Venomous Snakes
EVOLUTION AND PHYLOGENY OF SNAKES
Where snakes actually originate from still is unclear. Unquestionably, they belong to the squamates order of scaled reptiles; their relationship to lizards is also beyond dispute. The rise of snakes’ specific physical configuration is usually linked to hypothetical predecessors that led a subterranean life or inhabited cracks. However, alternative hypotheses concerning the aquatic ancestors of snakes also exist, winning increasing support from experts. Snakes are presumed to have evolved from anguimorphous, most probably varanid-like lizards within the process of diversification of ancestral scaled reptiles, in the Late Jurassic period about 150 million years ago. The oldest known fossil relics believed to be most likely related to snakes are highly fragmentary and come from the Early Cretaceous, which are over 120 million years old. The merging of the Lower and Middle Cretaceous produced Pachyrhachis problematicus in Israel, a very intriguing marine squamate reptile. Ranked as a member of the mosasaur family by some experts, this reptile is assumed to be an early snake with the remnants of rear legs. Thus, Pachyrhachis may support the idea of snakes originating from marine varanid-like lizards.
Described from the Algerian Middle Cretaceous from strata nearly 100 million years old, Lapparentophis defrennei can be indisputably considered a snake - most probably one of terrestrial form. In addition, other species appeared, for instance Simoliophis, supposedly a water snake. Any exact phylogenetic ranking of these prehistoric examples referred to as Cholophidia remains unclear for the moment. Shortly afterwards, the first family appeared: false coral snakes, Aniliidae; this monotypic group of a single species has survived until today. From the Upper Cretaceous, when apparently an important adaptive radiation of primeval snakes occurred, fossil documentation shows that snakes increased in diversity, although interpretation of their phyletic relationships greatly varies. A higher number of lineages may have originated, from which many more recently became extinct without any evolutionary successors. Genera that appeared at that time include Madtsoia and Gigantophis, members of a highly successful and widespread snake group that in some places (Wonambi genus in Australia) survived until the Pleistocene. These reptiles resemble later large boid snakes in the size and shape of their body. Sometimes considered an important phylogenetic node, Dinilysia, a reptile similar to a boa, is another Late Cretaceous form to emerge from Patagonia.
It was at the beginning of the Tertiary period when monumental forms resembling pythons or boas proliferated, while in the later Eocene the family of boas (Boidae) appeared, including genera that were either very close to recent members or still survive today (such as Charina). Evidence has been produced as regards members of the recent Tropidophiidae family as well. The earliest documented blind snakes (Scolecophidi) also come from the Eocene. In addition, the most archaic forms of so-called modern snakes Caenophidia appeared for the first time, from an ancestral relationship of extant wart snakes (Acrochordoidea) and ancient Colubroidea snakes, and maybe even the first colubrid snakes, Colubridae.
However, climatic changes at the meeting of the Eocene and Oligocene periods, along with less favorable conditions in the course of the Oligocene, brought about a vast reduction in prehistoric Eocene snake fauna, resulting in fundamental changes in its taxonomical structure. While some of the archaic lineages died out and Booidea and related forms shrank to some extent, Caenophidia took the lead and later developed at an enormous rate in the Miocene, which lasted until modern times. It is clear the evolution of snakes as such did not proceed independently, but in accord with the general transformation of the natural world at a time of climatic change, and in close correlation with alterations in other fauna as well, reflecting the changing pressure of rivalry in predators and their adversaries, and responding to diverse availability of diet. Even in the Oligocene, over 30 million years ago, colubrid snakes (Colubridae) were expanding throughout Asia, Europe, and North America. The oldest elapid snakes (Elapidae) in fossil records appeared in the Lower Miocene in Europe over 20 million years ago (Palaeonaja); more recent Miocene findings also show them inhabiting a number of places around North America, Europe, and Asia. Moreover, it is the Miocene period when vipers (Viperidae) first appear in fossil records over 20 million years old, these being snakes that soon proliferated as well. For instance, Vipera antiqua, a smaller close relative of the recent V. ammodytes, ranged throughout Europe as long ago as the lower Miocene together with V. platyspondyla, a snake closely related to the recent V. xanthina.
