AMPHIBIANS & REPTILOMORPHS
Fossilized remains of
fish-like vertebrates with four limbs and the earliest amphibians are found
before the end of the Devonian period (363 Ma). Reptile-like tetrapods evolved
during the Carboniferous period (363-290 Ma). From them, mammals differentiated
at the end of the Triassic period (208 Ma) and birds at the end of the Jurassic
period (146 Ma) (1995LundbergJG; 2023CohenKM_CarN).
The taxa of early
tetrapodomorphs are now arranged in the following order: Tinirau,
Eusthenopteron, Megalichthys, Panderichthys, Qikiqtania, Tiktaalik,
Elpistostege, Parmastega, Ventastega, Acanthostega, Elginerpeton, Ymeria,
Ichthyostega (2022StewartTA_ShubinNH).
The study of ligamentum
capitis femoris (LCF) in the listed extinct animal species is associated with
significant difficulties. They are primarily due to the long time that has
passed since the disappearance of the creatures. The situation is aggravated by
the fact that LCF is an anatomical structure of small size, formed from soft
tissues that are unstable in natural conditions. Confirmation of the fact of
its presence in the extinct species can be the presence of a similar element in
closely related living tetrapods.
Soft tissues can be preserved
in fossilized resin. Thus, in Myanmar, an insect was found in amber that is
approximately 99 Ma (2024CaiC_KundrataR). X-ray scanning of
fossils allows us to study the shape and internal structure of insects that are
approximately 300 Ma (2024LhéritierM_PerrierV). Perhaps, the
improvement of 3D tomography will eventually help to establish the fine
structure of hip joint tissue and LCF of dinosaur embryos in fossilized eggs
(2009LiangX_WuS). Preservation of LCF is possible during mummification of
bodies buried in conditions that promote dehydration with subsequent lack of
contact with water. A striking example of this is the Gebelein mummies, which
are approximately 3400 years old. Natural preservation of remains in peat bogs
in northern European regions is known. In particular, the body of the «Kølbjerg
man» was discovered, who died approximately 8000 years ago. It is possible to
detect LCF in bodies buried at sub-zero temperatures: in a glacier or frozen
ground. An example is the Sopkarginsky mammoth, which died almost 45000 years
ago. At longer periods, soft tissues decompose, and direct detection of LCF is
excluded.
Under certain conditions,
bones are preserved, being replaced by minerals (fossilization). LCF of a particular
species can be judged based on the analysis of fossilized bones of the pelvis
and femur. On the femur, these are the fossa of the head of the femur, the
groove of the head of the femur, the tuberosity of the head of the femur, the
gap of the head of the femur. In the pelvic area, the presence of LCF can be
indicated by: the notch of the acetabulum, the fossa of the acetabulum, the
opening of the floor of the acetabulum, irregularities on the articular surface
of the acetabulum, such as tuberosities and depressions. Some of the
best-studied early tetrapods: Acanthostega and Ichthyostega (clade
Ichthyostegalia) lived in the Upper Devonian, or 382.7-358.9 Ma (1993RieppelO;
2023CohenKM_CarN).
 |
| Ichthyostega stensiӧvi, skeleton reconstruction of the middle XX; exposition of the Orlov Paleontological Museum (Moscow), photo by the author. |
Their fossil remains are found
up to approximately 359.2 Ma (2012SmithsonTR_ClackJA). The acetabulum of
Acanthostega was rounded, subtriangular in shape, elongated in the horizontal
direction (1996CoatesMI). The elongated shape of the acetabulum in Acanthostega
had a maximum length-to-height ratio of 0.45 (2012SwartzB). A three-dimensional
reconstruction of the Ichthyostega skeleton showed that its hip joint had
significantly limited pronation-supination, and the femur with a
boomerang-shaped head swung in an elongated acetabulum in two planes
(2012PierceSE_HutchinsonJR).
 |
| A cast of the pelvic limb skeleton of Ichthyostega (Late Devonian, Famennian stage, 372.2-358.9 Ma), showing a flattened proximal end of the femur; exposition of the Orlov Paleontological Museum (Moscow), photo by the author. |
 |
| Articular fossae and pelvic limbs of some stem tetrapods (from 2012SwartzB: Fig. 5). |
Considering that the femur of
Acanthostega is also flattened, its hip movements were similar to Ichthyostega,
as was its gait. Apparently, the evolution of tetrapod locomotion between
Panderichthys - Acanthostega, went through a stage of locomotion with body
flexion, in which the pelvic fins played the role of anchors, before the
emergence of a propulsive force driven by the hind limbs (2005BoisvertCA).
