So far, we've travelled along the entire gnathostome stem within vertebrates, from our earliest chordate ancestors all the way to the living crown group gnathostomes. We briefly visited our chondricthyan cousins in the previous lesson, but for this lesson, we'll focus on the clade to which we belong, the bony fishes. You may not think of yourself as a fish, but taxonomically, we're all members of the Osteichthyes. You may be surprised by how many features you inherited from your aquatic ancestors. Osteichthyes means bony fishes. Which is a bit confusing considering we've been examining all kinds of fishes with bone up until this point. Most stem gnathostome groups had bone, but the vast majority of their bones were dermal. Some like the Osteostraci and placoderms also had perichondral bone, which is a thin layer of bone that forms around cartilage. Most of what we know of their internal anatomy comes from these perichondral ossifications. In the Osteichthyes, we see the addition of extensive endochondral bone, which is what we usually think of when we envision a bone. In lesson 1, we describe two types of bone. How does endochondral bone form? A, It replaces cartilage. B, It forms around cartilage. C, It's replaced by cartilage. Or D, it forms without cartilage. Most of our bones, especially our long bones like those in our limbs are endochondral. This type of bone starts off as a cartilage precursor which is replaced by bone, or ossified, during development. This means A is the correct answer. Endochondral means within cartilage, because this type of bone replaces the original cartilage skeletal element. Bone is deposited around the cartilage, but as replacement of the cartilage by bone continues, blood vessels invade the degenerating cartilage matrix allowing more bone cells to be deposited internally while other cells gradually dissolve the cartilage away. The cartilage ends up being completely replaced by a network of bone containing blood vessels and nerve canals. Endochondral bones are very spongy on the inside, and this spongy bone is surrounded by an outer rind of dense, hard, compact bone. This means the bones can remain very strong while being lighter and more flexible than they would be if they were dense bone all the way through. Osteichthyans have endochondral bones around the brain and in the girdles for fins or limbs. They also are the first group to have ossified vertebral centra, whereas chondrichthyans Have vertebral centra made from calcified cartilage. The endochondrial ossification is conserved in all bony fishes, tetrapods included. Aside from endochondrial bone, what makes and osteichthyan an osteichthyan? Osteichthyes is a well supported clade with several synapamorphies. Let's use a very famous early osteichthyan from Miguasha, to go through a few examples. This is <i>Eusthenopteron foordi</i>, one of the most well known Devonian osteichthyans. Swedish paleontilogist, Erik Jarvick, spent decades grinding away fossils of <i>Eusthenopteron</i> millimeter by millimeter to create beautifully detailed three dimensional re-constructions of the skull using thin wax plates. This detailed work means <i>Eusthenopteron</i> was incredibly well-known inside and out, long before the days of CT scanning and computer reconstructions. In the skull of <i>Eusthenopteron</i>, in the skull of all Osteichthyes, including ourselves, are complex assemblages of endochondral bones surrounded by dermal bone. The endochondral brain case of <i>Eusthenopteron</i> is covered over, both dorsally and ventrally, by dermal bones, many of which are homologous with our own skeletal bone. Dermal bone is the second type of bone we discussed in Lesson 1. How does it form? A, from cartilage. B, in the skin. C, around cartilage, and or D, without cartilage. More than one answer might be correct so check all that apply. Dermal bone forms without a cartilage precursor and forms in the skin. So B and D are both correct. The skull roof in <i>Eusthenopteron</i> is made up of a series of dermal bones that were inherited by tetrapods, including the parietal bones which make up a large part of our skulls. Dermal, tooth-bearing bones called the maxilla, premaxilla, and dentary form the margins of the mouth, covering the palatoquadrate and Meckel's cartilages. Osteichthyans also have a large plate of dermal bone that covers and protects the gill arches, called an operculum. In many osteichthyans, including <i>Eusthenopteron</i>, additional bones protect this area. These are the preopercular and subopercular. Osteichthyans have a well-developed pectoral girdle, which, like