Microanatomical tendenciesnext section
Within our placodont sample, five main distinct types of microanatomical organization are observed: 1) extremely compact humerus (CI=94.7%) and femur (CI=97.9%) with no medullary cavity in Paraplacodus; 2) strongly compact humeri (78.5% <CI<87.1%; mean value=83.1%) with a small open medullary cavity in Placodontia indet. aff. Cyamodus (no femur was available); 3) strongly compact humerus (CI=91.2%) with a small medullary cavity surrounded by a spongiosa and a compact outer cortex, and a rather compact femur (CI=78.2%) with a reduced medullary cavity surrounded by a thick spongiosa, with almost no outer compact cortex, in Psephoderma; and 4) humeri and femora with a large medullary region but no open medullary cavity and 5) femora with an open medullary cavity surrounded by a rather large spongiosa.
The extremely compact bones of Paraplacodus show strong osteosclerosis. Remodelling occurs in the core of the sections and is characterized by abundant endosteal bone deposits so that the medullary area is compacted. The humeri of Placodontia indet. aff. Cyamodus show a strong inhibition of primary bone resorption and remodelling process is initiated although only incompletely. They are similar to those of the diapsid Horaffia kugleri. However, if the latter are characterized by pachyosteosclerosis (Klein and Hagdorn, 2014), those of Placodontia indet. aff. Cyamodus, with no morphologically observable thickening, only display osteosclerosis (S2). As a result of the PCA Placodontia indet. aff. Cyamodus and the diapsid Horaffia kugleri along with Pistosaurus and the pachypleurosaur Anarosaurus are well separated from both the remaining placodonts on the one hand and sustained swimmers on the other (Fig. 3).
The Psephoderma femur is characterized by periosteal bone resorption in form of a perimedullary region containing large erosion cavities. This is to a lesser degree also visible in the much smaller and more gracile humerus. Periosteal remodelling is initiated by the deposition of a thin layer of lamellar bone around the large erosion cavities. The femur organization of Psephoderma rather resembles that of the femur, humerus and rib of the diapsid Claudiosaurus (Buffrénil and Mazin, 1989) that was an anguilliform swimmer (Houssaye, 2012). Some humeri and femora from our sample, referred to as Placodontia indet. have a large medullary region consisting of secondary trabecles. The occurrence of open medullary cavities in other femora of Placodontia indet. suggests different functional requirements (swimming styles) and slow swimming skills.
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.
Placodonts share several histological features. Numerous primary osteons occur in all bones that grew with fibro-lamellar bone. Primary osteons are not developed in Psephoderma alpinum and they are rare in Paraplacodus broilii, which both grew with lamellar-zonal bone. All placodonts share immature (i.e. incompletely lined) primary osteons that predominately occur in the inner cortices. Incompletely lined primary osteons are also described for other Sauropertygia such as a pachypleurosaurs and a pistosauroid (Klein, 2010), and for young Alligator mississippiensis (Woodward et al., 2014). There thus might be a correlation between aquatic lifestyle (and an inferred increased growth rate when compared to terrestrial forms; see White, 2011) and incompletely lined primary osteons.
Resorption occurs in all placodont samples. Psephoderma humerus and femur exhibit a perimedullary region in which periosteal bone is resorbed and partially replaced (remodelled). The entire medullary region of Paraplacodus is made of secondary (endosteal) bone, resulting in a very compact centre. Some Placodontia indet. show an extended medullary region filled by secondary trabecels. In fact, the Placodontia indet. sample can be divided into two groups based on the presence of a medullary cavity in some femora (MB.R. 961; MB.R 812; MB.R 814.2; SMNS 54578) and the presence of a large medullary region, respectively (IGWH 9, 23; SMNS 54585; MB.R. 454). The presence of a free cavity in some Placodontia indet. aff. Cyamodus and Placodontia indet. is not typical for aquatic tetrapods (e.g. Quemeneur et al., 2013; Hayashi et al., 2013).
In those samples that grew with fibro-lamellar bone remodelling is always only initiated, which means that secondarily widened (eroded) vascular canals or primary osteons are surrounded by a layer of circumferentially deposited lamellar bone but stay widely open and are not infilled. Mature secondary osteons are very rare and are restricted to the inner cortex of few samples. The absence of large amounts of mature secondary osteons (Francillon-Vieillot et al., 1990; Currey, 2002) in the placodont samples expressing fibro-lamellar bone is atypical. Ricqlès (1976) hypothesized a possible relation between fibro-lamellar bone and remodelling (i.e., the presence and number of secondary osteons), which is true for many extant and extinct vertebrates such as large herbivorous mammals, large birds, dinosaurs, and ichthyosaurs (e.g. Klein and Sander, 2008; Houssaye et al., 2014).