Currently, the type of locomotion with support on the front fins is
demonstrated by mudskippers (Periophthalmus) from the class of ray-finned
fishes. According to the graphical reconstruction, the acetabulum of
Acanthostega and Ichthyostega had a posterior notch (2018AhlbergP). More
precisely, the notch of the acetabulum in these tetrapodomorphs was located in
the posterosuperior section.
 |
| Pelvic morphology of elpistostegids and three early tetrapods, lateral, anterior, left view. Not drawn to scale. Tiktaalik, modified from Shubin et al. (Shubin, Daeschler and Jenkins 2014); Ichthyostega, new reconstruction based on data from Jarvik (Jarvik 1996); Acanthostega, new reconstruction based on data from Coates (Coates 1996); Eryops modified from Pawley & Warren (Pawley and Warren 2006); illustration from 2018AhlbergPE:Fig. 3, no changed. |
The presence of the acetabular
notch suggests that Ichthyostega and Acanthostega had at least one LCF in the
hip joint. This element, by restricting hip motion, contributed to more
effective support on the pelvic limb. It seems to us that the movements of the
hips in Ichthyostega and Acanthostega in the elongated acetabulum were circumduction,
which is well reproduced in the human wrist joint. In this case, the LCF
functioned as a flexible traction, limiting the adduction of the hip and the
tilt of the pelvis to the medial side. At the same time, it supplemented the
upper arch of the acetabulum, providing direct support for the upper surface of
the femoral head and contributed to the pressing of its to the acetabulum.
According to Б.З. Перлин et al.
(1977), the LCF functions in a similar way in humans, playing the role of «...
a supporting component of the bone-fibrous arch ...» of the hip joint.
It seems to us that the
proximal attachment area of the LCF in Ichthyostega and Acanthostega was
located above and behind the center of the hip joint. The distal attachment area
was probably located on the periphery of the upper part of the articular
surface of the femoral head or even on the neck. It could resemble the «capsular
ligament» (synovial fold) of Amantini, but was localized in the upper part of
the joint. According to our opinion, the LCF in Ichthyostega and Acanthostega
was formed from a portion of the articular capsule by its local intensive
stretching and gradual displacement in the direction of the central
longitudinal axis of the hip joint. A similar mechanism for the emergence of
LCF in phylogenesis was described by a number of authors of the XIX and XX
centuries (1902СаввинВН; 1927АнсеровНИ; 1939МаркизовФП; 1946ТонковВ).
Accordingly, in Ichthyostega
and Acanthostega, when supporting themselves on a leg extended forward, the LCF
was at an angle to the vertical. Being stretched by the weight of the pelvis
tilted downwards and medially, the element was similar to a deflected pendulum
thread with an upper suspension point. As a result of the ligament-bone-cartilage
interaction, a force directed forward appeared, facilitating the movement of
the animal.
The subsequent form of
tetrapods after Acanthostega and Ichthyostega is assumed to be Tulerpeton
curtum, which had longer and thinner bones than its evolutionary ancestors, but
with six fingers (2009ClackJA).
 |
| Artistic reconstruction of the external appearance of Tulerpeton curtum; exposition of the Orlov Paleontological Museum (Moscow), photo by the author. |
Tulerpeton curtum is
recognized as a stem-group reptiliomorph amniote that lived in an aquatic
environment; unfortunately, its pelvic girdle has not been preserved (1995LebedevOA_CoatesMI).
In the middle of the Visean stage (346.7-330.9 Ma) of the Carboniferous period,
tetrapods became effectively terrestrial (2012SmithsonTR_ClackJA;
2023CohenKM_CarN). Currently, it is assumed that the first truly terrestrial
vertebrate was Pederpes finneyae. Its fossilized remains were found in deposits
of the Tournaisian era, formed 354-344 Ma (2002ClackJA; 2023CohenKM_CarN). The
shape of the acetabulum of the individual could not be clarified, since this
part of the pelvis was irreversibly lost. However, it was noted that the shape
of the femur resembled that of Ichthyostega and is no different from the shape
of the femur of the closely related Whatcheeria (2005ClackJA_FinneySM). In
turn, Whatcheeria deltae, which lived somewhat later, approximately 340 Ma, had
a strongly concave acetabulum, an anteroposteriorly elongated and laterally
flattened femoral head (1995LombardRE_BoltJR).
