Cynodont Postcranial Anatomy and the “Prototherian” Level of Mammalian Organization

Jenkins, F. A.

doi:10.1111/j.1558-5646.1970.tb01756.x

Cited by 22 publications

(24 citation statements)

References 55 publications

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“…Modifications of their appendicular skeleton indicate that the locomotor capabilities of cynodonts steadily improved (Jenkins 1971), whereas changes in dentition and the structure of the lower jaw suggest that the rate offood assimilation increased (Kemp 1982). Although individually these features may have little direct bearing on the development of endothermy (Bennett and Ruben 1986), collectively they suggest that the metabolic capacities of cynodonts were expanding steadily.…”

Section: Discussionmentioning

confidence: 99%

“…The nasal turbinal ridges of the earliest mammals are essentially identical to those of extant mammals, suggesting that the development of "mammalian" respiration rates and endothermy was largely complete by this time (cf. Bennett and Ruben 1986;Jenkins 1990;Bennett 1991). Consequently, between the Late Permian and the Late Triassic, the process ofthe evolution ofendothermy may have required as much as 40 to 50 million yr.…”

Section: Discussionmentioning

confidence: 99%

“…The presence in these animals of an essentially mammalian turbinal complex, inclusive of extensive respiratory maxilloturbinates, therefore strongly indicates that even the earliest mammals possessed high ventilation and oxygen-consumption rates, similar to those of modern mammals. Most authors, in fact, agree that endothermy was most likely a primitive feature of mammals (McNab 1978;Bennett and Ruben 1986;Hopson 1987;Carroll 1988;Jenkins 1990).…”

Section: -'------------------------------------1mentioning

confidence: 99%

“…Their vertebral column became differentiated into cervical, thoracic, and lumbar regions, and the appendicular skeleton of both groups suggests an increased reliance on a more erect, rather than sprawled, gait (Jenkins 1970(Jenkins , 1971Kemp 1978Kemp , 1986. Their dentition became increasingly complex, and both therocephalians and cynodonts eventually developed a complete bony secondary palate (Kemp 1982).…”

Section: Therocephaliansmentioning

confidence: 99%

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1994

Evolution

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Abstract. -The structure and function of the nasal conchae of extant reptiles, birds, and mammals are reviewed, and the relationships to endothermy of the mammalian elements are examined. Reptilian conchae are relatively simple, recurved structures, which bear primarily sensory (olfactory) epithelium. Conversely, the conchae, or turbinates, of birds and mammals are considerably more extensive and complex, and bear, in addition, nonsensory (respiratory) epithelium. Of the mammalian turbinates, only the exclusively respiratory maxilloturbinal has a clear functional relationship with endothermy, as it reduces desiccation associated with rapid and continuous pulmonary ventilation. The other mammalian turbinates principally retain the primitive, olfactory function of the nasal conchae. The maxilloturbinates are the first reliable morphological indicator of endothermy that can be used in the fossil record. In fossil mammals and mammallike reptiles, the presence and function ofturbinates are most readily revealed by the ridges by which they attach to the walls of the nasal cavity. Ridges for olfactory turbinals are located posterodorsally, away from the main flow ofrespiratory air, whereas those ofthe respiratory maxilloturbinals are situated in the anterolateral portion of the nasal passage, directly in the path of respired air. The maxilloturbinal is also characterized by its proximity to the opening ofthe nasolacrimal canal. Posterodorsal ridges, for olfactory turbinals, have long been recognized in many mammallike reptiles, including early forms such as pelycosaurs. However, ridges for respiratory turbinals have not been identified previously in this group. In this paper, the presence of anterolateral ridges, which most likely supported respiratory turbinals, is reported in the primitive therocephalian Glanosuchus and in several cynodonts. The presence of respiratory turbinals in these advanced mammallike reptiles suggests that the evolution of "mammalian" oxygen consumption rates may have begun as early as the Late Permian and developed in parallel in therocephalians and cynodonts. Full mammalian endothermy may have taken as much as 40 to 50 million yr to develop.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: -'------------------------------------1mentioning

confidence: 99%

Section: Therocephaliansmentioning

confidence: 99%

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1994

Evolution

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“…Based on the appearance of epipubic bones in the fossil record, the lever system appeared in concert with the appearance of mammalian grade pelvis and the shift to an erect limb posture from the semi-upright posture of advanced cynodont ancestors (Jenkins, 1970;Pridmore, 1985). This also occurred in concert with the first evidence of a diaphragm (Jenkins, 1970;Crompton and Jenkins 1979) at a time when mammals are also thought to have shifted to expanded locomotor needs associated with higher metabolic rates (Crompton et al, 1978;Bennett and Ruben, 1979;Ruben et al, 1987). The shift to endothermy is considered to be related to increased stamina (Bennett and Ruben, 1986).…”

