AXIAL ELONGATION

Body elongation evolves through increase in region-specific number of vertebral though it can also be via increase in vertebral length. In fish, their shape and size due to evolution, influence their style of swimming, microhabitat use and escape response. Highly elongated fish swim in a snake like motion and perform a retraction response while escaping while other fish do a stereotyped C-start, (Ward and Mehta, 1016-1119). Limb loss is associated with axial elongation which has been witnessed in the evolution of tetra pods from limb reduction to limbless, (Cladwell, 573-588). Evolution in animals is characterized by body transformations which have led to beginning of new species. The rate of morphological developments is yet to be determined because most of these changes occurred a long time ago. Wiens and Slingluff () studied the morphological transformation of vertebrate in evolution from a lizard like body to a snake like body. The transition is replicated in closely related species. They researched on the evolution in Anguidae family in lizards where limb loss has occurred independently. Transition to snakelike body is characterized by expression changes in of Hog genes that relate elongation and loss of limbs causing sudden body transformations. In the research conducted by Mehta, Ward, Alfaro and Wainwright (1091–1105) explored evolution of axial skeleton of anguilliform taxa in addition to close relatives. The results indicated that precaudal and caudal vertebral numbers evolved independently in fishes. There is also diversity in the elongation mechanism across different species.

Research on axial elongation

Ward and Mehta (1106–1119) employed research on Actinopterygii in relation to anatomical characters in feeding and locomotion and their relation to body elongation. Fish elongation is characterized by loss of pelvic fins, increase in the length of the head, pectoral fin reduction and median fins expansion. Elongate fishes have more vertebral number. Studies indicate that vertebral number increases depending on region, abdomen and caudal. Actinopteregian fishes vary in their caudal verterbral number with some species increasing in both abdominal and caudal number with only a few varying in abdominal vertebral number. This indicates that both abdominal and caudal developments are controlled differently. Body elongation could also occur from increasing the length of vertebrates instead of addition of numbers. This modification however reduces flexibility due to few vertebral joints. Vertebral number is controlled by region whereas centrum length is global. Elongated fishes have also been shown to have an elongated skull. In species like congrid and ophichthid, the body length is contrary to their short skulls. It is not a universal characteristic. Increasing evidence indicates that these developmental changes in body elongation are controlled by genes. Hox genes control the development of the vertebrates.

Limb reduction is related to elongation in the transition of lizard like to snake like bodies. Wiens and Slingluff (2303-2318) research on morphometrics, phylogenic comparative and molecular phylogenetics supports that elongation is as a result of trunk elongation or trunk and tail elongation. The results indicate that these characteristics are correlated to different habitats. Limbless squamates move in a lateral manner that requires more energy and an elongated body. Limbs in this kind of locomotion are not required because they may curtail movement in burrows or grass which s dense, a common preference to snakes and lizard. This may be one of the reasons why lizards evolve to limbless snake like lizards. In amphibians, digit number in adults is related to that in developing limb bud. This indicates that there is a relationship between absolute limb size reduction and anguid digit loss. Study on Adriossaurus microbranchis, a 95 million old marine lizard like snake shows that it has preserved its anatomy as a result of limb reduction. This proves that developmental loss of girdle and forelimb occurs during evolution of species. Limb loss occurs also due to inactivity of these limbs in habitats, (Palci & Caldwell, 8-16).  According to Caldwell (573-588), long elongated Westlothiana lizziae evolved their limbless features in aquatic environment because none of their fossils were recovered in terrestrial environment but they were found in water. Pelvic girdle was retained even after he loss of other limbs like fore and rear limbs. He showed that body elongation and limb loss occur simultaneously though total limb loss takes time to occur.

Other suggestions suggest that vertebral column elongation alone does not contribute to body elongation but is also accompanied by decrease in depth of the body. Reduction of limbs or their total loss is so as to ease locomotion below surfaces. The elongated species in fish have smaller fins because they inhabit water areas mostly deep waters. Chalcides genus in lizards shows significant reduction of its limbs during evolution to acquire a snake like body form. The evolution has retained the number of presacral vertebrates with an increase in elongation. These modifications indicate why snake like chalcides laid fewer eggs due to ovary size reduction, (Vinceto, Guarino & Angelini, 385-391). Ward and Brainerd (96-116) accept that axial elongation can be due to increase in vertebral number, increase in vertebral centra length, body depth decrease and increase in length of the head.

Very many controversies surround the study on axial elongation. Most studies support the findings that regulation of Hox genes may affect elongation but research by Wiens and Slingluff (2303-2318) indicate that Hox gene expression is not related to body transformation. Authors have also indicated that body elongation, reduction in limb size and digit loss, occurs in this order during evolution while others studies indicate that all these changes occur simultaneously. Burrowing lifestyle has also been previously linked to evolution of snake like body though study in anguids show otherwise. Questions concerning antiquity of fossils that have already been recovered and used in this study remain an issue of controversy. Other researchers show that skull size in some fish species are associated with their body elongation where small skulls belong to stunted fish. These studies have been refuted since in other species, there are fishes with elongated bodies but they have short skulls.

Body elongation occurs due to either increase in vertebral number of lengthening of specific vertebral. It can also be due to skull elongation accompanied by decrease in depth of the body. Animals with slender bodies lay fewer eggs in relation to those with bigger circumference. Hox genes regulation affects elongation of the body by determining where elongation occurs. Limb loss and reduction is related to body elongation which depends on adapting to particular habitat. These studies have been done on various species of snakes, lizards and fishes. In lizards, elongation due to lengthening of vertebral reduces flexibility, a useful characteristic for the species that live on trees to enable proper navigation.



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