All placodonts, thanks to an inhibition of bone remodelling, have a good and fairly complete growth record preserved, which is the subject of another study (Klein et al., unpubl. data).
Paraplacodus, one humerus of Placodontia indet. aff. Cyamodus, and some humeri of Horaffia show pockets of calcified cartilage at midshaft. Retention of calcified cartilage at midshaft is often coupled with osteosclerosis and is recognized in many secondarily aquatic vertebrates, e.g. in pachypleurosaurs and plesiosaurs (Buffrénil et al., 1990; Ricqlès and Buffrénil, 2001; Hugi et al., 2011; Krahl et al., 2013). It is also described in some armour plates of placodonts (Scheyer, 2007).
Two major histological groups can be distinguished among placodonts, which do not follow the classical phylogenetic distinction into armoured vs. non-armoured Placodontia. The armoured Psephoderma and the non-armoured Paraplacodus both grew with compact, low vascularized or avascular lamellar-zonal bone, indicating a slow growth rate and rather low basal metabolic rate comparable to that of modern amphibians and reptiles. The armoured Henodus, non-armoured aff. Placodus, and the armoured Placodontia indet. aff. Cyamodus grew with plexiform to radiating fibro-lamellar bone tissue, indicating high growth rates and a high basal metabolic rate. For Placodontia indet. and for Horaffia it is unknown if the individuals had carried an armour or not, also grew with fibro-lamellar bone tissue but here the organization is more circumferential, indicating somewhat lower growth rates (Margerie et al., 2004). Fibro-lamellar bone tissue is known from modern birds, dinosaurs, most synapsids, and from other extinct marine reptiles such as ichthyosaurs and plesiosaurs (e.g. Chinsamy-Turan 2005, 2011; Houssaye et al., 2014; Wiffen et al., 1995). Thus, fibro-lamellar bone tissue clearly originated several times within different vertebrate lineages.
The fibro-lamellar bone of placodonts has the typical scaffolding of woven bone surrounded by lamellar bone. However, sometimes the woven bone component is replaced or grades into parallel-fibred bone with specially thick and coarse fibers. Both always contain a high amount of thick and numerous osteocyte lacunae. A replacement or modification of the woven bone component by parallel-fibred bone in the fibro-lamellar tissue was described before for archosaurs (Ricqlés et al., 2003), the ornithopod dinosaur Gasparinisaura (Cerda and Chinsamy, 2012), and the titanosaur dinosaur Ampelosaurus (Klein et al., 2012). An atypical parallel-fibred bone was mentioned for mosasaurs (Houssaye et al., 2013) and for the temnospondyl Plagiosaurus (Konietzko-Meier and Schmitt, 2013), which both did not grow with fibro-lamellar bone. It also occurs within the fibro-lamellar bone of ichthyosaurs (Houssaye et al., 2014).
Locally, a high amount of radial vascular canals occurs within the fibro-lamellar bone of some placodonts. Radial bone tissue reveals according to Margerie et al., (2004:869) in the king penguin chick the highest growth rates. In ichthyosaurs, a radial arrangement of vascular canals was interpreted as a consequence of the insertion of Sharpey’s fibers and thus linked to mechanical reasons (Houssaye et al., 2014). The local radial trabecle-like architecture in the inner cortex of some placodont samples may also indicate mechanical properties in addition to high overall growth rates. The fibro-lamellar bone tissue in combination with the plexiform to radiating organization indicates for Placodontia indet. aff. Cyamodus the highest growth rates among placodonts but also among other Triassic Sauropterygia and is comparable to that of ichthyosaurs (Buffrénil and Mazin, 1990; Houssaye et al., 2014).
In all placodonts that grew with fibro-lamellar bone, primary tissue is in general highly vascularized. Locally it is even spongeous-like or trabecular-like. Primary osteons are secondarily widened by successive resorption processes, resulting in an overall secondary spongeous tissue, which is similar to some ichthyosaurs (Houssaye et al., 2014).
In spite of certain inter- and intraspecific variability within the placodont sample that grew with fibro-lamellar bone, the primary bone tissue is in general very similar to that of ichthyosaurs. Differences include the lack of a medullary cavity, high endosteal and periosteal remodelling, and an overall more spongeous organization, in ichthyosaurs (Houssaye et al., 2014). The similarities are notable, because ichthyosaurs were efficient and sustained swimmers that lived in the open marine sea with a comparable body shape and lifestyle to dolphins or tunas. Placodonts have a cylindrical, sea cow-like body shape (non-armoured forms) or resemble the shape and morphology of flat aquatic turtles (armoured forms).
