Ornamentation of dermal bones of Metoposaurus krasiejowensis and its ecological implications

Diagnosis: clavicles

Clavicles of metoposaurids from Krasiejów showed diversity in ornamentation, having fine, regular and sparse, or coarse, irregular and dense sculpture. After this observation, the clavicles ornamentation was examined in greater detail.

Some of the analysed features show random variation or none; however, most are distributed bimodally. Therefore, in every specimen one or the other set of characteristics occur, and two types of ornamentation can be distinguished (Tc1 and Tc2).

Specimens classified as type 1 (Tc1) are characterised by more regular ornamentation of the clavicles: the borders of the ossification centre (polygonal sculpture) are easily recognised, the polygonal sculpture field has a square shape, and the ornamentation is fine and sparse, moreover, nodal points are more pronounced, being broader and higher than the ridges that connect them, ridges are usually narrow, hexagons with a low level of size diversity dominate, multipolygons are rare, clavicles, even when large, are relatively thin; the anterior process of the clavicle is usually flat and small (Fig. 2); while specimens classified as type 2 (Tc2) posses less regular ornamentation: the borders of the ossification centre (polygonal sculpture) are difficult to recognise, the polygonal sculpture field is characterised by a rectangular shape (elongated posteriorly), and the ornamentation is thicker and denser, moreover, nodal points are only slightly broader and higher than the ridges that connect them, ridges are wide or narrow, often rounded, polygons are more often pentagonal or irregular, multipolygons are frequent, clavicles are relatively thick, independently of their size or age, and the anterior process is usually round in cross section and expanded (Fig. 2).

Both types of Metoposaurus dermal bones ornamentation are however distinct from Cyclotosaurus sculpture (Sulej & Majer, 2005). Cyclotosaurus can be characterised by relatively large and rhomboidal polygons (sometimes elongated pentagons). Radial ornament is very sparse (spaces between ridges are wide). Ossification centre is large and posses distinct borders, but the polygon number is low (25). Clavicle is thick. Ridges are round and thick (ZPAL/AbIII/397, personal observation).

The distribution of certain characteristics according to the relative individual age or type assignment is presented in Figs. 4–7. All plots show bimodal distribution of the parameters, which are independent of estimated relative individual age of specimens. UOPB1165 (Fig. 2) specimen not fitting any of this types might be the representative of a different taxon, although it was the most incomplete specimen, which may affect the result of its description. Some features were not described for this specimen, i.e., borders and shape of the ossification centre or radial ridges character (Table 1). Estimated size of the complete specimen is small, but the ossification degree is high. Its possible assignment to species other than M. krasiejowensis would be difficult without other findings.

In Table 5, the results of conducted statistical test are presented –F and T or U, dependent on the data distribution. Considering described types as different groups, quantitative and qualitative data shows that they differ significantly (α = 0.05).

Micro/nanoscale

Two types can also be distinguished according to the micromorphology of the ornamentation ridges and bone structure in cross-section. Clavicles assigned to type 1 do not possess striations (or striations, if present, are barely visible and sparse) and possess a low number of small capillary foramina at the slopes of the ridges (less than 7 per 100 µm²). Usually they also have less than one foramen per 1 mm of ridge length and no distinct bumps or roughness at the top of the ridge (Figs. 8 and 9, Table 2). In cross-section they possess growth marks in close proximity within poorly vascularised upper cortex (Fig. 8).

Clavicles assigned to type 2 possess striations on the ridges and a greater number of small foramina (more than 7 per 100 µm²). Usually they also have more than one foramen per 1 mm of ridge length and distinct bumps and roughness at the top of the ridge (Figs. 8 and 9, Table 2). In cross-section they possess growth marks separated by well-vascularised zones (Fig. 9). This difference in histological patterns is analogous to different growth strategies described for the example of long bones (Teschner, Sander & Konietzko-Meier, 2017).

