Any practical use for Universal Common Ancestor?

1. THE THERAPSID--MAMMAL TRANSITIONAL SERIES
   ​

   quote:

   ---

   Not only is this explanation not "merely wishful conjecture", but it can be clearly seen in a remarkable series of fossils from the Triassic therapsids. The earliest therapsids show the typical reptilian type of jaw joint, with the articular bone in the jaw firmly attached to the quadrate bone in the skull. In later fossils from the same group, however, the quadrate-articular bones have become smaller, and the dentary and squamosal bones have become larger and moved closer together. This trend reaches its apex in a group of therapsids known as cynodonts, of which the genus Probainognathus is a representative. Probainognathus possessed characteristics of both reptile and mammal, and this transitional aspect was shown most clearly by the fact that it had TWO jaw joints--one reptilian, one mammalian:

   ​

   "Probainognathus, a small cynodont reptile from the Triassic sediments of Argentina, shows characters in the skull and jaws far advanced toward the mammalian condition. Thus it had teeth differentiated into incisors, a canine and postcanines, a double occipital condyle and a well-developed secondary palate, all features typical of the mammals, but most significantly the articulation between the skull and the lower jaw was on the very threshhold between the reptilian and mammalian condition. The two bones forming the articulation between skull and mandible in the reptiles, the quadrate and articular respectively, were still present but were very small, and loosely joined to the bones that constituted the mammalian joint . . . Therefore in Probainognathus there was a double articulation between skull and jaw, and of particular interest, the quadrate bone, so small and so loosely joined to the squamosal, was intimately articulated with the stapes bone of the middle ear. It quite obviously was well on its way towards being the incus bone of the three-bone complex that characterizes the mammalian middle ear." (Colbert and Morales, 1991, pp. 228-229)

   ​

   In a slightly later group, known as the ictidosaurians, the mammalian part of the double jaw joint seen in Probainognathus was strengthened, while the old reptilian part was beginning to become reduced in size. In describing a member of this group known as Diarthrognathus, paleontologists Colbert and Morales point out: "The most interesting and fascinating point in the morphology of the ictidosaurians (at least, as seen in Diarthrognathus) was the double jaw articulation. In this animal, not only was the ancient reptilian joint between a reduced quadrate and articular still present, but also the new mammalian joint between the squamosal and dentary bones had come into functional being. Thus, Diarthrognathus was truly at the dividing line between reptile and mammal in so far as this important diagnostic feature is concerned." (Colbert and Morales, 1991, p. 128)

   ​

   The therapsid-mammal transition was completed with the appearence of the Morganucodonts in the late Triassic. This group is described by paleontologist T.S. Kemp:

   ​

   "The axes of the two jaw hinges, dentary-squamosal and articular-quadrate, coincide along a lateral-medial line, and therefore the double jaw articulation of the most advanced cynodonts is still present . . . The secondary dentary-squamosal jaw hinge had enlarged (in the Morganucodonts) and took a greater proportion if not all of the stresses at the jaw articulation. The articular-quadrate hinge was free to function solely in sound conduction." (Strahler, 1987, p. 419)

   ​

   Thus, the fossil record demonstrates, during the transition from therapsid reptile to mammal, various bones in the skull slowly migrated together to form a second functional jaw joint, and the now-superfluous original jaw bones were reduced in size until they formed the three bones in the mammalian middle ear. The reptilian quadrate bone became the mammalian incus, while the articular bone became the malleus. The entire process had taken nearly the whole length of the Triassic period to complete, a time span of approximately 40 million years. Since the determining characteristic of a mammal in the fossil record is the structure of the jaw bone and joint, all of the therapsids up to the Morganucodonts are classified as reptiles, and all those after that are considered to be mammals. As Romer puts it, "We arbitrarily group the therapsids as reptiles (we have to draw a line somewhere) but were they alive, a typical therapsid probably would seem to us an odd cross between a lizard and a dog, a transitional type between the two great groups of backboned animals." (Romer, 1967, p. 227)

   ---

   ​
2. Therapsida
   ​

   quote:

