Morphology of Acanthostega
The morphology of Acanthostega is transitional between what we typically would call fish and amphibians. It has multiple traits typical of tetrapods while retaining features typical of fully aquatic fish. It is very likely that Acanthostega interacted briefly with the water's edge but was primarily an aquatic carnivore.
Feeding
Acanthostega was a carnivore that may have been feeding on arthropods off the shoreline, but most likely it fed on fish via suction feeding. Suction feeding is used by most fish, prehistoric and modern. Suction feeding is the act of fish generating a flow of water into a rapidly expanding mouth cavity (Day et al., 2015). The upper jaw protrudes during the expansion phase of the jaw, which allows the fish to rapidly move its jaw forward and engulf prey (Shadwick et al, 2006). The expansion phase is the expansion of the opercular cavity, which is coupled with compression of the mouth cavity and the momentum of the water. These act to continue to flow water through the mouth of the fish and out of the gills.
Cranial morphology during opening of mouth for feeding in a largemouth bass, highlighting (1) the oral jaws, including the premaxilla (green), maxilla (orange), lower jaw (blue), (2) the composite hyoid (yellow), and (3) the pectoral girdle (purple). From Shadwick et al., 2006.
Skull of Acanthostega highlighting the bones of the jaw and showing the teeth. Modified from original figure from Porro et al., 2015. From Farke, 2015.
Acanthostega is hypothesized to have used its teeth and front of its skull to grab live prey. The sturdy connections between bones on the front and roof of the skull as well as the teeth present in the fossil suggest that Acanthostega was actively feeding this way. However, the mid section of the jaw is composed of more loosely connected bones on the roof of the mouth and in the cheek. This suggests that Acanthostega was primarily feeding underwater (Farke, 2015). Acanthostega was probably a suction feeder like most fish, but most likely had reduced suction feeding in comparison to most fish. The biting force of Acanthostega was also relatively low, suggesting that its jaws were not functioning exactly the same way as aquatic organisms, but also not the same as later terrestrial tetrapods (Neenan et al., 2014).
The Lateral Line
The lateral line system is a series of pores filled with highly sensitive nerves that detect vibrations and pressure changes in the water. This allows fish to hear underwater and navigate their surroundings. Since their bodies have a density nearly the same as water, fish do not rely on specialized ears to concentrate air vibrations the way we do to a small, bony, focal point, instead their entire body operates as an auditory receptor.
Sarcopterygians more adapted for life in the water, such as Eusthenopteron (Whiteaves,1881) have lateral line systems that run down the length of their body. This effectively makes the entire body an ear.
Lateral Line in Acanthostega
The lateral line system of Acanthostega is apparently reserved to the skull wrapping below and behind the eyes. This is interesting when compared to other Sarcopterygian fish as it indicates the focusing and processing of sound occurred primarily in the head as opposed throughout the entire body as described above.
The lateral line system on Acanthostega indicates that most of its hearing was done underwater. The specialized Tympanic Membrane (Ear Drum) that frogs use to hear outside of the water was unlikely to be present.
Ears and Hearing
Acanthostega's skull anatomy shows multiple lines of evidence of being well suited for detecting soundwave vibrations in the water.
Hearing above the surface was highly limited, if done at all, but the basic arrangements of bones that would become the tetrapod ear were beginning to take place.
The Hyomandibular and The Stapes
In the fishiest of fish (e.g., sharks, tuna, and your beloved goldfish) the bones that are analogous to the ear bones of tetrapods are used primarily for opening and closing the jaws in a variety of complex ways. Today sound is processed on land by a wide variety of Tetrapod organisms including humans like you and I.
Shark Hyomandibular and Hearing
In most fish (like sharks) most "hearing" is received throughout the body, where it is processed by the brain.
The Hyomandibular (Blue), in tandem with the palatoquadrate and mandible (red and green) operate in union to open and close the jaw.
The spiracle and otic capsule aid in the sensation of both orientation and direction of movement to allow the shark to successfully navigate and feed in its ecosystem.
Bones of a Frog's Ear
In many non-mammalian tetrapods, like frogs, the tympanic membrane connects directly to the stapes. The homologues to the Malleus and Incus, Articular and Quadrate respectively, aid in opening and closing the Jaws.
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Bones of our Ear
Our bodies are hundred of times more dense than air. Vibrations in the air tend to bounce off of our dense bodies and make it difficult to interpret them as sound. Specialized bones, the Malleus, Incus, and Stapes work in threefold succession and focus vibrations received from the disc-shaped tympanic membrane (The Ear-Drum) onto the auditory nerve. The Stapes can trace its evolutionary origin to pieces of the Hyomandibular bone in fish. The Incus can trace its origin to the caudal end of the Palatoquadrate . Lastly, the malleus can trace its origin to the caudal portion of the mandibular in fish.
