Is Morganucodon a transitional fossil?
Evolutionists claim to have good fossil evidence for the transition of synapsids to modern mammals. This includes the key evolutionary transition from creatures with one ear ossicle and a quadrate-articular jaw joint to mammals with three ear ossicles and a dentary-squamosal jaw joint. The evolution of the mammalian jaw joint and middle ear is said to be well-documented and includes Morganucodon, which is believed to show a key step in this transition. This review re-examines Morganucodon and the claim that it had a transitional double jaw joint and a mandibular middle ear. This claim is essentially guesswork, driven by a prior commitment to evolutionary thinking and is not well supported by the evidence. An alternative view of the evidence is that Morganucodon was a mammal and not transitional.
Evolutionists tell us that mammals evolved from therapsids (which were a type of synapsid) living about 200 million years ago. They claim to have fossil evidence from Mesozoic rocks, showing the evolutionary transition from creatures with one ear ossicle and a quadrate-articular jaw joint to mammals with three ear ossicles and a dentary-squamosal jaw joint. The evolution of the mammalian jaw and middle ear is said to be well-documented and important as a demonstration of transitional forms and exaptation.
Scientists who are not committed to the theory of evolution as an explanation of mammal origins have shown that there is no convincing evolutionary account for the origin of mammals. Bill Mehlert, writing in 1993, reviewed the available evidence and identified many problems which are still relevant today.1 Eight years later John Woodmorappe published an analysis of the character traits of ‘mammal-like reptiles’ (now referred to as non-mammalian synapsids) and fossil mammals. He showed that instead of gradual evolutionary progression, there are major discontinuities and reversals which seriously undermine the theory that mammals evolved from non-mammals.2 Evidence from development is also consistent with creation biology.3
This article looks again at Morganucodon (figure 1), which was claimed to have a double jaw joint.4 This was purported to be a key step in the evolution of mammals and in particular the evolution of the jaw and middle ear bones, which are some of the defining characteristics of mammals.
What is a mammal?
A mammal has (among other things) a single jaw bone (the dentary), a dentary-squamosal jaw joint and three ear bones (the malleus, incus, and stapes) which transmit sound from the eardrum, as shown diagrammatically in figure 2. Reptiles, on the other hand, have a jaw made up of several bones, a jaw joint formed by the articular and quadrate bones and only one ear bone, the columella (sometimes called the stapes), transmitting sound from the eardrum.
Evolutionists believe that mammals evolved from reptilelike therapsid ancestors, which had a jaw similar to reptiles. The ear bones are believed to have been part of the jaw (the mandibular ear) which were connected to a stapes.5,6 The current evolutionary theory is that the dentary bone increased in size and formed a secondary jaw joint with the squamosal bone. At the same time the two jaw bones involved in hearing (articular and quadrate) are believed to have separated from the jaw and become the malleus and the incus, respectively, of living mammals. Simultaneously, an eardrum evolved as a new sound receiver. However, there are problems with the theory that the mammalian middle ear structures which developed at the angle of the lower jaw were transferred to the basicranium, and there is no clear consensus on the origin and evolution of the mammalian eardrum.5
Morganucodon (‘Glamorgan tooth’) is a mammalian genus which supposedly lived during the Late Triassic. According to evolutionists it first appeared about 205 million years ago. It was small, with a skull only 2–3 cm in length and is believed to have looked like a shrew or mouse.
Morganucodon is represented by abundant and well preserved, though in the vast majority of cases, disarticulated, material. Most of this came from Glamorgan, in Wales (Morganucodon watsoni), but fossils have also been found in the Yunnan province in China (Morganucodon oehleri).
The skull of Morganucodon
A detailed description of the skull published in 19814 is a key publication which is widely quoted. Diagrams from this paper are repeatedly redrawn as evidence of a transitional stage in the evolution of the mammalian jaw and ear bones. However, it should be noted that the skull was damaged, and the jaw joint and ear bones were not attached.
The jaw joint of Morganucodon
Morganucodon is claimed to have had a double jaw joint; that is, both a dentary-squamosal (mammalian) joint and a quadrate-articular (reptilian) joint, which were side by side. However, key parts were missing or disarticulated. The complete lower jaw (dentary) was missing from this specimen and only partial dentary bones have been found. Furthermore, the squamosal, quadrate and articular were not attached, a complete squamosal bone has never been found and all were damaged. Therefore, key parts of the double jaw joint were inferred. The supposed double jaw joint is shown in figure 3, which is based on figure 91 from reference 4. Only part of the stapes (yellow) was recovered. The quadratojugal (pink) was never recovered. In addition, the authors write that “the correct orientation of the squamosal was difficult”, which makes their interpretation of the dentary-squamosal joint somewhat subjective.
The quadratojugal bone having never been found presents a very serious problem. Yet the authors stated that “its existence is certain from the presence of the facets on the quadrate and the squamosal with which it articulated” [emphasis added]. They go on to write that “The quadratojugal formed the main articulation between the quadrate complex and the squamosal.” It is noteworthy that later publications, which include diagrams redrawn from the publication, simply omit the quadratojugal, while at the same time maintaining that Morganucodon had a double jaw joint. The fact that the reconstruction was based on the belief in the existence of the missing quadratojugal and that it linked the quadrate and squamosal adds to the doubts about the position of the squamosal and seriously undermines the idea of the double jaw joint. If only the quadrate is involved in the proposed joint, this requires a more robust articulation between the quadrate and the articular, which would have limited its movement and consequently reduced the effectiveness as part of the middle ear. However, there is a more serious problem because the quadrate is not large enough to fill the gap between the stapes and the squamosal. Neither can it completely articulate with the articular, which reportedly has two facets, one for the quadrate and another for the quadratojugal. Simply increasing the length of the stapes doesn’t help either, because the dorsal lamina of the quadrate has to fit into the quadrate notch of the squamosal. Not to mention that this would require a rather long stapes, which would be unusual. The question of what the second facet on the articular connects with remains unanswered. All of these problems with the joint between the quadrate and articular are good reasons to conclude that Morganucodon had a mammalian jaw joint between the dentary and squamosal alone and calls into question the nature of the quadrate.