   ---

   Synapsida
   
   Therapsida
   —Raranimus
   Biarmosuchia
   Dinocephalia
    —Anteosauria
    Tapinocephalia
   Anomodontia
    —Venyukovioidea
    Dicynodontia
   Theriodontia
   —Gorgonopsia
   Therocephalia
   Cynodontia

   ​

   The Therapsids were one of the great success stories of the Permo-Triassic. First appearing in the middle or even the early (if Tetraceratops is a member of this group) Permian, they very quickly dominated terrestrial and semi-aquatic environments, filling a number of ecological niches and guilds, including felid, canid, bear, otter, ungulate, and even mole analogues, as well as many forms with no contemporary counterparts. In keeping with the episodic nature of synapsid evolution, there seem to have been at least two distinct dynasties, a middle Permian fauna dominated by dinocephalians and other primitive forms, and a late Permian fauna characterised by a wide range of more advanced carnivorous and herbivorous groups. These animals were so abundant (especially the herbivorous yet bizaarely specialised dicynodonts with their toothless beaks) that one could easily refer to this period as the age of therapsids, with both anapsid reptiles and relict pelycosaurs playing second fiddle. Their evolutionary success was unfortunately cut short by the end Permian extinction event, and although a number of lineages made it through to the Triassic, their protomammalian metabolism put them at a disadvantage in a hot dry Triassic world far more suited to sauropsid reptiles, and increasingly dominated by thecodontian archosaurs. They survived by becoming progressively smaller and more mammal-like, except for the dicynodonts which if anything became larger. By the time the dinosaurs had appeared, the therapsids had given rise to the first mammals, although one lineage of rodent-like, non-mammalian therapsids, the tritylodonts, would continue to the Middle Cretaceous. MAK120127

   ---

quote:

---

Therapsida

Cynodontia
—Procynosuchidae
Galesauridae
Eucynodontia
—Cynognathia
 —Cynognathidae
 Tritylodontidae
Probainognathia
—Tritheledontidae
Mammaliaformes

​

In the unbroken evolutionary sequence from reptile to mammal, the cynodonts are intermediate between earlier and more primitive theriodont therapsids and the earliest mammals. These highly successful animals first appear in the late Permian, radiate quickly into a number of different forms (including both terrestrial and semi-aquatic) before the end of the period, reach their maximum diversity in the Early Triassic, and become increasingly mammal-like as the Triassic progresses, giving rise to true mammals in the later Triassic. By the start of the Jurassic, only the insectivore-like tritheledonts and rodent-like tritylodonts remained; the latter continuing alongside true mammals throughout the Jurassic and even into the early Cretaceous.

​

In cladistic nomenclature, the term "cynodont" is also used to include mammals, which evolved from cynodonts and hence are, phylogenetically speaking, derived cynodonts. MAK120206

---

quote:

---

Cynodontia

Mammaliformes
—Allotheria
 —Haramiyida
 Multituberculata
   —Paulchoffatiidae
   Gondwanatheria
     Cimolodonta
Morganucodontidae
Docodonta
 —Megazostrodontidae
 Docodontidae
Hadrocodium
Symmetrodonta
—Kuehneotheriidae

Mammalia

​

All of the early mammaliforms looked more or less like rodents and were about the same size as that most successful group of modern mammals. The mammaliform story is about internal, structural developments, many of which we have only begun to be able to study in the last decade. Understanding these changes unavoidably requires us to look at technical anatomical details. Some of the things to look for are these:

​

1) The dentary-squamosal jaw joint: All terrestrial vertebrates except mammaliforms form the jaw hinge between the articular and the quadrate. Mammaliforms form the joint between the dentary and the squamosal. This transition was well under way before mammaliforms got started.

​

2) The post dentary bones -- the articular and angular -- weren't lost. They, and the quadrate, became incorporated into the middle ear as the auditory ossicles. In mammals they are called the malleus, incus and stapes.

​

3) The inner ear was also re-engineered. The otic capsule became somewhat separated from the rest of the braincase as a pair of petrosals. Within the petrosal, one of the organs of hearing (the maculae) became first enlarged, and then coiled.

​

4) The teeth of mammals are almost unique in a number of respects. Instead of having lots of simple teeth that were replaced frequently, mammals have only one set of adult teeth which meet (occlude) in a very precise fashion.