Skulls

Bimodal differences were found also in skulls (Table 3), which have been divided in the Ts1 and Ts2 types. The main characteristic of ornamentation of ossifying centers resembles either Tc1 (large, hexagonal, sparse polygons, almost no multipolygons; six specimens; Ts1) or Tc2 (small, irregular and dense polygons with common multipolygons; seven specimens; Ts2). There is also a visible difference in the spatial distribution of polygonal and radial ornamentations between Ts1 and Ts2 (Fig. 10). In the first type, radial pattern covers large areas of the skulls roof in their both preorbital and postorbital (postfrontal, postorbital, supratemporal bones) parts, while in the second it occupies much smaller areas.

An important fact is that the skulls classified as Ts2 are relatively small (averaging 28 cm in length) in contrast to Ts1 skulls (averaging 35 cm in length). However, this was not a rule. Among analysed skulls were two 35 cm in length (UO/PP04, 35 cm; UO/PP18, 35.4 cm) with different ornamentation types (Fig. 10, Table 3, Fig. 4).

Ornamentation and lifestyle

The environmental differentiation is the most likely explanation regardless of whether caused by facultative neoteny or existence of two ecological types. Thus, described ornamentation types reflect more aquatic (Type 1) and more terrestrial (Type 2) morph of M. krasiejowensis. In modern limbless serpentine amphibians (Gymnophonia: Apoda) and lizard-like salamanders (Caudata: Urodela), larvae resemble miniature adult specimens. Metamorphosis is gradual and there is little reorganisation of body plan (Zug, 1993). In fossil amphibians, body plan reorganisation was also minimal and rather gradual (Boy, 1974; Boy, 1988; Boy, 1990; Schoch, 2002; Schoch, 2004), although its rate (trajectory: Schoch, 2010) might differ between taxa depending on their habitat (Schoch, 2009). This is also the cause that there are no other features suggesting more aquatic or more terrestrial lifestyle. Such changes, like differences in lateral line morphology, require more ‘evolutionary effort’, while changes in ornamentation are probably fast, reversible and do not require genetic changes (Rafiński & Babik, 2000; Babik & Rafiński, 2000).

Typically aquatic taxa are characterised by slow changes (low trajectory), sometimes with incomplete ossification of the pelvic region and limbs (last stages of ontogenetic trajectory). Terrestrial taxa are characterised by faster metamorphosis (high trajectory, with particular phases condensed within a short period of time), including final phases (limb ossification) enabling locomotion on land. The trajectory of semi-aquatic taxa lies between the two above-mentioned types.

This is an example of heterochrony. The length and composition of the temnospondyl ontogenetic trajectory is ecologically controlled (Schoch, 2010). Metamorphosis in this case might be described as extreme heterochrony, because many phases are condensed within a short time span (Alberch, 1989).

Ontogenetic trajectory and the morphology of adult specimens and their sizes may differ between various environments inhabited by representatives of the same taxon (Schoch, 2010). There are several examples of such diversity, such as differences observed in the length of the hind limbs of modern frogs (Schmidt, 1938; Dubois, 1982; Emerson, 1986; Emerson, Travis & Blouin, 1988; Rafiński & Babik, 2000) and the morphology of extinct temnospondyls: the ontogenetic rate and dentition of Apaeton (Schoch, 1995); the size of Micromelerpeton (Boy & Sues, 2000; Schoch, 2010); the morphology of Sclerocephalus (Schoch, 2010); the branchiosaurids (Werneburg, 1991; Werneburg, 2002; Werneburg, Ronchi & Schneider, 2007); and the plasticity of the plagiosaurid Gerrothorax (Schoch & Witzmann, 2012; Sanchez & Schoch, 2013). Polyphenism (environmentally controlled polymorphism) exists in a wide range of extant taxa (Roff, 1996) in adults (Whiteman, Krenz & Semlitsch, 2005) and tadpoles (Collins & Cheek, 1983; Pfennig, 1990; Pfennig, 1992; Walls, Belanger & Blaustein, 1993; Nyman, Wilinson & Hutcherson, 1993; Michimae & Wakahara, 2002; Pfennig & McGee, 2010).

