It has been relatively popular to imagine that mudskippers
look very much like how our semi-aquatic ancestors would have appeared
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Mudskippers and humans, however, are modern representatives of independent lineages that went down separate and very different evolutionary paths.
The most widely accepted phylogenetic reconstructions to date imply that our last common ancestor lived during the Silurian period, more than 400 million years ago.
Probably during the Silurian ancestral forms of bony fishes (osteichthyans) gave rise to two distinct lineages: sarcopterygians, the lobe-finned bony fishes (or as Clack suggested, "fleshy-limbed vertebrates": Clack, 2002) and actinopterigians, the ray-finned bony fishes.
The sarcopterygian lineage gave rise to tetrapods, including humans, while the actinopterygian one gave rise to the vast majority of modern fishes, including mudskippers (see also Systematics and Biogeography).
Drawing: G. Polgar (2005)
Sarcopterygians underwent several adaptive radiations during their history, as evidenced
by many diverse and different fossil forms.
At present, these aquatic forms include only the very derived modern lungfishes (dipnoans: 6 species included in three genera) and modern coelacanths
(genus Latimeria: two species).
But perhaps the most interesting part of sarcopterygian history concerns those among them that caught the "Devonian opportunity" (Murphy, 2005 - devoniantimes: Opportunity knocked), and eventually managed to invade terrestrial ecosystems: the tetrapods (see also The Devonian opportunity).
Today this group comprises the 'fish out of water' that are best adapted to terrestrial life: the amniotes
(reptiles, birds and mammals).
A few examples of extinct and modern sarcopterygians exemplify a complete range of adaptative strategies, from aquatic to terrestrial forms. a. reconstruction of Eusthenopteron sp., an extinct lobe-finned fish: Late Devonian;
b.Protopterusaethiopicus, a modern lungfish; c.Rana esculenta, an amphibian; d. Anolis carolinensis, a reptile;
e. Gallus gallus, a bird; f. Latimeria chalumnae, a modern coelacanth;
g. Homo sapiens, a mammal.
Several pre-existing anatomical and ecological features of tetrapodomorphs (Clack, 2002),
together with peculiar environmental conditions that occurred around the end of the Devonian period,
actually paved the way to vertebrate terrestriality.
During the Late Devonian transitional environments were already colonised by several groups of arthropods and by vascular
plants. In particular, this latter invasion led to the expansion of fine sediments' deposits, to the formation of the first deep organic soils and to the rapid increase
of atmospheric oxygen (Murphy, 2005; Clack, 2002; see also The Devonian opportunity).
Actinopterygians also underwent an extraordinary radiation, giving rise to many
Their living descendants are
much more diverse and numerous than tetrapods and their relatives, having colonised almost every
aquatic environment of the biosphere, from oceanic abyssal plains to muddy wallows, and from desert pools to Antarctic seas.
Air breathing (or better 'bimodal breathing': Graham, 1997; see also Bimodal Respiration) is often considered one of the most important preadaptations to amphibious lifestyle.
Graham (1997) calculated that more than 400 extant actinopterygian species in 45 different families breathe air.
Air breathing independently evolved dozens of times in this group.
The structures involved are as different as guts, branchial chambers, skin, or respiratorial gas bladders (Graham, 1997; Graham & Lee, 2004).
Nonetheless, only a minority of these species is capable of complex amphibious behaviour.
The gobiid ancestor of all oxudercines was completely aquatic.
Even in the absence of preadaptations to terrestriality, during their evolution the oxudercines were subjected to selective pressures that led them to 'come ashore'.
Their air-breathing organs are not particularly derived, but the complexity of their amphibious behaviour is unrivalled among modern fishes.
Is it possible to find any evolutionary convergent feature in these two independent pathways
Are at least some of the selective pressures operating on mudskippers similar to those ones that acted on aquatic tetrapodomorph sarcopterygians and on semi-aquatic early tetrapods 380-350 million years ago?
Could mudskippers teach us something about some of the evolutionary opportunities and challenges that our tetrapod ancestors faced
right "at the water's edge" in the Late Devonian?
The selective pressures that led vertebrates to the water-air transition, as well as those environmental conditions in which
this gradual evolutionary process took place, are both highly debated and subject to much speculation.
Nonetheless, some studies suggest that this process took place in periodically flooded alluvial plains and/or in estuarine intertidal
wetlands, environments very similar to those experienced by mudskippers (Murphy, 2005; Schultze, 1999; Clack, 2002).
Furthermore, mudskippers may have remained in such environmental conditions throughout their evolutionary history
(see also Systematics & Biogeography ).
On the other hand, other scientists (Graham & Lee, 2004) agree for the most part with Romer's most widely accepted theory,
that the origin of tetrapods took place in tropical lowlands, seasonally dried up by chronical water shortages (Romer &
According to this theory, the first prototetrapods came out of water not to 'move onto land', but simply to avoid inhospitable aquatic
conditions, e.g. moving from a drying pool to a bigger one, not unlike several modern freshwater bimodal breathers.
Examples of evolutionary convergence. a., b. a dolphin (Delphinidae: a mammal) and a carcharhinid shark (Carcharhinus melanopterus: a cartilagineous fish): both have independently evolved a dorsal fin to increase stability during fast swimming.
c., d. a frogfish (Antennarius commersoni: a completely aquatic antennariid bony fish) and a frog (Platymantis vitiensis: a ranid amphibian): both have independently evolved different types of limbs for locomotion on solid substrates; environmental conditions seem to be completely different, but in both cases a firm grip to the substrate is needed (Clack, 2002).