitudinal the microCT digital section, (h) right lateral view
(digital restoration), (i) left lateral view (digital restoration),
(k) digital longitudinal section (vertical plane), (l)
digital transverse section in anterior view, and (m) digital longitudinal
section (horizontal plane) in dorsal view. Broken lines show the border
between trunk vertebrae. Abbreviations: NA – neural arch; TP1 – transverse
process of more anterior vertebra; TP2 – transverse process of more
posterior vertebra.
The fused three anterior trunk vertebrae ZIN PH 5/243 of
Eoscapherpeton asiaticum. (a) Ventral view, (b)
dorsal view, (c) left lateral view, (d) right lateral view,
(e) ventral view (digital restoration) with the location of
longitudinal the microCT digital section, (f) left lateral view
(digital restoration) with the location of longitudinal the microCT digital
section, (g) right lateral view (digital restoration), (h)
digital longitudinal section (vertical plane), and (i) digital
longitudinal section (horizontal plane) in dorsal view. Broken lines show the
border between trunk vertebrae.
The specimen ZIN PH 1832/242 (Fig. 7) exhibits enlargements on the lateral
surfaces of the centra and the hypapophyses along the suture between the
atlas and the first trunk vertebrae that were formed during reactive bone
growth. The internal structure of these enlarged areas is highly porous. The
enlargements could be caused by osteomyelitis after trauma
(traumatic-infectious nature of pathology).
The specimen ZIN PH 1831/242 (Fig. 8) displays the fusion of the atlas and
the first trunk vertebrae without notable pathological enlargements on the
lateral surfaces of the centra and the hypapophyses (only low ridge is
present along the suture between atlas and the first trunk vertebrae). This
suggests the absence of the periosteal reactive bone growth. Both vertebrae
are shortened in comparison with normal vertebrae or that in the specimen
(ZIN PH 1832/242). The absence of any traces of trauma (e.g., fracture,
displacement, callus) and inflammation of bone (e.g., enlargements formed
during reactive bone growth, necrotic cavities) suggests the congenital
origin of the fusion.
The fused neural arches of two trunk vertebrae ZIN PH 8/243 of
Eoscapherpeton asiaticum. (a) Dorsal view, (b)
right lateral view, (c) left lateral view, (d) left lateral
view (digital restoration), (e) right lateral view (digital
restoration), (f) dorsal view (digital restoration) with the
location of longitudinal the microCT digital section, and (g) digital
longitudinal section (vertical plane).
The distal parts of the femora ZIN PH 9/243 (a–h) and ZIN
PH 10/243 (i–o) of Eoscapherpeton asiaticum. Specimen ZIN
PH 9/243: (a) ventral view, (b) dorsal view, (c)
ventral view (digital restoration), (d) dorsal view (digital
restoration), (e) anterior or posterior view (digital restoration)
with the locations of transverse histological sections and longitudinal the
microCT digital section, (f) digital longitudinal section, and
(g–h) histological transverse section. The arrow shows the border
between cortical bone and pathological tissue. Specimen ZIN PH 10/243:
(i) ventral view, (k) dorsal view, (l) ventral
view (digital restoration) with the locations of the microCT digital
sections, (m) anterior or posterior view (digital restoration),
(n) digital longitudinal section, and (o) digital transverse
section. Blue lines show the original bone position prior to the break.
Abbreviations: BE – bone enlargement; C – callus; CB – cortical bone.
The distal parts of the femora ZIN PH 11/243 (a–f) and ZIN
PH 12/243 (g–p) of Eoscapherpeton asiaticum. Specimen ZIN
PH 11/243: (a) ventral view, (b) dorsal view, (c)
dorsal view (digital restoration) with the locations of the microCT digital
sections, (d) anterior or posterior view (digital restoration),
(e) digital longitudinal section, and (f) digital transverse
section. Specimen ZIN PH 12/243: (g) ventral view, (h)
dorsal view, (i) ventral view (digital restoration), (k)
dorsal view (digital restoration), (l) anterior or posterior view
(digital restoration) with the locations of transverse histological sections
and longitudinal the microCT digital section, (m) digital
longitudinal section, and (n–p) transverse histological sections. Blue
lines show the original bone position prior to the break. Abbreviations: C –
callus.
