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Historically, the bowfin and the gars were recognised as a group Holostei in apposition to the Teleostei. When first established, this division was motivated primarily by the nature of their scales: the heavy, solid scales of the holosteans contrasted with the thinner, lighter scales of the teleosts. Hence the name 'Holostei' meaning 'entirely bone': the holosteans have both a completely bony skeleton on the inside (as opposed to the partially cartilaginous skeletons of more basal fishes) and a complete covering of bony scales on the outside. However, the heavy scales of the Holostei are a primitive feature, indicating that the two lineages diverged before the evolution of the lighter teleost scales but not indicating a direct relationship with each other.
With the increasing emphasis on evolutionary relationships as the primary informer of classifications, a different system was proposed. This saw the gars as the most divergent lineage of the Neopterygii with the bowfin being united with the teleosts as a clade Halecostomi. This time, the primary evidence for this division was in how their jaws worked. The ancestral condition for vertebrate jaws has them working much as our own still do. The upper jaw, the maxilla, is largely fixed in place against the base of the neurocranium (the brain-holding bit) while the movement of opening and closing the mouth is achieved by the lower jaw, the mandible, pivoting around its hinge towards the back of the skull. In the bowfin and teleosts, however, the maxilla is hinged with the skull at its anterior end and with the mandible at the back. When the mouth opens, the maxila pivots downwards from this anterior hinge, dropping the mandible as a whole downwards. The bowfin and teleosts also possess a bone in the cheek, the interopercular bone, that is not found in other fishes; a muscle attached to this bone rotates the gill operculum as the mouth opens (Lauder 1980). Functionally, the expansion of the mouth cavity in this manner of opening the jaws creates a suction that pulls prey or other food into the fish's mouth.
Though it was by no means universally accepted, it is probably fair to say that the halecostomes vs gars picture of neopterygian evolution became the majority view. But then came the advent of molecular phylogenetic analysis, all ready and willing to cast the proverbial spanner. Rather than confirming halecostome monophyly, molecular analyses pointed the other way, back towards a clade of the bowfin and gars. Following this, a detailed study of gar systematics published by Grande (2010) also supported a gar plus bowfin monophylum on morphological grounds and resurrected the concept of Holostei (albeit redefined on phylogenetic grounds).
Gar jaws, it should be noted at this point, are a bit weird. Rather than being primarily composed of a single maxilla on each side, the upper jaws are made up of a series of tooth-bearing bones, each bone carrying just a few teeth, that have been dubbed the lacrimomaxillaries. When the jaws open, as well as the lower jaw opening in the standard manner, the flexible upper jaw also bends upwards. Rather than using suction to draw in their food like other neopterygians, gars capture prey by sneaking up to it then using a quick sideways jerk of the head to bring the open jaws around the prey (Lauder 1980). Gars were excluded from the Halecostomi on the basis of their lack of a long, mobile maxilla but, as explained by Grande (2010), a mobile maxilla is indeed present in gars but reduced to a remnant splint at the back of the jaw (in mature alligator gars Atractosteus spatula, the maxilla does not ossify). In very young juvenile gars, the mobile maxilla remains a significant part of the upper jaw with the lacrimomaxillaries being added in front of it as the jaw lengthens. As for the interopercular, this is genuinely absent in modern gars but it is present in close fossil relatives of gars such as semionotids. Rather than retaining a primitive jaw structure that was superseded in the bowfin and teleosts, it appears that gars evolved their own derived jaw structure from 'halecostome' ancestors.
Given that suction-assisted feeding is generally regarded as a major advance in fish evolution, how did gars end up abandoning it? That I can only speculate about. Is it related to the evolution of their elongate rostra? Long beaks are certainly a thing for a number of teleosts, but I don't know if any have a beak as long and robust as a gar's. Could it be that the greater precision of gars' snapping mode of feeding is an advantage in the low-oxygen, muck-filled waters in which gars thrive? Or could it be a side effect somehow of gars' more heavily armoured condition than other early-diverging neopterygians?
It's only fair to note that monophyly of Holostei is still not universally accepted; there are sill researchers who are inclined to think the bowfin closer to teleosts. But even if the 'Halecostomi' hypothesis was to rise once more to the surface, it would not be for the same reasons it did before.
REFERENCES
Grande, L. 2010. An empirical synthetic pattern study of gars (Lepisosteiformes) and closely related species, based mostly on skeletal anatomy. The resurrection of Holostei. Copeia 2010 (2A): iii–x, 1–871.
Lauder, G. V., Jr. 1980. Evolution of the feeding mechanism in primitive actinopterygian fishes: a functional anatomical analysis of Polypterus, Lepisosteus, and Amia. Journal of Morphology 163: 283–317.
The recent work by Lemberg et al. (2019) found that despite their highly modified jaw anatomy, gars actually do retain suction feeding capabilities, which they use in conjunction with lateral snapping. Even more recently, Lemberg et al. suggested a similar feeding mechanism for the Tiktaalik, interestingly enough.
ReplyDeleteI confess I sort of prefer this arrangement simply because Holostei sounds better than Halecostomi.
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