The middle ear of Morganucodon
The questions raised above also have an impact on the claim that Morganucodon had a mandibular middle ear involving the quadrate, which functioned simultaneously as part of the jaw joint and middle ear. Almost all of the middle ear is actually inferred. As mentioned above, the stapes was incomplete as only the footplate has been recovered. Therefore, the length is unknown and the shape of the joint between the stapes and quadrate (q) is unknown. Furthermore, since it is now believed that Morganucodon did not have a quadratojugal, the reconstruction is invalid. The initial reconstruction shows a joint between the articular and the quadrate plus the quadratojugal, potentially allowing some freedom of movement for the quadrate as part of an acoustic transformer, but this is now known not to be the case.
Another feature of the reconstructed mandibular middle ear of Morganucodon is that the tympanic membrane has been placed in the mandible. But once again this was inferred, because the supporting bones, which are the reflected lamina of the angular and the manubrium, were not recovered. Their shape and location was based on a prior commitment to the theory of the mandibular eardrum as proposed by Allin and others.5,6
Furthermore, the evidence for the involvement of the articular is lacking since the complete articular is unknown. The ridge of the articular, which is supposed to have supported the eardrum, is straight, rather than curved, which would make the eardrum a rather odd semicircle. But the major problem is the lack of fossil evidence for the reflected lamina of the angular and the manubrium; this is enough to cast doubt on the whole idea of a mandibular eardrum. There is also the question of the gap between the quadrate and the squamosal, which suggests that if the quadrate was part of the jaw joint it had to be larger than the bone which had been identified as the quadrate, which in turn calls into question the effectiveness of sound transmission through such a large bone. The whole middle ear assembly is essentially guesswork!
The quadrate of Morganucodon
Given the fact that the quadrate is too small and does not have the facets required to articulate properly with the articular, it seems likely that it has been misidentified and incorrectly placed. If the bone in question was actually the incus, then it is possible that the middle ear of Morganucodon was essentially a mammalian middle ear. The incus of mammals is a complex structure which articulates with the stapes and the malleus,7 and the shape of the quadrate of Morganucodon is not dissimilar to the incus of extant mammals, even if it had some rather unusual features. Since the connection with the stapes is unknown and the malleus has never been found it is impossible to know for sure, but the evidence which is available is consistent with Morganucodon having had a mammalian middle ear.
In addition, the fact that the petrosal (which houses the inner ear) is described as a ‘fully mammalian petrosal’, also supports this possibility.
Given that the available evidence is more consistent with a mammalian jaw joint, then the suggestion that the element identified as the quadrate is actually the incus is not unreasonable. If this is correct, then the incus was relatively large, but this is consistent with hearing low-frequency sound and would support the idea that Morganucodon was a small burrowing mammal. Unfortunately, the malleus and tympanic bone have never been identified, but this is not surprising considering the fragility of these bones. However, it should be noted that there is as much evidence for a mammalian middle ear as there is for the mandibular middle ear. Both theories are valid until further evidence is forthcoming.
An alternative interpretation
There are many uncertainties and a lack of clear evidence for the transitional status of Morganucodon, so it seems appropriate to suggest an alternative which is consistent with a biblical worldview.
Mehlert examined Morganucodon and compared it with the so-called ‘more primitive’ cynodonts (a subgroup of therapsids) and concluded that it was most likely a cynodont. Thus, he considered that there was a major gap between Morganucodon and mammals. However, he did not consider later research on Morganucodon which was published in 1981.
This review looks at the more recent evidence and goes the other way. The weight of the evidence appears consistent with the suggestion that Morganucodon was a mammal. Thus, the gap was on the other side, separating Morganucodon from the cynodonts. In either case Morganucodon was not transitional.
The argument for Morganucodon as a transitional fossil with a double jaw joint, a tympanic membrane (eardrum) in the jaw, and mandibular middle ear bones is clearly a matter of interpretation. Therefore, the case for Morganucodon as a transitional fossil is not valid. The evidence is consistent with an alternative interpretation that Morganucodon had a mammalian dentary-squamosal jaw joint and mammalian middle ear bones. Therefore, Morganucodon was a mammal, and as a result a key step in the evolutionary account of the origin of the mammalian jaw and ears appears to be missing.
References and notes
- Mehlert, A., The origin of mammals: a study of some important fossils, J. Creation 7(2):122–139, 1993. Return to text.
- Woodmorappe, J., Mammal-like reptiles: major trait reversals and discontinuities, J. Creation 15(1):44–52, 2001. Return to text.
- Cserhati, M., Did the ear bones of mammals really evolve from the jawbones of reptiles? 2 July 2019, accessed 16 July 2020. Return to text.
- Kermack, K., Musset, F., and Rigney, H., The skull of Morganucodon, Zoological J. Linnean Society 71:1–158, 1981. Return to text.
- Maier, W. and Ruf, I., Evolution of the mammalian middle ear: a historical review, J. Anatomy 228:270–283, 2016. Return to text.
- Allin, E., Evolution of the mammalian middle ear, J. Morphology 147(4):403–437, 1975. Return to text.
- Charuta, A., Wysocki, J., and Wieczorek, U., Anatomical conditions of strengthening of the acoustic wave in the middle ear in selected species of mammals, Electronic J. Polish Agricultural Universities 14(2), ejpau.media.pl/volume14/issue2/art-12.html, 2011. Return to text.