Ecological inferences and possible swimming styles
According to Ricqlès and Buffrénil (2001), increase in skeletal density and mass is a clear advantage for poorly active aquatic tetrapods because the resulting additional ballast allows a hydrostatic (passive) control of body trim in water and counteracts lung buoyancy. As a result it facilitates diving and extended underwater stays and improves stability in rough water (Taylor, 1994). Conversely, this skeletal specialization increases the inertia of the body and induces limitations of the swimming speed and capabilities to perform rapid maneuvers (Ricqlès and Buffrénil, 2001). A different pattern is a spongeous general organization as a result of the combined absence of medullary cavity with an increase in cortical porosity due to the development of erosion bays, which are not entirely filled up by secondary bone (Ricqlès and Buffrénil, 2001). In general one could say that taxa that lived in shallow water display an increase in bone compactness (Laurin et al., 2004), whereas species that lived in open marine habitats tend to have spongy bone (Ricqlés, 1977), as is obvious in e.g. several ichthyosaurs (Buffrénil and Mazin, 1990; Talevi and Fernandez, 2012; Houssaye et al., 2014).
All placodonts sampled share a thick cortex and clearly display bone mass increase (BMI) via osteosclerosis. However, the processes involved are disparate, as are the same global swimming mode. All placodonts are known from marine sediment except for Henodus that was found in lagoonal or lake sediments. Thus, based on sediments and microanatomy an aquatic lifestyle in shallow marine habitats can be inferred for all. Due to their specialized durophageous dentition all placodonts have fed predominately on hard shelled mollusks that moved only slowly or were sessile. Thus, there was no need for placodonts to be fast swimmers. However, the discussion of swimming styles and capabilities remains difficult, because among modern aquatic vertebrates only sirenians display BMI, which makes interpretation of the placodont data difficult. Additionally, morphological, histological, and microanatomical inferences contradict each other in some taxa.
BMI is most intense in Paraplacodus. Importantly this taxon lacks any kind of armour, which could otherwise also serve as a source of body mass increase (e.g. Scheyer, 2007). If it was an inefficient swimmer, achieving long dives close to the bottom, a high increase in bone mass would have been advantageous to control buoyancy. BMI is generally concentrated in the anterior portion of the body when it is also involved in body trim control (Houssaye, 2009). The occurrence of strong BMI in Paraplacodus humeri and femora would be in accordance with a bottom-walker rather than shallow-swimmer ecology for this taxon.
Psephoderma alpinum also shows high bone compactness among the sample in spite of resorption processes in the cortex. Psephoderma was armoured with a dorsal shield consisting of a main carapace and a pelvic shield, which increases body mass, too. Its overall habitus and largely inflexible body axis would suggest a comparable swimming style to modern freshwater turtles that is a combination of bottom-walk and rowing with the limbs. However, Psephoderma has slender and short (reduced) limb bones making rowing with effective strokes unlikely. Microanatomy is similar to Claudiosaurus that was an anguilliform swimmer. However, an anguilliform swimming mode for Psephoderma is unlikely due to a largely inflexible body axis (dorsal armour). Although in modern interpretations the dorsal armour is divided at the pelvic region anguilliform movements are still restricted. The best interpretation is so far that Psephoderma has had a passive lifestyle sitting most of the time on the bottom/ground (or was maybe buried in analogy to some flat turtles). For feeding it slowly walked the ground. Certain propulsion by wriggling movements of the pelvic and tail region is conceivable. Higher resorption in the femur than in the humerus further indicates more active movements of the limbs and thus might support more active swimming when compared to Paraplacodus (that has a similar high BMI).
Humeri of Placodontia indet. aff. Cyamodus also show BMI although to a lesser extend when compared to Paraplacodus and Psephoderma. The humerus assigned to aff. Placodus, the humerus of Henodus, and the humeri of the diapsid Horaffia all share a similar microanatomy and histology. Thus, similar compactness values are assumed for aff. Placodus and Henodus (although they are both not tested in the PCA due to the lack of a thin section). The lower BMI indicates more efficient swimming in these taxa when compared to Paraplacodus and Psephoderma, because less BMI suggests more maneuverability.
Cyamodus hildegardis was armoured with a dorsal shield consisting of a main carapace and a pelvic shield (Rieppel, 2002; Scheyer, 2010). Other Cyamodus species most likely carried armour as well (as evidenced in C. kuhnschnyderi; see Nosotti and Pinna, 1996) but their exact configuration on the body is not known. In addition, Cyamodus hildegardis also carried a heavily armoured tail (Scheyer, 2010). Henodus was heavily armoured as well, with a closed dorsal and ventral shield (Huene, 1936). Thus, body mass was increased by armour in both taxa. Cyamodus has long and massive humeri, Henodus humeri are tiny (reduced). Based on its morphology, lifestyle and swimming capabilities of Henodus are interpreted as similar to those described above for Psephoderma, making Henodus mainly a bottom walker. No propulsion by wriggling seems possible due to its entirely enclosed body.
Humeri of Placodontia indet. aff. Cyamodus are long and massive (as it is the case in all Cyamodus spp.), allowing strong strokes and making a swimming style by rowing with the limbs possible. [Please note that it is unknown if the feet of any placodont were webbed.] Additionally, the divided dorsal shield would have allowed some (minimal) propulsion by wriggling with the rear and tail.
Placodus has only one single row of osteoderms along its vertebral column that has not much contributed to body mass. It was classified as a subcarangiform swimmer (Braun and Reif, 1985), which is supported by its laterally compressed tail (Drevermann, 1933) and the supposed BMI.
We did not know if armour was present or not for the taxa represented by humeri and femora assigned to Placodontia indet. All bones show a high compactness and have a large spongeous medullary area of a comparable size to that of some otariids (but combined with a much higher compactness in Placodontia indet.). In addition, some femora show an open medullary cavity. Thus, microanatomy indicates certain, but not very fast, swimming capabilities.
It is also unknown whether the diapsid Horaffia was armoured. Its humeri were large and pachyostotic (Klein and Hagdorn, 2014). Microanatomy clearly documents osteosclerosis (Klein and Hagdorn, 2014; current study). Both identify this taxon as a shallow marine inhabitant most likely a bottom walker or slow swimmer but due to the lack of a complete skeleton this remains rather speculative.