{"id":2544,"date":"2017-02-06T21:30:15","date_gmt":"2017-02-06T21:30:15","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&#038;p=2544"},"modified":"2024-04-26T00:47:02","modified_gmt":"2024-04-26T00:47:02","slug":"evolution-of-amniotes","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-biology2\/chapter\/evolution-of-amniotes\/","title":{"raw":"Evolution of Amniotes","rendered":"Evolution of Amniotes"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Discuss the evolution of amniotes<\/li>\r\n<\/ul>\r\n<\/div>\r\nThe first amniotes evolved from amphibian ancestors approximately 340 million years ago during the Carboniferous period. The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids and sauropsids.\u00a0<b>Synapsids<\/b> include all mammals, including extinct mammalian species. Synapsids also include therapsids, which were mammal-like reptiles from which mammals evolved. <b>Sauropsids<\/b> include reptiles and birds, and can be further divided into anapsids and diapsids. The key differences between the synapsids, anapsids, and diapsids are the structures of the skull and the number of temporal fenestrae behind each eye (Figure 1).\r\n\r\n[caption id=\"attachment_2549\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-2549\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06212922\/Figure_29_04_02-1024x217.jpg\" alt=\"The illustration compares three different skull types. All three skulls are elongated and similar in shape; the only difference between them is the number of holes behind the eye. The anapsid skull (left) has no openings. The synapsid skull (middle) has one opening, and the diapsid skull (right) has two openings, one on top of the other.\" width=\"1024\" height=\"217\" \/> Figure 1.\u00a0Compare the skulls and temporal fenestrae of anapsids, synapsids, and diapsids. Anapsids have no openings, synapsids have one opening, and diapsids have two openings.[\/caption]\r\n\r\n<b>Temporal fenestrae<\/b> are post-orbital openings in the skull that allow muscles to expand and lengthen. <b>Anapsids<\/b> have no temporal fenestrae, synapsids have one, and <b>diapsids<\/b> have two. Anapsids include extinct organisms and may, based on anatomy, include turtles. However, this is still controversial, and turtles are sometimes classified as diapsids based on molecular evidence. The diapsids include birds and all other living and extinct reptiles.\r\n<p id=\"fs-idp80739120\">The diapsids in turn diverged into two groups, the\u00a0<em data-effect=\"italics\">Archosauromorpha<\/em>\u00a0(\u201cancient lizard form\u201d) and the\u00a0<em data-effect=\"italics\">Lepidosauromorpha<\/em>\u00a0(\u201cscaly lizard form\u201d) during the Mesozoic period (Figure 2). The\u00a0<span id=\"term1195\" data-type=\"term\">lepidosaurs<\/span>\u00a0include modern lizards, snakes, and tuataras. The\u00a0<span id=\"term1196\" data-type=\"term\">archosaurs<\/span>\u00a0include modern crocodiles and alligators, and the extinct ichthyosaurs (\u201cfish lizards\u201d superficially resembling dolphins), pterosaurs (\u201cwinged lizard\u201d), dinosaurs (\u201cterrible lizard\u201d), and birds. (We should note that clade Dinosauria includes birds, which evolved from a branch of maniraptoran theropod dinosaurs in the Mesozoic.)<\/p>\r\n<p id=\"fs-idp80730219\">The evolutionarily derived characteristics of amniotes include the amniotic egg and its four extraembryonic membranes, a thicker and more waterproof skin, and rib ventilation of the lungs (ventilation is performed by drawing air into and out of the lungs by muscles such as the costal rib muscles and the diaphragm).<\/p>\r\n\r\n\r\n[caption id=\"attachment_2550\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-2550\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06212953\/Figure_29_04_03-1024x636.png\" alt=\"The trunk of the amniote phylogenetic tree is the ancestral amniote. Initially, the tree branches into diapsids, anapsids, and synapsids. Synapsids give rise to mammals, which are therapsids. Anapsids are all extinct. Diapsids are subdivided into two groups, lepidosaurs and archosaurs. Lepidosauria includes plesiosaurs, ichthyosaurs, Sphenodontia and Squamata, which includes lizards and snakes. Archosauria branches into Crocodilia, pterosaurs, dinosaurs, and birds.\" width=\"1024\" height=\"636\" \/> Figure 2.\u00a0This chart shows the evolution of amniotes. The placement of Testudines (turtles) is currently still debated.[\/caption]\r\n\r\n<section id=\"fs-idp190324400\" data-depth=\"1\">\r\n<p id=\"fs-idp176720736\">In the past, the most common division of amniotes has been into the classes Mammalia, Reptilia, and Aves. However, both birds and mammals are descended from different amniote branches: the synapsids giving rise to the therapsids and mammals, and the diapsids giving rise to the lepidosaurs and archosaurs. We will consider both the birds and the mammals as groups distinct from reptiles for the purpose of this discussion with the understanding that this does not accurately reflect phylogenetic history and relationships.<\/p>\r\n\r\n<\/section><section id=\"fs-idp67260192\" data-depth=\"1\"><\/section>\r\n<div class=\"textbox exercises\">\r\n<h3>Practice Question<\/h3>\r\nMembers of the order Testudines have an anapsid-like skull with one opening. However, molecular studies indicate that turtles descended from a diapsid ancestor. Why might this be the case?\r\n\r\n[practice-area rows=\"2\"][\/practice-area]\r\n[reveal-answer q=\"540837\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"540837\"]The ancestor of modern Testudines may at one time have had a second opening in the skull, but over time this might have been lost.[\/hidden-answer]\r\n\r\n<\/div>\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/635a0f34-32c6-4d21-9219-117ba49db97a\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Discuss the evolution of amniotes<\/li>\n<\/ul>\n<\/div>\n<p>The first amniotes evolved from amphibian ancestors approximately 340 million years ago during the Carboniferous period. The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids and sauropsids.