DYNAMIC MODEL OF MUSCLE EVOLUTION AND DEVELOPMENT

In previous studies, authors have categorised musculature into box-like schemes, an approach that was necessary to handle and to bring order to these elusive structures. As an example, I demonstrate the confusion that occurred for the categorisation of n. trigeminus (V) innervated jaw muscles in turtles (Figure 2, Figure 3, and Figure 4). In every study, authors had different scientific approaches, and they were only aware of a certain taxonomic sample. That resulted in different assumptions of hierarchies and hence in divergent interpretations of homology and evolution.

To avoid this categorical approach I propose a novel approach to study muscular structures. Therefore, the above-defined idea of the muscular unit – the smallest macroscopic part of a muscular structure – forms the basic element. I noticed in the literature review as well as in my own observations, that muscular structures display a very "fluid" material. It develops and evolves very plastically. Parts of the muscular material separate from a Mutterboden at one point, but they may refuse to the same structure in other taxa or developmental stages. Elements of these parts may also fuse to a muscular structure of a different origin or innervation, such as m. intermandibularis (No. 31) and m. constrictor colli Pars intermandibularis (No. 42) in turtles (Figure 16.4). Lubosch (1933, p. 589) predicatively called this behaviour of muscular structures biomyotaxis. As many separations, re-fusions, and partial rearrangements occur in ontogeny and phylogeny, categorisations generally proposed as working tools by several authors are no longer tenable. In Figure 16, all possible arrangements of n. trigeminus (V) and n. facialis (VII) innervated muscular units that I detected in the presented study are represented. Every unit represents either a separated muscle (Figure 16.1) or a muscle portion that is still correlated to other muscle portions – a scheme that literary can be best described as a "lotus blossom shape". Every petal only represents a temporary (ontogenetic) or spatial (phylogenetic) potential that is seen e.g., in turtles. Some petals are connected to the "lotus stem" (tendon), some are "shed" (separated) and directly attach to the "soil" (bones, etc.). The size and mightiness of every "petal" (unit) may change, or the structure may be completely absent. Muscle heads may be "translated" as "leaf margins".

As exposed to the development of cNCC migration and possibly other factors too, cranial musculature may best fit to the lotus scheme. As I personally do not have detailed experiences with trunk muscle development and evolution, I can only assume a comparable pattern in that region. The somitogenic neck musculature here studied at least behaves like this.

The herein presented fluctuating system of appearing and dissolving muscular units expands to a fluid, somehow "irregularly" oscillating system of partitions, fusions, demerging "muscle drops", which themselves split, refuse with other parts, etc. (Figure 16). Any box-like thinking must be refused, and although the muscular units as illustrated in Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13, Figure 14, and Figure 15 show some rectangles, they must be taken as pure descriptive simplifications, a tool kit for a more integrated discussion of fluid-lotus-like muscle evolution and development (Figure 16).