The tympanic membrane in frogs rests on the outside of the skull and is visible to the unaided eye.
Acanthostega Hyomandibular
The Hyomandibular bone in Acanthostega is in a position in the skull similar to the Stapes. However, the "Stapes" of Acanthostega is very large and not likely great at focusing high frequency noises. This has lead many to argue for the absence of a Tympanic membrane in Acanthostega. If it was listening to sounds above the water's surface they were likely very limited in scope of pitch relative to the broad auditory spectrum we as derived tetrapods take for granted.
Acanthostega skeletal by Matthew Bonnan PhD 2016
Pectoral Girdle and Forelimbs of Acanthostega
The pectoral girdle and forelimbs of Acanthostega show a mixed bag of typical terrestrial tetrapod features as well as features more typical of fish. The girdle itself is only loosely connected to the spine indicating a primarily aquatic lifestyle.
Skeletal Diagram of Acanthostega Pectoral Girdle and Forelimbs (Bonnan 2016)
Color coded forelimb of Acanthostega (modified from Bonnan 2016)
The forelimb skeletal anatomy of Acanthostega is virtually indistinguishable from that of modern tetrapods. A single long-bone, the humerus, connects the two smaller long-bones, the radius and ulna, which in turn connect to a series of wrist-bones and fingers. Instead of the typical 5-fingered arrangement seen in modern tetrapods, Acanthostega had an 8-fingered hand.
The Pectoral Girdle of Acanthostega retains a lot of fish-like traits. The Scapulocoracoid retains grooves for gills to rest on (in green). The Cleithrum is still distinct unlike that in derived tetrapods. However, the arrangement of these bones is more similar to that of basal tetrapods instead of fish, highlighting a highly transitional bodyplan.
Pectoral Girdle of Acanthostega (modified from Bonnan 2016)
Hindlimb of Acanthostega showing the basic anatomy of modern tetrapod hindlimb arrangement (modified from Coates 1996)
In derived tetrapods the ilium of the pelvis is fused to a complex of fused vertebrae called a Sacrum. This fusion between the Ilium and Sacrum provides rigidity along the hindlimbs to allow for tetrapods to walk on land.
Acanthostega lacked a sacrum and its ilium was not fused to the vertebral column, further indicating a primarily aquatic lifestyle as its hindlimbs could not overcome the force of gravity for extended periods of time.
The Pelvic Girdle of Acanthostega
The ​pelvis and hindlimbs of Acanthostega, much like the rest of the animal, are in an arrangement very similar to modern tetrapods with a few residual fish-like traits (again indicating a primarily aquatic lifestyle). The bones of the pelvis are robust but are not fused to the spine indicating a primarily aquatic lifestyle.
The arrangement of the hindlimb bones follows the same basic pattern as the forelimb. With a single long-bone, the femur, connecting to two smaller long-bones, the tibia and fibula, followed by a series of ankle and toe bones. Again an 8-toed foot rested at the end of Acanthostega's hindlimb
Terapod sacral complex (modified from Bonnan 2016)
Ossification of Acanthostega
The limb bones of Acanthostega for the first 8 or so years of its life were made from soft unossified cartilage (Sanchez 2016). Soft cartilaginous limbs would likely not support the animal on land for extended periods of time. When Acanthostega's limbs began to ossify as they entered sexual maturity the end bones were not particularly well adapted for terrestrial locomotion. Cortical bone is the compact weight-bearing material in terrestrial tetrapods. Inside of the cortical bone ring lies the lamellar or spongey-bone that provides internal cushioning from shock.
Diagram of Acanthostega forelimb indicating thin cortical layer surrounding vast lamellar bone complex (Modified from Bonnan 2016)
In Acanthostega the cortical bone (shown in pink) was an incredibly thin veneer indicating minimal terrestrial locomotion in mature individuals. The lamellar bone (grey) dominated the limbs of Acanthostega indicating a lot of tensile stresses acting on the limbs with minimal influence from gravity.
Could Acanthostega walk?
Only for very brief moments
Given the lack of pectoral and pelvic support from the vertebral column and the consistent fish-like adaptations such as apparent gills and a derived lateral line system, it seems unlikely that Acanthostega would interact with dry ground for extended periods of time and was likely a primarily aquatic creature.