Tetrapods Roald Smeets

English: Illustration of Tiktaalik roseae, a S...

English: Illustration of Tiktaalik roseae, a Sarcopterygiin fish found at Ellesmere Island, Canada. Famous as one of the best candidates to the title as a missing link between lobe-finned fishes and tetrapods. (Photo credit: Wikipedia)

The tetrapods (Greek τετραπόδηs tetrӑpódēs, “four-footed”) are the amphibians, reptiles, birds and mammals; these four groups are united by the fact that most animals in each group have four limbs. Some animals with only two limbs such as pygopod lizards and some with no limbs at all like snakes and caecilians are considered tetrapods because they are classified among the four groups. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian. They are now a dominant part of the terrestrial fauna, representing all known larger land animals. Some groups have even returned to an aquatic existence, including the largest animal known, the blue whale.

The evolution of the first tetrapods marked the moment when the two basic forms of vertebrates, fishes and tetrapods, diverged. This transition, from a body plan for breathing and navigating in water and a body plan enabling the animal to move on land, involved a series of changes taking place throughout most of the 56.8 million years that make up the Devonian period. While it is one of the most profound evolutionary changes known, it is also one of the best understood, largely thanks to a number of amazing fossil finds in the late 20th century combined with improved phylogenetic analysis.

The Devonian period is traditionally known as the “Age of Fishes”, marking the diversification of numerous extinct and modern major fish groups. Among them were the early bony fishes, who diversified and spread in freshwater and brackish environments at the beginning of the period. The early types resembled their cartilaginous forefathers in many aspects of their anatomy, including a shark-like tailfin, spiral gut, large pectoral fins stiffened in front by skeletal elements and a largely unossified axial skeleton.

They did however have certain traits separating them from cartilaginous fishes, traits that would become pivotal in the evolution of terrestrial forms: With the exception of a pair of spiracles, the gills did not open singly to exterior like in sharks, rather they were hidden behind a bony operculum. The gill chamber was bound posteriorly by a stout cleithrum bone, also functioning as anchoring for the pectoral fins. As part of the overall armour of rhomboid cosmin scales, the skull had a full cover of dermal bone, constituting a skull roof over the otherwise shark-like cartilaginous inner cranium. Importantly, they also had a swim bladder/lung, a feature lacking in all other fishes.


Herbivores Roald Smeets Plant Life

Herbivores (The making of)

Herbivores (The making of) (Photo credit: leesean)

Roald Smeets, herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in general are known as primary consumers.

Herbivory usually refers to animals eating plants; fungi, bacteria and protists that feed on living plants are usually termed plant pathogens (plant diseases),and microbes that feed on dead plants are saprotrophs. Flowering plants that obtain nutrition from other living plants are usually termed parasitic plants.

A herbivore is not the same as a vegetarian, a human who voluntarily undertakes a primarily herbivorous diet.

Herbivore is the anglicized form of a modern Latin coinage, herbivora, cited in Charles Lyell’s 1830 Principles of Geology. Richard Owen employed the anglicized term in an 1854 work on fossil teeth and skeletons. Herbivora is derived from the Latin herba meaning a small plant or herb, and vora, from vorare, to eat or devour.

Our understanding of herbivory in geological time comes from three sources: fossilized plants, which may preserve evidence of defence (such as spines), or herbivory-related damage; the observation of plant debris in fossilised animal faeces; and the construction of herbivore mouthparts.

Long thought to be a Mesozoic phenomenon, evidence for herbivory is found almost as soon as fossils which could show it. Within under 20 million years of the first land plants evolving, plants were being consumed by insects. Insects fed on the spores of early Devonian plants, and the Rhynie chert also provides evidence that organisms fed on plants using a “pierce and suck” technique.

Herbivory among terrestrial vertebrates (tetrapods) came much later. Early tetrapods were large amphibious piscivores. While amphibians continued to feed on fish and later insects, reptiles began exploring two new food types, tetrapods (carnivory), and later, plants (herbivory). Carnivory was a natural transition from insectivory for medium and large tetrapods, requiring minimal adaptation. In contrast, a complex set of adaptations was necessary for feeding on highly fibrous plant materials).

During the ensuing 75 million years, plants evolved a range of more complex organs – from roots to seeds. There is no evidence for these being fed upon until the middle-late Mississippian, 326.4 million years ago. There was a gap of 50 to 100 million years between each organ evolving, and it being fed upon; this may be due to the low levels of oxygen during this period, which may have suppressed evolution. Further than their arthropod status, the identity of these early herbivores is uncertain. Hole feeding and skeletonisation are recorded in the early Permian, with surface fluid feeding evolving by the end of that period.

Arthropods have evolved herbivory in four phases, changing their approach to herbivory in response to changing plant communities.

Another stage of herbivore evolution is characterized by the evolution of tetrapod herbivores, with the first appearance in the fossil record near the Permio-Carboniferous boundary approximately 300 MYA. The earliest evidence of herbivory by tetrapod organisms is seen in fossils of jawbones where dental occlusion (process by which teeth from the upper jaw come in contact with those in the lower jaw) is present. The evolution of dental occlusion lead to a drastic increase in food processing associated with herbivory and provides direct evidence about feeding strategies based on tooth wear patterns. Examination of phylogenetic frameworks reveals that dental occlusion developed independently in several lineages through dental and mandibular morphologes, suggesting that the evolution and radiation of tetrapod herbivores occurred simultaneously within various lineages.