The evolution of growth patterns in mammalian versus nonmammalian cynodonts

Abstract

One of the major evolutionary transitions of the mammaliaform lineage was the origin of a typically mammalian pattern of growth. This is characterized by rapid juvenile growth followed by abrupt cessation of growth at adult size and may be linked with other important mammaliaform apomorphies of dental replacement and morphology. Investigation of growth patterns in the tritylodontid cynodont Oligokyphus and the basal mammaliaform Morganucodon provides insight into this crucial transition. We collected mandibular depth measurements from large samples of Morganucodon and Oligokyphus and constructed distributions of mandibular depth versus frequency for each species. These were compared with distributions from species from three different growth classes of extant amniote: testudines + crocodilians, mammals + birds, and lepidosaurs. Discriminant function analysis was used to differentiate between known growth classes by using different combinations of three measures of mandibular depth distribution shape (skew, kurtosis, and coefficient of variation) as proxies for different juvenile and adult growth patterns. Classification of the fossil species showed that Morganucodon closely resembled extant placental mammals in having rapid juvenile growth followed by truncated, determinate adult growth. Oligokyphus showed intermediate growth patterns, with more extended adult growth patterns than Morganucodon and slightly slower juvenile growth. This suggests a gradual evolution of mammalian growth patterns across the cynodont to mammaliaform transition, possibly with the origin of rapid juvenile growth preceding that of truncated, determinate adult growth. In turn, acquisition of both these aspects of mammalian growth was likely necessary for the evolution of diphyodont tooth replacement in the mammaliaform lineage.

The dentary of Wareolestes rex (Megazostrodontidae): a new specimen from Scotland and implications for morganucodontan tooth replacement

“…There is clear evidence that, among morganucodontans, the molars in more posterior positions are never replaced, erupting as the individual reaches maturity as in most modern mammals (Parrington ; Crompton & Parker, ; Luo et al . ; O'Meara & Asher ). The wear facet patterns on many hundreds fragmentary mandibles of Morganucodon watsoni show no evidence of replacement in the posterior molars (Parrington ; Young ; Crompton & Luo ); with molars frequently heavily worn in larger and older individuals, suggesting that they had not been replaced (Parrington ).…”

Tissue Mechanical Forces and Evolutionary Developmental Changes Act Through Space and Time to Shape Tooth Morphology and Function

“…The loss of polyphyodonty in mammals is potentially linked to changes in the growth patterns of the skull. Initially rapid skull growth that slows or stops in adulthood is a hallmark of placental mammal development . Teeth generally cannot change size after eruption, and thus multiple tooth replacements in species with skulls that grow continuously throughout life serve the purpose of allowing larger teeth to fill the dentary .…”

“…Initially rapid skull growth that slows or stops in adulthood is a hallmark of placental mammal development . Teeth generally cannot change size after eruption, and thus multiple tooth replacements in species with skulls that grow continuously throughout life serve the purpose of allowing larger teeth to fill the dentary . Rapid early skull growth that does not continue into adulthood truncates the period during which an intermediate jaw requires intermediate sized teeth, thus reducing the pressure to form multiple generations of intermediately sized teeth .…”

“…[78] Rapid early skull growth that does not continue into adulthood truncates the period during which an intermediate jaw requires intermediate sized teeth, thus reducing the pressure to form multiple generations of intermediately sized teeth. [77,78] Consequently, reducing the number of generations of teeth may also be related to changes in tooth attachment. Early mammals had teeth that fully ankylosed to the bones of the jaw, whereas the teeth of crown mammals are attached to the bone by the periodontal ligament, called a gomphosis.…”

Enamel formation and growth in non-mammalian cynodonts

“…[ 60 , 61 ]) or comparison of size ranges of specimens (e.g. [ 36 , 37 , 46 , 62 ]). In this study, we further explore the link between growth patterns and dentition by investigating absolute rates of growth within the dentition itself, a method not previously applied to these animals.…”