the skull, is composed of both endochondral and dermal elements. Their scapulocoracoids were endochondrally ossified. But osteichthyans also have a series of paired dermal bones in the pectoral girdle. The cleithrum is a dermal bone that connects to the scapulocoracoid. The dermal bones dorsal to the cleithrum, reaching up to attach to the back of the skull, are the supracleithrum and posttemporal bones. Ventral to the cleithra are the clavicles, and a single interclavicle between them. How do you think all these dermal bones affect the movement of the fish? A, They have no effect. B, they slow the fish down. C, they restrict the movement of the head. Or D, they restrict the movement of the pectoral fin. All these bones mean the pectoral girdle is very firmly connected to the back of the skull, so they restrict the movement of the head. This makes C the correct answer. Non-tetrapod bony fishes have no neck. If your pet goldfish wants to turn in another direction, it has to turn its whole body. Osteichthyes is a clade made up of two sister groups, the Actinopterygii and the Sarcopterygii, and the split between these two groups occurred very early after the evolution of bony fishes. And even the earliest fossil Osteichthyes are either actinopterygians or sarcopterygians. Both actinopterygians and sarcopterygians have fins supported by lepidotrichia, dermal fin rays. Their median fins are supported by long thin bones called radials. The most obvious difference between the fins of actinopterygians and sarcopterygians give these two groups their names. In the sarcopterygians, the lobe fins or fleshy fins, the bones supporting the fin rays and their associated muscles are on the outside of the body of the fish. In the actinopterygians, or ray fins, the paired fins are supported by radials that, along with their associated muscles, are embedded inside the bodies of the fish. This means all of their external fins are webs of skin supported by fin rays. Actinopterygians are incredibly diverse, comprising nearly 99% of all living fishes and occupying every aquatic environment. Most living ray fin fishes are extremely derived and specialized. But some of their primitive features can be seen in <i>Cheirolepis</i>, another fish from Miguasha. Actinopterygians like <i>Cheirolepis</i> have only one dorsal fin and their tails are heterocercal. Instead of an epicercal lobe of the tail, the part of the fin that is dorsal to the notochord, these primitive actinopterygians have rows of large scales. In later actinopterygians, the tail is modified and its external shape is symmetrical but the tail skeleton remains asymmetrical. In other words, heterocercal. Another synapomorphy of Osteichthyes may surprise you: the ancestors of all bony fishes had lungs. We don't usually think of fish as having lungs, but they do, or at least they did, and some still do. Lungs in fishes are outpocketings of the esophagus that will allow fishes to swallow air to supplement the oxygen they absorb through their gills. Most sarcopterygians retain their lungs. But, in almost all living actinopterygians, the lungs have been replaced by a swim bladder that is probably homologous to the lung. The swim bladder's main function is to help regulate the fish's buoyancy in the water column. The earliest branching lineage of ray fin fish, the Bicher <i>Polypterus</i>, is derived in many other ways, but it lacks several synapomorphies of more derived actinopterygians, like the modified upper jaw bones. It also retains many primitive characteristics, like lungs. Adult <i>Polypterus</i> can actually drown if they're not able to breathe air. They can survive out of water for quite some time. Actinopterygians also had heavy diamond shaped scales made of dentine and deep layers of acellular bone. The surface of the scales is covered with thick layers of a shiny, hard substance called ganoine, which is a form of enamel. In many living groups of actinopterygians, these scales have been modified to become lighter and some groups lose their scales entirely. But other groups, like polypterids and gars, retain their heavy, shiny ganoid scales. These scales preserve well, and so are extremely common fossils. Actinopterygians are arguably the most successful group of vertebrates. The oldest actinopterygian fossils are from the Late Silurian. They rapidly diversified towards the end of the Devonian and expanded into every aquatic environment in the world. They are a truly fascinating vertebrate success story. However, our own story is mostly concerned with the other group of osteichthyans, the sarcopterygians.