The fused two anterior trunk vertebrae ZIN PH 4/243 (Fig. 9) are
characterized by the enlargements on the lateral surfaces of the centra and
the hypapophyses along the suture between vertebrae and by deformation of the
hypapophyses. CT data show that the more posterior vertebra is slightly
displaced in relation to the more anterior vertebrae (Fig. 9l), so the trauma
took place before the forming of pathological enlargements. The internal
structure of enlarged areas is highly porous and contains large necrotic
cavities.
The fused two anterior trunk vertebrae ZIN PH 6/243 (Fig. 10) are
characterized by shortening of the centra, asymmetry of the centra (due to
the unequal development of the anterior and posterior portions) and
asymmetric arrangement of the transverse processes (widely separated on the
left side and closely spaced on the right side of the fused centra). The
bases of neural arches are fused and a large spinal nerve foramen is present.
Similarly to the specimen ZIN PH 6/243, the fused two posterior trunk
vertebrae ZIN PH 7/243 (Fig. 11) exhibit the vertebral shortening with
asymmetry of the centra and asymmetric arrangement of the transverse
processes. The absence of any traces of trauma and inflammation of bone
suggests the congenital origin of the fusion. The asymmetric arrangement of
the transverse processes and vertebral shortening could be the result of
abnormal segmentation during embryogenesis.
The specimen ZIN PH 5/243 consists of three fused anterior trunk vertebrae
(Fig. 12). The vertebrae exhibit the vertebral shortening and do not display
any signs of trauma and/or inflammation of bone. We suggest the congenital
origin of the fusion in ZIN PH 5/243.
The neural arches of two trunk vertebrae ZIN PH 8/243 (Fig. 13) are fused
without any trace of suture. The more anterior neural arch is lower than the more
posterior one. There are no pathological enlargements (no signs of periosteal
reactive growth), and the fusion of these neural arches is most likely of
congenital origin.
Femoral pathologies
The distal fragment of the medium-sized femur ZIN PH 9/243 (Fig. 14a–h)
displays the presence of large smooth focal bone enlargement (i.e., periosteal
elevation) in the metaphyseal region. The internal structure of this bone
enlargement is porous due to the presence of large cavities of irregular
shape (i.e., erosion bays) (Fig. 14f–h). The tissue of the enlargement is
entirely composed of a parallel-fibered primary bone of periosteal origin. The
parallel-fibered primary bone in the outer part of the enlargement is more
organized than in the inner porous part and is similar to that of the normal
cortical bone of this femur (Fig. 14h). No evidence of fracture was
revealed by CT scanning and histological analysis. Similar bone enlargement
in modern reptiles was identified as hematoma (Rothschild et al., 2010,
2012). We also interpret the femoral pathology on ZIN PH 9/243 as hematoma
that could have been caused by trauma (traumatic origin).
All other femoral specimens in the sample (two distal fragments ZIN
PH 10-11/243 of medium-sized femora and one distal fragment ZIN PH 12/243 of
large femora; Table 1, Figs. 14i–o, 15) display the signs of healed
fractures that are traceable in CT images. Externally, all the specimens
demonstrate displacements of the distal end of the bone and have large smooth
focal bone enlargements (i.e., calluses) in the metaphyseal region that were
formed during the healing of traumatic fractures. The internal structure of
the calluses is porous and contains numerous
longitudinal and obliquely oriented vascular canals that are typical for these structures (Fig. 15n–p).
