Hydrodynamics of fossil fishes

Abstract

From their earliest origins, fishes have developed a suite of adaptations for locomotion in water, which determine performance and ultimately fitness. Even without data from behaviour, soft tissue and extant relatives, it is possible to infer a wealth of palaeobiological and palaeoecological information. As in extant species, aspects of gross morphology such as streamlining, fin position and tail type are optimized even in the earliest fishes, indicating similar life strategies have been present throughout their evolutionary history. As hydrodynamical studies become more sophisticated, increasingly complex fluid movement can be modelled, including vortex formation and boundary layer control. Drag-reducing riblets ornamenting the scales of fast-moving sharks have been subjected to particularly intense research, but this has not been extended to extinct forms. Riblets are a convergent adaptation seen in many Palaeozoic fishes, and probably served a similar hydrodynamic purpose. Conversely, structures which appear to increase skin friction may act as turbulisors, reducing overall drag while serving a protective function. Here, we examine the diverse adaptions that contribute to drag reduction in modern fishes and review the few attempts to elucidate the hydrodynamics of extinct forms.

Keywords: biomechanics; comparative anatomy; fishes; functional morphology; hydrodynamics; locomotion.

Figure 1.

Stages of boundary layer development on a flat plate, subjected to an adverse pressure gradient. Arrows show flow direction, with length indicating velocity and mean flow velocity emboldened, boundary layer in blue and zone of vortex formation or ‘wake’ in red.

Figure 2.

Boundary layer development and separation across a fish-like form, showing the effect of a turbulisor on flow regime and wake formation.

Figure 3.

Flank scale of the osteichthyan Lophosteus: (a) scanning electron microscope (SEM) image of large buttressed tubercles on upper surface; (b) lateral view (surface rendering of µCt scan); and (c) dorsal view (SEM image). Scale bar: (a) 100 µm, (b–c) 0.5 mm.

Figure 4.

Hypothesized drag reduction, abrasion resistance and parasitic defence functions of the flank scales of (a) Phlebolepis elegans, (b) Nostolepis striata, (c) Lophosteus, (d) Oniscolepis sp., (e) Thelodus laevis, (f) Andreolepis, (g) Thelodus parvidens, and (h) Loganellia cuneata. Based on Reif's scheme of shark scale classification [19]. Background SEM images courtesy of Sue Lindsay, Australian Museum: top; Carcharhinus obscurus, left; Orectolobus ornatus, right; Deania calcea.

Figure 5.

Examples of hypothesized swimming morphotypes of extinct and extant fishes: (a) Saurichthys (Triassic), (b) Aspidorhynchus (Mid-Jurassic–Late Cretaceous), (c) Belone belone (extant garfish), (d) Dorypterus (Permian), (e) Proscinetes (Jurassic), (f) Stromateus fiatola (extant pomfret), (g) Trachinotus falcatus (extant permit), (h) Bobasatrania (Triassic), (i) Cheirodus (Carboniferous), (j) Chaetodon (extant butterflyfish), (k) Tarrasius (Carboniferous), (l) Clinoporus biporosus (extant ladder klipfish), (m) Rebellatrix divaricerca (Early Triassic), (n) Hypsocormus (Mid-Late Jurassic), (o) Scomber scombrus (extant atlantic mackerel), (p) Parasemionotus (Early Triassic), (q) Mesolepis (Carboniferous), (r) Oncorhynchus mykiss (extant rainbow trout), (s) Carpiodes cyprinus (extant quillback), (t) Perleidus (Early–Middle Triassic), (u) Paracentrophorus (Early Triassic), (v) Serranus hepatus (extant brown comber). After [,,,–84].