Osteichthyes

Taxa on This Page

1. Osteichthyes

Fish Bones

The Devonian is frequently called, with ample reason, the "Age of Fishes." The stage for this explosion of aquatic vertebrates was undoubtedly set in the Silurian, perhaps due to the duplication of ancestral hox genes. But, whatever the cause, the earliest Devonian saw the rapid development of chondrichthyans, placoderms, acanthodians, and the two most successful groups of vertebrates ever known: the actinopterygians and sarcopterygians -- collectively, the bony fish, the Osteichthyes. Much has been written on the unique adaptations of the osteichthyans. Some of these points are summarized in the main entry in these Notes. However, what everyone remembers is that osteichthyans were the first group with endochondral bone.

In the more general vertebrates there are various types of calcified tissues: dentine, enamel (or "enameloids" to be precise) and bone, plus variants, characterized by their ontogeny, chemistry, form and location. However, endochondral bone is said to be unique because it begins life as cartilage. In more basal vertebrates, cartilaginous structures can become superficially calcified in at least two different modes. Chondrichthyans develop microscopic bone platelets which cover the surface of the cartilage to create prismatic calcified cartilage. All gnathostomes and probably some more basal groups can also produce layers of bone over the surface of cartilage to create dense lamellar or perichondral bone. However, the bone-forming osteoblasts are eventually entombed in their own work product, cut off from all living connections, and drowned in their own metabolic wastes -- rather like post-modern philosophers.

In osteichthyans, the circulatory system actually invades the cartilaginous matrix. This permits the local osteoblasts to continue bone formation within the cartilage and also recruits additional, circulating osteoblasts (who have, presumably, been cruising around looking for some action). Other cells gradually eat away at the surrounding cartilage. The net result is that the cartilage is replaced from within by a somewhat irregular vascularized network of bone. Structurally, the effect is to create a relatively lightweight, flexible, "spongy" bone interior, surrounded by an outline of dense, lamellar periostial bone (since this bone now surrounds other bone, rather than cartilage, it is referred to as periostial rather than perichondral). Developmentally, in a sort of teleological, nineteenth century way, the process is viewed as "pre-forming" bone in cartilage, since the effective result is that a structure created in cartilage is replaced by the same structure composed of bone. This is the unique endochondral bone from which the osteichthyans derived their name, as well as countless structural advantages.

The problem here is that, like a lot of things, it ain't necessarily so. That is, endochondral bone is not really unique to the osteichthyans. Endochondral bone is found, for example, in the rhinocapsular ossifications of some arthrodire placoderms. Janvier (1996). In the Chondrichthyes there may be a long-term evolutionary trend to incorporate small, but increasing amounts of endochondral bone. Maisey 1988). There have even been reports of endochondral bone in acanthodians, although it seems that these are now discredited. Smith & Hall (1990). Smith & Hall, as well as Janvier, tend to dismiss these occurrences as not really comparable to the wholesale use of endochondral bone in osteichthyans, and certainly there is a significant quantitative difference. However, it would seem that Maisey has the better argument and that endochondral bone is plesiomorphic, a generalized gnathostome character.

Second, it is not clear that endochondral bone was much of an advantage. If one ignores the aberrant tetrapods, the indiscriminate internal calcification of cartilage may be viewed as a relatively short-lived experiment in evolving a fish strong enough to carry around massive armored scales while maintaining a relatively high degree of mobility. The actinopterygians quickly jettisoned their ganoid scales and thus dispensed with the need for most endochondral bone. Sarcopterygians retained their heavy cosmine scales, and endochondral bone, but -- except for the tetrapods -- became virtually extinct in the process.

Note added in disproof:  An excellent textbook writer, who presumably would not wish to be quoted here by name, has kindly suggested that we go back to the drawing board on this essay. APW 040806.

Descriptions

Osteichthyes: Andreolepis 

Image: Modified from Zhu et al. (1999).

Note: Zhu et al.'s (1999) description of Psarolepis from the Upper Silurian and Lower Devonian of China threatens to create chaos out the conventional, orderly treatment of this group. Psarolepis shares a number of characteristics previously believed to be unique to actinopterygians or sarcopterygians. In addition, it has several features, such as the fin spines and characters of the shoulder plate, which are associated with placoderms, chondrichthyans, or acanthodians. In overall appearance, Psarolepis most resembles a sarcopterygian, but the plates in isolation look strikingly like placoderm material. Depending on the data set used, PAUP suggests that it is either (a) the sister group of all (other) osteichthyans or (b) part of a paraphyletic Actinopterygii and sister of all (other) Sarcopterygii. The many strikingly primitive features and the pattern of cranial dermal bones probably favor (a), but these are admittedly a poor basis on which to make major phylogenetic decisions. We have elected to follow current fashion and place Psarolepis among the Sarcopterygii, as the sister of all other sarcopterygians.