Intermediate Forms Between Classes

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[edit] Definition

In this article we shall document intermediate forms between classes amongst the vertebrates; and intermediate forms between the invertebrates and the vertebrate group.

There are five classes in the group of vertebrates:

  • Fish (Pisces)
  • Amphibians (Amphibia)
  • Reptiles (Reptilia)
  • Birds (Aves)
  • Mammals (Mammalia)

Some people count fish as three classes: jawless fish (agnatha); cartilaginous fish (chondrichthyes); and bony fish (osteichthyes), so we shall have something to say about them, and in particular about the subclass of bony fishes known as lobe-finned fishes, from which amphibians are descended. The proposed evolutionary family tree is that invertebrates gave rise to jawless fishes; jawless fishes gave rise to cartilagenous fishes and bony fishes; bony fishes gave rise to amphibians; amphibians gave rise to reptiles; reptiles gave rise to mammals and birds. We shall therefore demonstrate the existence of the following intermediate forms:

  • Invertebrate-fish intermediates
  • Fish-amphibian intermediates
  • Amphibian-reptile intermediates
  • Reptile-bird intermediates
  • Reptile mammal intermediates

It is not the aim of this article to give the complete set of transitional fossils known; rather, we intend to list the defining differences in anatomy between the beginning and end of each transition, and to demonstrate that on these anatomical criteria there are indeed intermediate forms.

Anyone faced with creationist obfuscation about the significance of intermediate forms, or about the meaning of "intermediate forms" should consult the article defining Intermediate Forms. This article merely rebuts the commonest creationist claim of all concerning intermediate forms --- that there aren't any.


[edit] Note on the morphological tables

To demonstrate the intermediacy of intermediate forms, we have drawn up tables in the sections below cataloguing morphological traits for various forms and their intermediates. We should explain why the tables list the particular traits given and not others.

We have seen creationists complaining bitterly because in such tables features which seem --- to them --- to be important, such as, and I'm not joking, leg length and size of mouth. These are, of course, not morphological criteria for distinguishing vertebrate classes. Let the legs of Palaeothyris be longer than those of anthracosaurs --- or shorter, if the creationists wish it. It can have no possible bearing on whether Palaeothyris is an amphibian-reptile intermediate form.

So much for the differences we haven't tabulated. We have also not tabulated some important similarities. For example, we have not added to the dinosaur-to-bird table a row recording the fact that a dromaeosaur has a radius and an ulna in its forelimbs, and so does Archaeopteryx, and so do primitive birds, and so do modern birds.

Now these homologies of structure between limbs with different purposes is of course powerful evidence for evolution, but it is not in particular evidence for the fact that Archaeopteryx is a dinosaur-bird intermediate, because such structures exist in all tetrapods.

Therefore our tables show those features which distinguish one end of a transition from another; or those features of interest which are shared between the group at the beginning of the transition, the group at the end, the intermediates, and no other vertebrates.


[edit] Invertebrate-fish intermediates

We shall begin the history of the origin of vertebrates with the tunicates (or "sea squirts"). While adult tunicates are sessile (stay in one place) the infant form of the tunicates is free-swimming and has a number of chordate characteristics, including pharyngeal slits and arches, a dorsal hollow nerve cord, a notochord and a post-anal muscular (unsegmented) tail.

It is supposed, then, that the chordates evolved from tunicates through mutations causing the retention of these characteristics into adulthood (a process known as paedomorphism).

The fossil record is not kind to soft-bodied organisms, especially those of early date. An interesting demonstration of this is given by the conodonts. The hard feeding structures of conodonts (conodont structures) were first described in 1856, and it was presumed that they were the feeding parts of some animal which, being soft-bodied, had not been preserved. Although conodont structures are abundant in the early fossil record, no fossils with soft parts were discovered until 1983.

Despite the rarity of such fossils, species intermediate in form between tunicate larvae and fish can be found in the fossil record, as tabulated below.

We have omitted for now the primitive agnathids Myllokunmingia and Haikouichthys: these are each represented by incomplete specimens, and so we shall leave these off our chart until better fossils are found.

We may note that while there are gills throughout the series, at the start of it they are used for feeding, and at the end of it principally for gas exchange (respiration). Similarly, the endostyle at the start of the process is a thyroid gland by the end of it.