​

5) Most importantly, mammals developed separate, specialized molars. Molars not only have points (cusps) which shear past each other, but have a certain cusps which grind food on relatively flat regions of the opposite molar. This system seems to have evolved separately at least three times and probably more often. Since most mammaliform remains consist of teeth, we have a great many examples of tooth forms. Unfortunately, the degree of convergent dental evolution has also greatly confused the picture.

​

6) The brain itself, and the surrounding bone, seems to have undergone some profound reorganization. After hundreds of millions of years of gradual retreat, the anterior part of the old palatoquadrate (the epipterygoid of reptiles) makes a strong come-back in mammaliforms in vatious guises such as the alisphenoid and orbitosphenoid -- structures that provide central support for the anterior skull.

​

7) these changes in the organization of the skull are accompanied by soft tissue changes in the distribution of the cranial nerves and the major blood vessels. These transitions remain poorly understood.

​

8) Post-cranially, as in reptiles, the limbs tended to move under the body. Surprisingly, mammals seem to have been very slow to acquire a truly erect stance; and the fore- and hind-limbs seemed to have evolved erect postures almost independently.

​

Of course, a great many other things were going on at the same time: lactation, increased body metabolism, body hair, and so on. But these are almost impossible to study directly in the fossil record. They remain, for the most part, matters of speculation.

---

quote:

---

Mammaliformes

Mammalia
—Australosphenida
 —Ausktribosphenidae
 Monotremata
Triconodonta
Spalacotheroidea
Cladotheria
—Dryolestoidea
Theria

—Metatheria
Eutheria

1. Metabolic rate: transition to more or less full homeothermy inferred from geographic range, nocturnal habit, etc.
2. Temporal fossa: increase in size; confluence with orbit.
3. Zygomatic Arch: development; replacement of jaw adductor by masseter as principal jaw muscle; greatly increased capacity for oral processing of food.
4. Reflected lamina of angularLower jaw: dentary becomes only significant element of mandible; development of coronoid process; reduction of post-dentary elements; reflected lamina of angular; dentary-squamosal jaw articulation. See infra, ear.
5. Dentition: development of heterodont dentition with incisors, canines, pre-molars and molars; "permanent" (diphyodont) teeth with prismatic enamel; increasingly fixed pattern of occlusion; definite dental formula.
6. Palate: full secondary palate.
7. Ear: reflected lamina of the angular (tympanic); retroarticular process of articular; conversion of post-dentary bones to sensory use in middle ear. Reflected lamina may have been resonating chamber, followed by development of tympanic membrane framed by increasingly small and gracile reflected lamina and/or retroarticular process
8. Pineal foramen: pineal foramen initially becomes more conspicuous, then recedes and is lost.
9. Skull table: development of parietal crest with muscular attachment on outside of dermal bones.
10. Braincase: parietal and squamosal spread downward to cover braincase, gradually replacing (in advanced mammals) neurocranium while providing muscle attachment on lateral (formerly dorsal) surface; epipterygoid changes from pillar supporting parietal and braincase to alisphenoid element of skull, continuous with parietal, squamosal, & petrosal (fused otic capsule). Brain size does not increase relative to diapsids.
11. Skull fusions: fusion of parietals and frontals, otic capsule, occipitals, numerous other elements in therians; loss of dermal bones, e.g. post-orbitals.
12. Skull attachment: double occipital condyle, condyles move ventrally, development of mammalian circular form.
13. Spine: loss of lumbar ribs (reversed in advanced cynodonts & lost again in Mammalia); increase in number of sacral vertebrae (??); reduction of tail; vertebral articulations changed to accommodate dorsoventral undulation; vertebrae amphiplatyan (flat on both ends), implying loss of notochord remnants (?).
14. Limb girdles: reduction (e.g. pubes, coracoids) or loss of ventral elements; more vertical orientation of limbs.
15. Limbs: more vertical orientation; elongation of humerus & femur; digits shorter; calcaneal heel
16. Integument: fur?; mammalian muzzle
17. Habit: primitively large carnivores; great reduction in size; development of omnivorous and herbivorous adaptations. --ATW 001202

---