Throughout the Miocene adaptive radiation of modern snakes, numerous extant colubrid, elapid and viperid genera or species evolved. In Europe, not only were there colubrid and viperid species as more recently, but even elapids and boids could be found. Pliocene snake fauna already featured high diversity and taxonomic similarity to that of recent times, developments which were further affected by Pleistocene glaciations as well as marine transgressions and shrinkage. All of the above was accompanied by changes in snake fauna populations in different parts of the world, resulting in the map of zoogeographical distribution of snakes and their taxonomical structure as it is known today. Currently, snakes are present on every continent except Antarctica, including a number of islands. Approximately 3,000 species have been described to date, mostly colubroids (Colubroidea), in addition to the most abundant family in terms of species, Colubridae, which covers elapids (Elapidae), viperids (Viperidae), and the less-numerous Atractaspididae family including burrowing asps and other snakes.
The exact time when snakes became venomous is not known. However, indications of venomousness existed as long ago as the Cretaceous Period in the Pachyrhachis genus that can be considered a sister taxon linked to all extant snakes. This is noteworthy as it suggests in some snake lineages venomousness could have present in their earliest evolutionary phases. After all, laboratory experiments with saliva from recent non-venomous snakes with archaic origins indicate a certain level of toxicity. Nevertheless venomousness, as is understand from a practical aspect, only exists in colubroids (Colubroidea) within the variety of contemporary snakes. The presence of venom secretion from modified salivary glands in the upper jaw may even have been a common trait from the very start, being apparently one of the reasons for the extraordinary evolutionary success of these snakes. The non-existence of venom secretion in some colubrids might obviously be only a secondary feature. In colubroid snakes, the process of evolution was accompanied by extensive modifications to the venomous gland, as well as the growth and development of specialized venom-delivering apparatus. The wonderful adaptive radiation of these snakes brought about venom-specialist families, namely in two basic parallel lineages: the solenoglyphous (viperids, Viperidae) and proteroglyphous (elapid snakes, Elapidae). Since the Miocene period, these two families have presented two successful alternative approaches in maximizing the potential of snake venom.
- VENOMOUS SNAKES & ENVENOMING THERAPY. By JIRI VALENTA
Nova Science Publishers, Inc.
New York
Where snakes actually originate from still is unclear. Unquestionably, they belong to the squamates order of scaled reptiles; their relationship to lizards is also beyond dispute. The rise of snakes’ specific physical configuration is usually linked to hypothetical predecessors that led a subterranean life or inhabited cracks. However, alternative hypotheses concerning the aquatic ancestors of snakes also exist, winning increasing support from experts. Snakes are presumed to have evolved from anguimorphous, most probably varanid-like lizards within the process of diversification of ancestral scaled reptiles, in the Late Jurassic period about 150 million years ago. The oldest known fossil relics believed to be most likely related to snakes are highly fragmentary and come from the Early Cretaceous, which are over 120 million years old. The merging of the Lower and Middle Cretaceous produced Pachyrhachis problematicus in Israel, a very intriguing marine squamate reptile. Ranked as a member of the mosasaur family by some experts, this reptile is assumed to be an early snake with the remnants of rear legs. Thus, Pachyrhachis may support the idea of snakes originating from marine varanid-like lizards.
Described from the Algerian Middle Cretaceous from strata nearly 100 million years old, Lapparentophis defrennei can be indisputably considered a snake - most probably one of terrestrial form. In addition, other species appeared, for instance Simoliophis, supposedly a water snake. Any exact phylogenetic ranking of these prehistoric examples referred to as Cholophidia remains unclear for the moment. Shortly afterwards, the first family appeared: false coral snakes, Aniliidae; this monotypic group of a single species has survived until today. From the Upper Cretaceous, when apparently an important adaptive radiation of primeval snakes occurred, fossil documentation shows that snakes increased in diversity, although interpretation of their phyletic relationships greatly varies. A higher number of lineages may have originated, from which many more recently became extinct without any evolutionary successors. Genera that appeared at that time include Madtsoia and Gigantophis, members of a highly successful and widespread snake group that in some places (Wonambi genus in Australia) survived until the Pleistocene. These reptiles resemble later large boid snakes in the size and shape of their body. Sometimes considered an important phylogenetic node, Dinilysia, a reptile similar to a boa, is another Late Cretaceous form to emerge from Patagonia.