Dimorphism in bone characteristics of metoposaurids from Krasiejów can be seen in dermal bones as well as in non-dermal skeletal elements from Krasiejów. Two types connected with growth trajectory were seen in histological observations of metoposaur skulls (K Gruntmejer, 2018, pers. comm.), humeri (Teschner, Sander & Konietzko-Meier, 2017), and the morphology of femora (Konietzko-Meier & Klein, 2013).

New facts about metoposaurids from Krasiejów show that they were not fully aquatic animals. 3D computational biomechanics analysis of the skull of Metoposaurus show that it was capable of biting prey in the same manner as semi-aquatic and terrestrial animals like Cyclotosaurus or modern crocodiles (Gruntmejer, Konietzko-Meier & Bodzioch, 2016; Fortuny, Marcé-Nogué & Konietzko-Meier, 2017; Konietzko-Meier et al., 2018).

The described diversity is consistent with the experiment of Schoch (1995) and the results of Werneburg (2002) and Schoch (2010). One of the Metoposaurus ornamentation types from Krasiejów (T2) thus represents a more terrestrial form (associated with the more variable and unstable environment of a river or a small lake or the metamorphic adult form of facultative neotenic population), while the other (T1) represents forms more closely related to water (a large lake habitat or partially paedomorphic aquatic adults) (ecological populations—as stated by Witzmann et al. (2010); but described as species-specific; neoteny as described by Whiteman, Krenz & Semlitsch (2005)).

The adaptations in T2 favouring a more terrestrial lifestyle are:

1. The increased mechanical strength of the bones (Rinehart & Lucas, 2013) (coarser, denser, irregular sculpture, thicker clavicles);

2. Protection from a greater number of blood vessels, improving thermoregulation (Gadek, 2012) (denser sculpture, more numerous polygons and radial rows, more numerous microforamina);

3. Stronger integration of bone and skin, which is thicker in terrestrial amphibians and exfoliates (Zug, 1993; Schoch, 2001) (coarser, denser sculpture, microstriations);

4. Stronger connection of the pectoral girdle elements and, potentially, limbs (expanded anterior projection of the clavicle);

5. Faster growth revealed by histological structure (growth marks separated by zones of highly vascularised bone).

The more terrestrial character of one of the population may also be proved by:

1. Faster (at younger age) metamorphosis revealed by smaller skulls;

2. The length of limb bones not correlated with individual age (Teschner, Sander & Konietzko-Meier, 2017) or a slender or robust femur (Konietzko-Meier & Klein, 2013); 10% elongation of limbs in Anura distinctly increases migration capabilities (Pogodzinski, Hermaniuk & Stepniak, 2015; M Pogodziński, pers. comm.).

The dimorphic character of clavicles described herein and the two growth patterns of dermal and long bones (humeri) (Teschner, Sander & Konietzko-Meier, 2017) suggests that the ontogeny of specimens assigned to Metoposaurus krasiejowensis could have proceeded via a different growth rate and time span of metamorphosis, caused by differing environmental conditions. The similar number of specimens from both populations (Tc1/Tc2—44%/56% and Ts1/Ts2—53%/47%) suggests stable populations.

Apart from dermal bone ornamentation, the degree of ossification and variation in skull sizes divides metoposaurids into two groups. Smaller skulls occur in the more terrestrial type, as in Micromelerpeton from Germany, where smaller specimens represent an unstable lake environment (Boy & Sues, 2000). The described type T2 reflects a more terrestrial or riparian habitat, where environmental conditions are variable and amphibians are forced to change their dwellings more often (migration between watercourses or ‘stream-type’ small, drying lakes; Werneburg, Ronchi & Schneider, 2007). It does not mean that ‘more terrestrial/stream’ metoposaurids moved efficiently on land. Modern salamanders can migrate between rivers and lakes by ‘pond-hopping’ (Zug, 1993). The first type reflects a more stable habitat, possibly a large lake, where animals are not forced to migrate (‘pond-type’; Werneburg, Ronchi & Schneider, 2007).

