/* */ Rosa Rubicondior: More of Those 'Missing' Transitional Forms

Sunday, 22 February 2015

More of Those 'Missing' Transitional Forms

Mammalian and non-mammalian jaws. In the mammal configuration, the quadrate and articular bones are much smaller and form part of the middle ear. Note that in mammals the lower jaw consists of only the dentary bone.
Evolutionary development in basal mammaliaforms as revealed by a docodontan

An arboreal docodont from the Jurassic and mammaliaform ecological diversification

Two papers published in last week's Science by the same team of researchers, describe two more of those 'non-existent' transitional fossils on which so much creationism depends.

They both deal with fossils of two members of an early and now extinct order of vertebrates which seems to sit somewhere between pre-mammalian reptiles and true mammals in that they have a transitional jaw joint closer to the 'squamous' joint typical of reptiles, where the mandible articulates with the squamous bone, and the mammalian jaw joint in which some bones have become reduced and now form the ossicles of the mammalian inner ear, allowing a new jaw joint to form between the quadrate bone and the mandible.

In all other respects these early proto-mammals have a mammalian skeleton and dentition. Because of this difference in jaw joint, the order is not universally accepted as a true mammal but are normally referred to an 'mammaliaforms', i.e., mammal-like - exactly the sort of taxonomic problem we would expect of something which is part-way between two others.

They are unlikely to have been placental but were probably egg-laying, like the monotremes of today. The fossils of these two docodontans were found in China and already show wide diversification. One of them was probably subterranean, like the golden mole whilst the other was probably arborel like the tree shrew. Other docodontans are known to have been aquatic.

A new Late Jurassic docodontan shows specializations for a subterranean lifestyle. It is similar to extant subterranean golden moles in having reduced digit segments as compared to the ancestral phalangeal pattern of mammaliaforms and extant mammals. The reduction of digit segments can occur in mammals by fusion of the proximal and intermediate phalangeal precursors, a developmental process for which a gene and signaling network have been characterized in mouse and human. Docodontans show a positional shift of thoracolumbar ribs, a developmental variation that is controlled by Hox9 and Myf5 genes in extant mammals. We argue that these morphogenetic mechanisms of modern mammals were operating before the rise of modern mammals, driving the morphological disparity in the earliest mammaliaform diversification.

A new docodontan mammaliaform from the Middle Jurassic of China has skeletal features for climbing and dental characters indicative of an omnivorous diet that included plant sap. This fossil expands the range of known locomotor adaptations in docodontans to include climbing, in addition to digging and swimming. It further shows that some docodontans had a diet with a substantial herbivorous component, distinctive from the faunivorous diets previously reported in other members of this clade. This reveals a greater ecological diversity in an early mammaliaform clade at a more fundamental taxonomic level not only between major clades as previously thought.

Qing-Jin Meng, Qiang Ji, Yu-Guang Zhang, Di Liu, David M. Grossnickle, and Zhe-Xi Luo
An arboreal docodont from the Jurassic and mammaliaform ecological diversification
Science 13 February 2015: 347 (6223), 764-768. [DOI:10.1126/science.1260879]

The significance of this diversification is that the main skeletal adaptations needed are known in modern mammals to be controlled by a few genes such as the Hox9 and Myf5 genes, which is highly suggestive that these 'modern' genes were present in at least our earliest ancestors and enabled evolution to quickly cause diversification into available niches once key adaptations had evolved.

The significance from a creationist point of view is that we have yet more examples of transitional fossils, this time showing a clear intermediate form between the reptiles and the mammals so showing how an earlier order evolved and diversified into two, one of which went on to give rise to all the different mammals, carrying with them genetic remnants of this earlier ancestor.

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