It was at the beginning of the Tertiary period when monumental forms resembling pythons or boas proliferated, while in the later Eocene the family of boas (Boidae) appeared, including genera that were either very close to recent members or still survive today (such as Charina). Evidence has been produced as regards members of the recent Tropidophiidae family as well. The earliest documented blind snakes (Scolecophidi) also come from the Eocene. In addition, the most archaic forms of so-called modern snakes Caenophidia appeared for the first time, from an ancestral relationship of extant wart snakes (Acrochordoidea) and ancient Colubroidea snakes, and maybe even the first colubrid snakes, Colubridae.
However, climatic changes at the meeting of the Eocene and Oligocene periods, along with less favorable conditions in the course of the Oligocene, brought about a vast reduction in prehistoric Eocene snake fauna, resulting in fundamental changes in its taxonomical structure. While some of the archaic lineages died out and Booidea and related forms shrank to some extent, Caenophidia took the lead and later developed at an enormous rate in the Miocene, which lasted until modern times. It is clear the evolution of snakes as such did not proceed independently, but in accord with the general transformation of the natural world at a time of climatic change, and in close correlation with alterations in other fauna as well, reflecting the changing pressure of rivalry in predators and their adversaries, and responding to diverse availability of diet. Even in the Oligocene, over 30 million years ago, colubrid snakes (Colubridae) were expanding throughout Asia, Europe, and North America. The oldest elapid snakes (Elapidae) in fossil records appeared in the Lower Miocene in Europe over 20 million years ago (Palaeonaja); more recent Miocene findings also show them inhabiting a number of places around North America, Europe, and Asia. Moreover, it is the Miocene period when vipers (Viperidae) first appear in fossil records over 20 million years old, these being snakes that soon proliferated as well. For instance, Vipera antiqua, a smaller close relative of the recent V. ammodytes, ranged throughout Europe as long ago as the lower Miocene together with V. platyspondyla, a snake closely related to the recent V. xanthina.
Throughout the Miocene adaptive radiation of modern snakes, numerous extant colubrid, elapid and viperid genera or species evolved. In Europe, not only were there colubrid and viperid species as more recently, but even elapids and boids could be found. Pliocene snake fauna already featured high diversity and taxonomic similarity to that of recent times, developments which were further affected by Pleistocene glaciations as well as marine transgressions and shrinkage. All of the above was accompanied by changes in snake fauna populations in different parts of the world, resulting in the map of zoogeographical distribution of snakes and their taxonomical structure as it is known today. Currently, snakes are present on every continent except Antarctica, including a number of islands. Approximately 3,000 species have been described to date, mostly colubroids (Colubroidea), in addition to the most abundant family in terms of species, Colubridae, which covers elapids (Elapidae), viperids (Viperidae), and the less-numerous Atractaspididae family including burrowing asps and other snakes.
The exact time when snakes became venomous is not known. However, indications of venomousness existed as long ago as the Cretaceous Period in the Pachyrhachis genus that can be considered a sister taxon linked to all extant snakes. This is noteworthy as it suggests in some snake lineages venomousness could have present in their earliest evolutionary phases. After all, laboratory experiments with saliva from recent non-venomous snakes with archaic origins indicate a certain level of toxicity. Nevertheless venomousness, as is understand from a practical aspect, only exists in colubroids (Colubroidea) within the variety of contemporary snakes. The presence of venom secretion from modified salivary glands in the upper jaw may even have been a common trait from the very start, being apparently one of the reasons for the extraordinary evolutionary success of these snakes. The non-existence of venom secretion in some colubrids might obviously be only a secondary feature. In colubroid snakes, the process of evolution was accompanied by extensive modifications to the venomous gland, as well as the growth and development of specialized venom-delivering apparatus. The wonderful adaptive radiation of these snakes brought about venom-specialist families, namely in two basic parallel lineages: the solenoglyphous (viperids, Viperidae) and proteroglyphous (elapid snakes, Elapidae). Since the Miocene period, these two families have presented two successful alternative approaches in maximizing the potential of snake venom.
- VENOMOUS SNAKES & ENVENOMING THERAPY. By JIRI VALENTA
Nova Science Publishers, Inc.
New York