Geological, sedimentological, and other analysis of the Krasiejów site shows that both of these habitats—episodic rivers and ponds at the excavation site and a large reservoir in close proximity—may have occurred there (redeposited charophytes and Unionidae bivalves; Szulc, 2005; Szulc, 2007), and that conditions changed over time (Dzik & Sulej, 2007; Gruszka & Zieliński, 2008; Bodzioch & Kowal-Linka, 2012). Differences in dermal bone ornamentation constitute an adaptive answer to changes in the environment (temperature, water level, food availability) over time or to geographical differentiation of habitats, i.e., faster metamorphosis (at smaller size) as an answer to higher temperatures; or metamorphosis into terrestrial adult vs. transformation into aquatic paedomorphic individuals.

Rapid changes in the ornamentation morphology in one population (or part of the population, when weather conditions favour such solution) are possible because they are the effects of hormonally induced metamorphosis. The water temperature in which larvae live strongly affects ectothermic animals. The growth of amphibians and larval development both depend on external environmental factors. At higher temperatures, not only metabolic rate but also development rate increases (Motyl, 2008). Low temperatures reduce development rates to a greater extent than they reduce growth rate, as a result of which amphibians metamorphose after achieving larger size (Wilbur & Collins, 1973) (Ts1 skulls are usually larger than Ts2 skulls). Prey abundance might exert some influence as well (Motyl, 2008), but probably not as much (Blouin & Loeb, 1990).

The Krasiejów ecosystem changed over time. The Late Triassic climate favoured evolution of freshwater environments. In Krasiejów, small periodic reservoirs, probably also inhabited (as in the environments of the Saar-Nahe Basin), occurred along with larger more stable ones (Szulc, 2005; Szulc, 2007; Gruszka & Zieliński, 2008; Szulc, Racki & Jewuła, 2015). Small reservoirs (and potentially with higher temperature) or periodic rivers forced earlier metamorphosis, dwelling on land, or migration between lakes and watercourses. On the other hand, larger lakes or the proximity of a large reservoir enabled the development of a fully aquatic (Szulc, 2005), possible neotenic population.

Large reservoirs, stable over long periods of time, enable the development of a fully aquatic (neotenic?) ecotype T1 (Tc1, Ts1), reducing the need to dwell on land by virtue of providing:

• enough room for numerous large specimens;

• shelter from mainland carnivores;

• stable, invariable conditions;

• potential lower temperatures.

The ontogenetic trajectories of the two metoposaurid ecotypes from Krasiejów cannot differ on a large scale, because they are assigned to the same semi-aquatic species. However, between types there was clearly some deflection into a more aquatic or more terrestrial form. In the case of a more terrestrial (stream-type) ecomorph, the trajectory would be more condensed (Schoch, 2001).

According to the described observations, it is possible to introduce an argument about the function of temnospondyl ornamentation into the discussion. There are several hypotheses as to the function of the ornamentation, which may have been:

1. mechanical strengthening of the bone (Coldiron, 1974; Rinehart & Lucas, 2013);

2. water-loss reduction (Seibert, Lillywhite & Wassersug, 1974);

3. integration of the bone and skin (Romer, 1947; Bossy & Milner, 1998);

4. improvement of dermal respiration (Bystrow, 1947);

5. thermoregulation (Seidel, 1979; Grigg & Seebacher, 2001);

6. acting as a metamorphosis marker (Boy & Sues, 2000);

7. buffering of acidosis and lactic acid build-up in tissues due to anaerobic activity (Janis et al., 2012).

The microstructural observations described in this manuscript support two hypotheses. Ornamentation increases the surface area of the bone (Rinehart & Lucas, 2013) and thus improves its thermoregulatory abilities and probably its integration with the skin, as histological thin sections show many Sharpey’s fibres residing deep in the ridges (Gadek, 2012). Moreover SEM photographs presented herein show more or less numerous striations (skin and bone contact) and vascular foramina.

The hypothesis put forward by Janis et al. (2012) of dermal bone ornamentation developed in primitive tetrapods for the purpose of buffering acidosis and lactic acid build-up in their tissues due to anaerobic activity is also plausible. This would enable the amphibians to spend longer times on land and thus better exploit the terrestrial environment. This statement is in agreement with a study by Witzmann et al. (2010), who stated that terrestrial forms (according to species or population) show more pronounced sculpture than aquatic forms.