We continue the table through from the agnathans, which are considered vertebrates, to the osteolepiform (bony) fishes. If instead we had placed the cartilagenous fish (Sharks, skates and rays) in the right hand column of the table, the difference you would notice is that where primitive agnathans had a mixture of bone and cartilage in their skeletal structure, cartilagenous fish have reverted to an all-cartilage skeleton. It is hard to say why, exactly, the early agnathans had such heavy bony armour on their heads in the first place; it is not hard to see why a shark might be in no need of such weighty and cumbersome protection.


Larval tunicates Cephalocordates (eg Pikaia) Haikouella Conodonts (eg Clydagnathus) Agnathans Osteolepiforms
Postanal tail Yes Yes Yes Yes Yes Yes
Gills Yes Yes Yes Yes Yes Yes
Notochord and dorsal nerve cord In tail only Yes Yes Yes Yes Yes
Myotomes No Yes Yes Yes Yes Yes
Heart No Yes Yes Yes Yes Yes
Fins No No Yes Yes Yes Yes
Vertebrae No No No No Basidorsal and interdorsal elements (cartilagenous). All elements present.
Hard parts No No Pharangeal denticles for feeding Conodont structures for feeding Teeth; a range of braincases from partly closed and cartaliganous to fully enclosed and bony. Bony skeleton, teeth.
Eyes No Eye spots Eye spots Primitive eyes Primitive eyes "Fishlens eye"
Jaws No No Yes Yes No Yes
Paired fins No No No No No Yes
Lungs or swimbladder No No No No No Yes


The bony fish diverged into ray-finned and lobe-finned fishes, of which the most interesting to us will be the lobe-finned fish, since the tetrapods descended from this subclass.

We should mention on the route to the lobe-finned fishes the recent fossil discoveries Psarolepis and Achoania. Psarolepsis, amongst other primitive features, has paired pectoral spines in the same place that osteolepiforms have true pectoral fins; whereas the earlier agnathan fishes lacked any such structures. It is considered to be transitional between poroforms and true osteolepiforms. Achoania ia a primitive sarcopterygian fish which, as the name suggests, lacks the choana (airway between nostrils and mouth) that we find in more advanced lobe-finned fish such as Panderichthys and Eusthenopteron, and which otherwise fill the morphological gap between Psarolepis and higher sarcoptergyians.

Examples of primitive ray-finned fishes include such examples as Limnonis, Cheirolespsis, Mimia, Moythomasia, Redfieldius, Platysomus and Cheirodus. Their most obvious primitive feature is the continuation of the spine into the top margin of the caudal fin, which is highly asymmetric, a feature not found in modern bony fish.

The record for early cartilagenous fish is worse even than that for conodonts, and for the same reason: a cartilagenous fish, by definition, has no hard parts but its teeth. About a hundred million years separate the first sharks' teeth in the fossil record from the first soft-bodied preservation.


References:


[edit] Fish-amphibian intermediates

The amphibians are descended from lobe-finned fish, the fins of which (uniquely among fish) have a central appendage in their fins containing many bones and muscles: the structure from which legs evolved.

In fishes such as Eusthenopteron and Panderichthys we see lungs, and also a choana --- an airway connecting the nostrils and the mouth. They have, in short, adaptations for breathing air. In Eusthenopteron the dorsal and anal fins are still present, while in Panderichthys these are gone. The bones of the pectoral and pelvic fins (corresponding to the arms and legs) fit the tetrapod pattern of but humerus, ulna and radius in the forelimb and femur, tibia and fibula in the hindlimb; this is true of no other fish outside the osteolepiform fish. In their cranial anatomy they are so similar to tetrapods that the first fossils found, which didn't have the fins preserved, were identified as tetrapods.

Later, in creatures such as Ichthyostega and Acanthostega, we see the pectoral and pelvic fins converted to very primitive legs; the joints were not solid, nor could the toes flex independently, and the legs couldn't bear the weight of the animals, so that the must have moved on land by dragging themselves along (they have scales on their bellies, which seems to confirm this). Besides these rudimentary limbs and a few adaptations to half-walking, they resemble the osteolepiform fish, and fish in general, in a number of archaic features of anatomy not found in modern amphibians.

The table below sets out some of the important anatomical features involved.


