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Introduction to the Therapsids
2, 3, 3, 3, 3. That’s the number of bones in your thumb, index finger, middle finger, ring finger and little finger respectively. The same numbers apply to the phalangeal bones in your toes, starting with your big toe. This basic structural plan (common to most land mammals) has a very ancient pedigree – the therocephalia, a group of carnivorous “mammal-like reptiles” that walked the earth over 250 million years ago during the Permian period, had hands and feet constructed in just this fashion. Four out of the five digits on each extremity contained 3 bones while the remaining digit (corresponding to the thumb or big toe) contained only 2.
Many of the most unique and widely conserved features of mammalian anatomical design and physiology – including a bony secondary palate, a three-ossicle ear, endothermy, an erect limb-posture, and quite probably hair and lactation – originated among ancestral groups of “mammal-like reptiles” during the Permian and early Triassic periods.
So how did the evolutionary process derive a class of highly active, fur-clad, milk-producing animals from ancestral forms that were so fundamentally reptile-like in appearance? This is the question that I shall attempt to answer in a series of posts over the course of this winter.
We discussed the biology and classification of some of the earliest “mammal like reptiles” in my post on Pelycosaurs. In this post, we shall begin to deal with their Permian heirs, the Therapsids. The first few installments of this series will deal with specific groups of mammal-like reptiles that lived during the Middle and Late Permian, while the remaining parts will deal with some of the long-term evolutionary ‘trends’ we can glean from the Therapsid fossil record.
What is a Therapsid?
Therapsida is a clade that owes its evolutionary origins to a pelycosaurian-grade ancestor and includes all of the most ‘advanced’ synapsids, including modern mammals. If that sentence is gibberish to you, perhaps reading the first few paragraphs of my post on pelycosaurs will help demystify things. To summarize:
- Birds, mammals and modern reptiles are amniotes by descent. The first amniotes were terrestrially-adapted animals that laid water-proof eggs on dry land.
- Synapsida is a subgrouping of amniotes that includes modern mammals and their various extinct proto-mammal ancestors and relatives, but excludes birds and reptiles. Early synapsids can be distinguished from other groups of amniotes in the fossil record on the basis of a number of skeletal features, the most important of which is the presence of a single hole on each side of the skull behind the eye-orbit. These holes are called temporal fenestrae. By contrast, other early amniote skulls bear either no holes (anapsids), two holes (diapsid) or a single highly placed hole (euryapsid, similar to the synapsid condition, but differing in the location of the hole) behind each eye-orbit. The temporal fenestrae provide secure anchorage points for muscles associated with jaw function. Mammals are the only surviving synapsids.
- ‘Pelycosaur‘ is a somewhat informal term applied to a range of basal/’primitive’ non-mammalian synapsids.
What differentiates early Therapsids from pelycosaurs?
The most important differences lie in the anatomy of the skull.
Therapsids have larger temporal fenestrae than pelycosaurs.
An enlarged canine-like tooth is present on the lower and upper jaws of Therapsids, clearly demarcating the boundary between a set of incisor-like teeth in front and a set of post-canine teeth behind. Heterodonty – or the presence of morphologically distinct sets of teeth in the jaws (e.g. incisors, canines, molars, premolars) – is one of the signature features of mammalhood. Most bony fish, amphibians and modern reptiles have ‘homodont’ teeth that are essentially morphologically identical.
The back of the skull in Therapsids is also more vertical and the jaw-joint is more anteriorly placed than in pelycosaurs.
The mammalian jaw consists of a single bone – the dentary. Earlier Therapsids had jaws constructed from a number of bones in addition to the dentary, including the angular bone (refer to diagram). One diagnostic feature of the therapsid jaw is the presence of a wide leaf of bone called the “reflected lamina” that protrudes from the angular. The reflected lamina may have played some part in hearing. We will discuss the evolution of the mammalian auditory system in a future post in this series.
The post cranial skeletons of early Therapsids show signs of improved locomotory ability. They appear to have traded in the permanent sprawling posture of their pelycosaur ancestors for a more erect gait and developed shoulder and hip joints that permitted a greater range of limb movements.
Most of the traits listed above were present in some incipient form in one group of Pelycosaurs in particular- the Sphenocodontidae. This group includes the famous sail-backed Dimetrodon. The Sphenocodontids are nearly universally posited as the closest relatives of the Therapsids.
Up next, the Dinocephalia.
1. Kemp, Thomas Stainforth. The origin and evolution of mammals. Oxford: Oxford University Press, 2005.
2. Kemp, Thomas Stainforth, and T. S. Kemp. Mammal-like reptiles and the origin of mammals. London: Academic Press, 1982.
3. Chinsamy-Turan, Anusuya, ed. Forerunners of Mammals: Radiation• Histology• Biology. Indiana University Press, 2011.
4. MacLean, Paul D., Jan J. Roth, and E. Carol Roth. The ecology and biology of mammal-like reptiles. Washington, DC: Smithsonian Institution Press, 1986.
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