Discussion
General discussion
Eoscapherpeton asiaticum demonstrates a wide spectrum of osseous
anomalies/pathologies, resulting from trauma, possible infection due to trauma, and
congenital disorders. Traumatic origins of pathologies are suggested for the dentary
ZIN PH 2/243 (the presence of the fracture and the callus) and for femoral
fragments ZIN PH 9-12/243 (hematoma and healed fractures with the calluses on
the distal portions of the bones) (Table 1). The possible
traumatic-infectious pathologies are associated with bone enlargements formed
during reactive bone growth and/or the presence of necrotic cavities are
suggested for the prearticular ZIN PH 1/243, the anterior trunk vertebra ZIN
PH 13/243, the fused atlas and the first trunk vertebrae ZIN PH 1832/242, and
the fused two anterior trunk vertebrae ZIN PH 4/243 (Table 1). The congenital
pathologies of E. asiaticum are represented by the hemivertebra
(specimen ZIN PH 1829/24; see discussion below) and by fusion of two (fused
atlas and anterior trunk vertebra ZIN PH 1831/242, fused anterior trunk
vertebrae ZIN PH 6/243, fused posterior trunk vertebrae ZIN PH 7/243 and for
fused neural arches 8/243) or three (fused anterior trunk vertebrae ZIN
PH 5/243) vertebrae (Table 1). The total number of these vertebral congenital
pathologies in E. asiaticum is very low (n= 5) in comparison
with thousands of collected vertebrae of this salamander (only collection of
atlases ZIN PH 242 contains 1909 specimens, while there are only two
specimens of congenital pathologies with atlas – ZIN PH 1829/24 and ZIN
PH 1831/242. This frequency of vertebral congenital pathologies in E. asiaticum (for congenital pathologies with atlas < 0.1 %) is in
accordance with the previous estimations for Paleozoic–Mesozoic temnospondyls
and Holocene frogs (Rothschild and Laub, 2013; Witzmann et al., 2013).
Hemivertebra in Eoscapherpeton asiaticum
A hemivertebra is a congenital malformation in which one or more lateral
halves of vertebrae are intercalated between neighboring vertebrae. Formation
of hemivertebrae is a result of abnormal segmentation during embryogenesis
(i.e., failure of somitogenesis before chondrification and ossification of the
vertebral anlagen; e.g., Witzmann et al., 2008, 2013; Burnham et al., 2013).
In amniots, single vertebrae are derived from the sclerotome of two adjacent
somites and this process is widely known as resegmentation (Piekarski and
Olsson, 2014, and references therein). Resegmentation have been proposed to be
present in all modern lissamphibians, but embryological and molecular
evidence for this process was provided only for one salamander species –
ambystomatid Ambystoma mexicanum (Mexican axolotl) (Piekarski and
Olsson, 2014). The development of the described hemivertebra in more basal
cryptobranchid salamander Eoscapherpeton asiaticum can also be
explained by a failure of resegmentation. If this is correct, then it is
significant for the following reasons: (1) it is the oldest occurrence of
resegmentation in salamanders, and (2) it is evidence that resegmentation
is a common process for salamanders (including relatively basal
cryptobranchids and advanced ambystomatids).
A hemivertebra in E. asiaticum was found in a relatively large (according to the large size of
the atlas that was co-ossified with hemivertebra) and, presumably, adult
individual (Fig. 6). The presence of a hemivertebra in a large individual suggests that this pathology did not
cause its death.
The hemivertebrae have been reported earlier for some Paleozoic and Mesozoic
temnospondyls (Witzmann et al., 2013). The finding of hemivertebra in the
Late Cretaceous cryptobranchid E. asiaticum is the oldest (and the
first described) occurrence of this pathology among modern groups of
amphibians (i.e., lissamphibians: salamanders, frogs and caecilians).
Femoral pathologies in Eoscapherpeton: possible insight into
behavior
The traumatic origin of pathologies on the distal parts of the femora
(Figs. 14–15) could be evidence of frequent occurrence of hind limb injuries
in Eoscapherpeton asiaticum. In modern cryptobranchids such as
Cryptobranchus, limb injuries are the most common physical
abnormalities in wild populations (Miller and Miller, 2005). The proposed
main cause of limb injuries in Cryptobranchus is intraspecific
aggression during the reproductive season (e.g., defending of nesting sites),
because fresh injuries were found only during the reproductive season (namely in
August and September) (Hiler et al., 2005; Miller and Miller, 2005). Taking
into account the phylogenetic position of Eoscapherpeton as a member
of the cryptobranchid clade and their aquatic lifestyle (see Skutschas, 2009,
2013), the reason for the frequent occurrence of hind limb injuries in E. asiaticum
could be the same (intraspecific aggression during reproductive
season), and this behavioral element could have appeared early in the
evolution of cryptobranchids, i.e., at least in the Late Cretaceous.