Non-rhipidistian fish Panderichthys Acanthostega Modern amphibians
Dorsal and anal fins Yes In some species No No
Choana No Yes Yes Yes
Lungs, external nostrils No Yes Yes Yes
Gills Yes Yes Internal gills No
Sacral rib No No Yes Yes
Legs No No Yes, non-weight-bearing. Weightbearing limbs
Digits Many bony rays in fins Many bony rays in fins Free digits, 8 at front, 7 at back 5 digits
Pectoral girdle Part of skull Part of skull Attached to skull Separate from skull
Caudal fin Yes Yes Yes No
Lateral line Yes Yes Yes No
Teeth Fishlike Fishlike Fishlike Amphibian
Notochord enters braincase Yes Yes Yes No
Opercular chamberYesYesYesNo


Besides the forms intermediate between amphibians and reptiles, which will be covered in the next section, we might draw the reader's attention to Triadobatrachus, a transitional form between ancient labyrynthodont amphibians and modern frogs.


References:


[edit] Amphibian-reptile intermediates

The most important change from amphibian to mammal is the development of the aminotic egg. It is not hard to see how this developed; there is a smooth gradient in nature between being completely submersed in water and being bone-dry --- for example, the eggs of turtles, which are leathery rather than hard-shelled, are laid in damp sand at the shoreline. It is trivial to observe that if eggs are at risk of drying out, characteristics which make them less likely to do so will, other things equal, be selected for.

The changes in the bony anatomy during the same period reflect the necessities of a life spent more on land (hence the fusion of bones to produce the astralagus, for example) and less time in the water (hence the loss of the lateral line). Some of these changes are tabulated below.


Early amphibians Anthracosaurs (eg Proterogyrinus) Palaeothyris Modern reptiles
Vertebrae Rachitomous Gastrocentrous Gastrocentrous Gastrocentrous
Contact between dental and parietal No Yes Yes Yes
Intertemporal Yes Yes No No
Otic notch Yes Yes No No
Mesotarsal No No Yes Yes
Astralagus No No Yes Yes
Lateral line Yes Yes No No
Sarcopterygian skull hinge Yes Yes No No
Sphenethmoid Yes Yes No No
Sacral vertebrae 1 1 2 2-3
Stapes Massive Massive Slender Slender
Teeth Labyrinthodont Labyrinthodont Intermediate Reptilian
Pineal opening Yes Yes Yes No
Eardrum No No No Yes


References:


[edit] Dinosaur-bird intermediates

Birds evolved from a group of small dinosaurs called coelurosaurs. The resemblances between coelurosaurs and modern birds is closer than you might think. In addition to the resemblances (themselves numerous) which birds share with all other tetrapods, both have a bipedal stance with their legs directly under their bodies; both have a hinged ankle joint; both have wishbones; both have bird-like lungs; both have scales (on the legs of modern birds); it is almost certain from their anatomy, which is built for speed, that coelurosaurs must have had fast reflexes and warm blood (endothermy). What little we know about dinosaurs suggests that they had nesting habits like birds --- though not in trees! --- brooding on their eggs, nesting in colonies, and possibly caring for their young after birth. Moreover, some dinosaurs, such as Caudipteryx had feathers. We may note that no such dinosaurs were known when the dinosaur-bird link was first proposed: these recent finds are a startling verification of the theory.

So much for the major similarities between birds and dinosaurs. To pick out some of the major differences: dinosaurs have no wings; they have a bony jaw with teeth instead of a modern bird's edentate beak; dinosaurs have a long tail of unfused vertebrae whereas in modern birds the distal caudal vertebrae are fused into a stubby structure known as a pygostyle; in dromaeosaurs the skull attaches at the back of the skull; in birds it attaches at the base; a modern bird's foot has a reversed hallux (big toe); a coelurosaur has three fully independent digits on its forelimb, whereas a bird does not. These are the chief criteria by which a birdwatcher might distinguish a feathered dinosaur such as Caudipteryx from a flightless modern bird. An anatomist would have rather more detail at his disposal, and many of the more subtle distinguishing anatomical features are listed in the table below. One feature of particular interest is the semilunate carpal, an anatomical feature found exclusively in manitoraptors and in extinct birds, and in no other species: in particular, in no living bird.

Creationists are fond of denying that there are primitive birds. As the table below shows, there are primitive birds: the confuciusornithids are primitive birds. They are also fond of denying in particular that Archaeopteryx is a primitive bird. We concur, but for reasons different than theirs: Archaeopteryx is not a primitive bird, it is a winged dinosaur, as the table below will demonstrate.

We have given the confuciusornithids as an example of primitive birds. Confuciusornithids are enantiorthines, a sister group of the group which includes all modern birds, the ornithurines. For an example of an early ornithurine, see the articles on Hesperornis below.