Long bone histology allows the identification and assignment of isolated bone fragments to Placodontia. Neither the combination of the avascular to low vascularized lamellar-zonal bone tissue of Psephoderma and Paraplacodus nor the special plexiform to radiating fibro-lamellar bone tissue of aff. Placodus, Placodontia indet. aff. Cyamodus, and Henodus or circumfertential fibro-lamellar bone of Placodontia indet. has so far been reported in the here described combinations in any long bone of any other marine reptile (e.g. Sander, 1990; Wiffen et al., 1995; Pellegrini 2007; Klein, 2010; Hugi, 2011; Hugi et al., 2011; Krahl et al., 2013; Houssaye et al., 2014).
Different bone tissue types and growth strategies
As summarized in Table 3, long bone histology indicates two major groups, which do not follow the classical phylogenetic distinction into armoured vs. non-armoured Placodontia (Fig. 1). The armoured Psephoderma and the non-armoured Paraplacodus both grew with lamellar-zonal bone tissue indicating slow growth rates and a low basal metabolic rate, comparable to modern reptiles. Henodus, aff. Placodus, Placodontia indet. aff. Cyamodus, and Placodontia indet. grew with fibro-lamellar bone tissue combined with a high, though extremely variable, vascular density, indicating high growth rates and an increased (sustained high) basal metabolic rate. Both tissue types represent two completely different growth strategies in these closely related taxa. Differences in growth rate and basal metabolic rate as well as the somewhat lower BMI in Henodus, Placodontia indet. aff. Cyamodus, aff. Placodus, and Placodontia indet. could point to a more active lifestyle when compared to Psephoderma and Paraplacodus. Perhaps a more active lifestyle also includes sustained swimming (?migration) over long distances for aff. Placodus, Placodontia indet. aff. Cyamodus, and Placodontia indet. but not for Henodus.
[ *image not found: m8401a05:TAB3 ]
That the two different growth strategies are the result of gross differences in habitat/environment or climate can be excluded. All bones exhibiting fibro-lamellar bone originate from localities within the Germanic Basin, whereas Psephoderma and Paraplacodus samples originate from localities settled in the Alpine Triassic realm. All localities were interpreted to represent shallow marine environments. During the Middle Triassic, climate was subtropical warm with alternating dry and wet seasons (megamonsoonal intervals) (Parrish et al., 1982; Röhl et al., 2001). Accordingly, water temperatures of the Muschelkalk Sea and the Alpine Triassic lagoonal settings have been correspondingly high and less metabolic effort was needed for marine reptiles to sustain a high growth- and basal metabolic rate in both realms. Thus, strong differences in paleoclimate can be excluded as explanation why some placodonts grew with fibro-lamellar bone whereas others grew with lamellar-zonal bone.
High predatory pressure that makes faster growth necessary, although conceivable as a trigger for the presence of fibro-lamellar bone in those placodonts from the Germanic Basin, appears very unlikely since predators also occurred in the environments of the Alpine Triassic. Predatory pressure might even be higher in environments of the Alpine Triassic due to the presence of a higher number of possible predators such as ichthyosaurs, large nothosaurs, and thalattosaurs.
So far we can only observe that Henodus, Placodontia indet. aff. Cyamodus, aff. Placodus, and Placodontia indet. followed a different life history strategy when compared to Psephoderma and Paraplacodus the exact reasons for this have yet to be fully understood.
The study of the morphology, long bone microanatomy, and histology of Placodontia clearly shows that extinct taxa are not ‘simply similar’ to modern taxa but that they could have had a variety of features, which are in certain combinations sometimes without a modern analogy so that the possibilities for interpretation are limited.
Clear evolutionary trends within Placodontia regarding microanatomical and histological features are not observable but identification might be hampered due to still limited sample size. Fibro-lamellar bone tissue already occurs in placodonts from the late early Anisian and persists until the early Carnian with Henodus. The lamellar-zonal bone type is so far only documented in stratigraphically younger placodonts from the Anisian-Ladinian boundary and the Rhaetian. A similar distribution of lamellar-zonal bone and fibro-lamellar bone is documented in pachypleurosaurs (Sauropterygia). Neusticosaurus ssp. from the Anisian-Ladinian boundary of the Alpine Triassic show lamellar-zonal bone (Sander, 1990; Hugi et al., 2011) whereas the stratigraphically older Anarosaurus heterodontus from the Lower Muschelkalk of Winterswijk (Germanic Basin) grow with incipient fibro-lamellar bone (Klein, 2010). This could indicate that the fibro-lamellar bone tissue was inherited from the unknown terrestrial ancestor of Sauropterygia and was later abandoned in some taxa.
In conclusion, all placodonts sampled followed an essentially or probably exclusive aquatic lifestyle. However, we observe distinct differences in histology and microanatomy that lead to different growth strategies and life (swimming) styles in Placodontia independent from phylogenetic relationships, ontogenetic stages or the presence or absence of armour. The differences imply a high inter- as well as intraspecific variability, most likely depending on the environment each individual lived in (developmental plasticity), and diverse lifestyles among the group.