It can be assumed that
Pederpes finneyae had an acetabulum similar to Whatcheeria deltae.
 |
| Right pelvic bone of Whatcheeria deltae, lateral view (schematic), cranial end on the right; the image is based on the drawing from 1995LombardRE_BoltJR, our notations). |
Analyzing the graphic
reconstruction of the pelvis of Whatcheeria deltae from the work of R.E.
Lombard, J.R. Bolt (1995: Fig. 8), we see the notch of the acetabulum in the
upper-posterior section, caudal to the buttress located above. Accordingly,
based on this observation, it can be assumed that Whatcheeria deltae and
Pederpes finneyae had LCF. It was probably located, as in Acanthostega,
attached above the center of the acetabulum and followed forward-outward-downward
to the periphery of the head or to the neck of the femur. The most ancient
amphibian anthracosaur from the lower stages of the Carboniferous period
(358.9-315.2 Ma) is Eoherpeton watsoni (1975PanchenAL). T.R. Smithson (1985)
provides a detailed description of its pelvis and the acetabulum region. The
author notes that in Eoherpeton watsoni, below the blade of the ilium and its
process under the transverse line, there is a large bulbous tubercle, which
forms a supraacetabular buttress. Its incomplete surface faces
posterolaterally, and its posterior margin forms the anterior border of a deep
cleft: the supra-acetabular notch, which is probably the site of a ligament
[LCF] similar to that found in the acetabular notch in mammals (see A.S. Romer,
1922:581). The supra-acetabular notch and buttress in Eoherpeton watsoni are
much more strongly developed than in most early tetrapods, especially in
Archeria and Gephyrostegus. The researcher points out that the significance of
this observation is unclear, but it may indicate a closer relationship between
the head of the femur and the acetabulum, providing a more powerful ligamentous
connection between the hindlimb and the pelvic girdle. Neglecting the
supraacetabular buttress, the acetabulum appears to have originally formed a
deep ovoid depression at the junction of the ilium, ischium, and pubis. The
acetabulum is oriented forward and has a greatest diameter of about 60 mm.
Ventrally, there is a prominent labrum, approximately 12 mm high, running the
length of the concave ventral margin and oriented at right angles to the
posterior aspect of the acetabulum. At its lowest point, the labrum is
horizontal. The bony surface of the acetabulum was apparently covered with
cartilage (1985SmithsonT:Fig. 27).
An early tetrapod,
Crassigyrinus scoticus, is an amphibian from the Namurian stage of the
Carboniferous period, i.e., it lived approximately 326-313 Ma. Its ilium formed
a strong articulation with the spine through the sacral rib. The ischium was
separated from the ilium by cartilage, ornamented like a skin bone. The pubic
bone is not preserved. The femur bears a strongly developed internal trochanter
on the surface. The described structure of the femur suggests swimming, not
walking (1990PanchenAL_SmithsonTR). The shape of the acetabulum of the
mentioned ancient amphibian remains unknown. We do not exclude that the
structure of its hip joint was similar to that of a modern salamander
(1934FrancisETB). A temnospondyl (primitive amphibian) Eryops megacephalus,
which flourished in the Permian, or 298.9-251.9 Ma, had significant
similarities with salamanders (2022HerbstEC_HutchinsonJR; 2023CohenKM_CarN).
In the posterosuperior region
of the acetabulum of Eryops megacephalus, a large notch is clearly visible. Its
configuration and location resembles that of Acanthostega and Whatcheeria
deltae. In our opinion, in Eryops megacephalus, the proximal end of the LCF was
attached in the region of the acetabular notch and was located behind and above
the center of the hip joint. Its distal part, apparently, was attached near the
edge of the articular surface of the head or even to the neck of the femur. We
propose that in the line from Acanthostega to Eryops megacephalus, the LCF was
oriented in a mirror-opposite manner to modern mammals. It was located at the
top of the hip joint. This suggests similar biomechanics of the hip joint and
the gait pattern of the earliest tetrapods.
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Keywords
ligamentum capitis femoris, ligamentum teres, ligament of head of femur, doctrine, animals, reptile, amphibian
The original text in Russian is available at the link: Амфибии и рептиломорфы
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