CoelurosaursArchaeopteryxPrimitive birds (eg confuciusornithids)Modern birds
Bipedal gaitYesYesYesYes
Bird-like lungsYesYesYesYes
WishboneIn some speciesYesYesYes
FeathersIn some speciesYesYesYes
WingsNoYesYesYes
Reversed halluxNoProbablyYesYes
Neck attaches at base of skullNoNoYesYes
PygostyleNoNoYesYes
Loss of abdominal ribsNoNoYesYes
Keeled sternumNoNoIn some speciesYes
EdentateNoNoNearlyYes
BeakNoNoYesYes
Clawless forelimbsNoNoNoYes *
No semilunate carpalNoNoNoYes
AlulaNoNoNoYes
Triosseal canalNoNoNoYes
Fused ischium and iliumNoNoNoYes
No "foot" to the pubisNoNoNoYes

Footnote: * Modern birds have claws on their wings when they are embryos ("Intelligent Design", hey?) and some have claws on their forelimbs during infancy: just as human embryos have tails, and juvenile amphibians have gills. These claws are lost in adulthood.


Meanwhile, here is what creationists say about primitive birds:

"We see all the complex structures of birds appearing suddenly in Archaeopteryx. There are no feathered "primitive birds."" [1]

"All the complex structures" evidently excludes the beak, the pygostyle, the keel on the sternum, the reversed hallux, the fused pubis, the alula, the triosseal canal, lots of extra sacral vertebrae, etc. If the creationists have leant nothing from Archaeopteryx, and they haven't, they seem to have learned a great deal from the ostrich.

Compsognathus, Archaeopteryx, Confuciusornis, and a chicken. Images are not necessarily to scale.
Compsognathus, Archaeopteryx, Confuciusornis, and a chicken. Images are not necessarily to scale.

References:


[edit] Reptile-mammal intermediates

The reptile-to-mammal transition is amongst the most beautifully documented in the fossil record. The similarities between reptiles and mammals are, we take it, self-evident. Some of the most significant differences between them, such as warm blood and suckling of young, are not, of course, preserved in the fossil record. However, there are many anatomical criteria in the bony anatomy which distinguish modern mammals from the reptiles, including a bony secondary palate, cusped and differentiated teeth which occlude (that is, fit together neatly), in place of reptiles' identical coniform teeth; the lateral temporal fenestrae; and the loss of the lumbar ribs. Most interesting is the simplification of the jaw to two articulated bones, and the progressive use of the "spare" jawbones as part of the auditory apparatus, leading to the development of the modern mammalian ear.

For a closer look at the transitions involved, the table in the first reference below gives a sample of thirteen genera in a morphological and temporal sequence from the protorothyrids to the modern mammals. The table below uses slightly more morphological criteria, but due to considerations of space we have confined ourselves to picking out, between reptiles and mammals, those beasts which we think come closest, anatomically, to the halfway mark between the two anatomical extremes.


Reptiles Procynosuchids (eg Procynosuchus) Epicynodonts (eg Thrinaxodon) Early mammals (eg Morganucodon) Modern mammals
Lateral temporal fenestrae Absent Present Present Merged with eyesocket Merged with eyesocket
Secondary palate No Yes, partly ossified Yes, fully ossified Yes, fully ossified Yes, fully ossified
Multicuspate teeth No Yes, but they don't occlude Yes, but they don't occlude Yes, with some occlusion Yes, occluding
Differentiated teeth No Canines differentiated Canines differentiated Canines, molars, premolars differentiated Canines, molars, premolars
Dentary bone 65% of jaw length 75-80% of jaw length 85% of jaw length 100% of jaw length 100% of jaw length
Post-dentary bones 70% of jaw length 40% of jaw length 30% of jaw length 20% of jaw length No post-dentary bones
Lumbar ribs lost No No Yes Yes Yes
Joints in jaw Quadrate-arcticular Quadrate-arcticularQuadrate-arcticularQuadrate-arcticular and squamosal-dentary Squamosal-dentary
Ear bones Stapes Stapes Stapes (with articular and quadrate conducting sound) Stapes (with articular and quadrate conducting sound) Stapes, incus, malleus


References:


[edit] Quotations

"So many intermediate forms have been discovered between fish and amphibians, between amphibians and reptiles, between reptiles and mammals, and along the primate lines of descent that it often is difficult to identify categorically when the transition occurs from one to another particular species. Actually, nearly all fossils can be regarded as intermediates in some sense; they are life forms that come between the forms that preceded them and those that followed." (National Academy of Sciences: Science and Creationism, page 21. National Academy Press, 1999)


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