Section: The Loss Of Epipubic Levers and The Basal Condition In Theriansmentioning

confidence: 99%

Abdominal muscle function in ventilation and locomotion in new world opossums and basal eutherians: Breathing and running with and without epipubic bones

Reilly

McElroy

White

2009

Journal of Morphology

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All tetrapods have the same four basic abdominal hypaxial muscle layers that wrap around the abdomen between the pelvis, ribcage, and spine. However, the marsupials and our immediate mammalian ancestors have epipubic bones extending anteriorly into the ventral hypaxial layers with two additional muscles connecting them to the ventral midline and femur. Studies of two marsupials have shown that all of the abdominal hypaxials play a part bilaterally in resting ventilation and during locomotion there is an asymmetrical pattern of activity as the hypaxial muscles form a cross-couplet linkage that uses the epipubic bone as a lever to provide long-axis support of the body between diagonal limb couplets during each step. The cross-couplet epipubic lever system defines the earliest mammals and is lost in placental mammals. To expand our understanding of the evolution of mammalian abdominal muscle function and loco-ventilatory integration we tested the generality of the cross-couplet system in marsupials and conducted the first formal studies of hypaxial abdominal motor patterns in generalized placental mammals focusing on a representative rodent and insectivore. These new data reveal 1) that continuous abdominal muscle tonus during resting ventilation and a 1:1 breath to step cycle during locomotion appear to be the basal condition for mammals, 2) that the loss of epipubic bones in eutherians is associated with a shift from the cross-couplet dominated motor pattern of marsupials to a shoulder-to-pelvis system with unilateral activation of abdominal muscles during locomotion and 3) that hypaxial function in generalized eutherians is more similar to marsupials than cursorial mammals.

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The fossil record of appendicular muscle evolution in Synapsida on the line to mammals: Part I—Forelimb

Bishop,

Pierce

2023

The Anatomical Record

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This paper is the first in a two‐part series that charts the evolution of appendicular musculature along the mammalian stem lineage, drawing upon the exceptional fossil record of extinct synapsids. Here, attention is focused on muscles of the forelimb. Understanding forelimb muscular anatomy in extinct synapsids, and how this changed on the line to mammals, can provide important perspective for interpreting skeletal and functional evolution in this lineage, and how the diversity of forelimb functions in extant mammals arose. This study surveyed the osteological evidence for muscular attachments in extinct mammalian and nonmammalian synapsids, two extinct amniote outgroups, and a large selection of extant mammals, saurians, and salamanders. Observations were integrated into an explicit phylogenetic framework, comprising 73 character–state complexes covering all muscles crossing the shoulder, elbow, and wrist joints. These were coded for 33 operational taxonomic units spanning >330 Ma of tetrapod evolution, and ancestral state reconstruction was used to evaluate the sequence of muscular evolution along the stem lineage from Amniota to Theria. In addition to producing a comprehensive documentation of osteological evidence for muscle attachments in extinct synapsids, this work has clarified homology hypotheses across disparate taxa and helped resolve competing hypotheses of muscular anatomy in extinct species. The evolutionary history of mammalian forelimb musculature was a complex and nonlinear narrative, punctuated by multiple instances of convergence and concentrated phases of anatomical transformation. More broadly, this study highlights the great insight that a fossil‐based perspective can provide for understanding the assembly of novel body plans.

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Cynodont Postcranial Anatomy and the “Prototherian” Level of Mammalian Organization

Cited by 22 publications

References 55 publications

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Untitled

Abdominal muscle function in ventilation and locomotion in new world opossums and basal eutherians: Breathing and running with and without epipubic bones

The fossil record of appendicular muscle evolution in Synapsida on the line to mammals: Part I—Forelimb

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