\u00a0<b>Synapsids<\/b> include all mammals, including extinct mammalian species. Synapsids also include therapsids, which were mammal-like reptiles from which mammals evolved. <b>Sauropsids<\/b> include reptiles and birds, and can be further divided into anapsids and diapsids. The key differences between the synapsids, anapsids, and diapsids are the structures of the skull and the number of temporal fenestrae behind each eye (Figure 1).<\/p>\n<div id=\"attachment_2549\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2549\" class=\"size-large wp-image-2549\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06212922\/Figure_29_04_02-1024x217.jpg\" alt=\"The illustration compares three different skull types. All three skulls are elongated and similar in shape; the only difference between them is the number of holes behind the eye. The anapsid skull (left) has no openings. The synapsid skull (middle) has one opening, and the diapsid skull (right) has two openings, one on top of the other.\" width=\"1024\" height=\"217\" \/><\/p>\n<p id=\"caption-attachment-2549\" class=\"wp-caption-text\">Figure 1.\u00a0Compare the skulls and temporal fenestrae of anapsids, synapsids, and diapsids. Anapsids have no openings, synapsids have one opening, and diapsids have two openings.<\/p>\n<\/div>\n<p><b>Temporal fenestrae<\/b> are post-orbital openings in the skull that allow muscles to expand and lengthen. <b>Anapsids<\/b> have no temporal fenestrae, synapsids have one, and <b>diapsids<\/b> have two. Anapsids include extinct organisms and may, based on anatomy, include turtles. However, this is still controversial, and turtles are sometimes classified as diapsids based on molecular evidence. The diapsids include birds and all other living and extinct reptiles.<\/p>\n<p id=\"fs-idp80739120\">The diapsids in turn diverged into two groups, the\u00a0<em data-effect=\"italics\">Archosauromorpha<\/em>\u00a0(\u201cancient lizard form\u201d) and the\u00a0<em data-effect=\"italics\">Lepidosauromorpha<\/em>\u00a0(\u201cscaly lizard form\u201d) during the Mesozoic period (Figure 2). The\u00a0<span id=\"term1195\" data-type=\"term\">lepidosaurs<\/span>\u00a0include modern lizards, snakes, and tuataras. The\u00a0<span id=\"term1196\" data-type=\"term\">archosaurs<\/span>\u00a0include modern crocodiles and alligators, and the extinct ichthyosaurs (\u201cfish lizards\u201d superficially resembling dolphins), pterosaurs (\u201cwinged lizard\u201d), dinosaurs (\u201cterrible lizard\u201d), and birds. (We should note that clade Dinosauria includes birds, which evolved from a branch of maniraptoran theropod dinosaurs in the Mesozoic.)<\/p>\n<p id=\"fs-idp80730219\">The evolutionarily derived characteristics of amniotes include the amniotic egg and its four extraembryonic membranes, a thicker and more waterproof skin, and rib ventilation of the lungs (ventilation is performed by drawing air into and out of the lungs by muscles such as the costal rib muscles and the diaphragm).<\/p>\n<div id=\"attachment_2550\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2550\" class=\"size-large wp-image-2550\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/06212953\/Figure_29_04_03-1024x636.png\" alt=\"The trunk of the amniote phylogenetic tree is the ancestral amniote. Initially, the tree branches into diapsids, anapsids, and synapsids. Synapsids give rise to mammals, which are therapsids. Anapsids are all extinct. Diapsids are subdivided into two groups, lepidosaurs and archosaurs. Lepidosauria includes plesiosaurs, ichthyosaurs, Sphenodontia and Squamata, which includes lizards and snakes. Archosauria branches into Crocodilia, pterosaurs, dinosaurs, and birds.\" width=\"1024\" height=\"636\" \/><\/p>\n<p id=\"caption-attachment-2550\" class=\"wp-caption-text\">Figure 2.\u00a0This chart shows the evolution of amniotes. The placement of Testudines (turtles) is currently still debated.<\/p>\n<\/div>\n<section id=\"fs-idp190324400\" data-depth=\"1\">\n<p id=\"fs-idp176720736\">In the past, the most common division of amniotes has been into the classes Mammalia, Reptilia, and Aves. However, both birds and mammals are descended from different amniote branches: the synapsids giving rise to the therapsids and mammals, and the diapsids giving rise to the lepidosaurs and archosaurs. We will consider both the birds and the mammals as groups distinct from reptiles for the purpose of this discussion with the understanding that this does not accurately reflect phylogenetic history and relationships.<\/p>\n<\/section>\n<section id=\"fs-idp67260192\" data-depth=\"1\"><\/section>\n<div class=\"textbox exercises\">\n<h3>Practice Question<\/h3>\n<p>Members of the order Testudines have an anapsid-like skull with one opening. However, molecular studies indicate that turtles descended from a diapsid ancestor. Why might this be the case?<\/p>\n<p><textarea aria-label=\"Your Answer\" rows=\"2\"><\/textarea><\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q540837\">Show Answer<\/span><\/p>\n<div id=\"q540837\" class=\"hidden-answer\" style=\"display: none\">The ancestor of modern Testudines may at one time have had a second opening in the skull, but over time this might have been lost.<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_635a0f34-32c6-4d21-9219-117ba49db97a\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/635a0f34-32c6-4d21-9219-117ba49db97a?iframe_resize_id=assessment_practice_id_635a0f34-32c6-4d21-9219-117ba49db97a\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-2544\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology 2e. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":15,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology 2e\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction\"}]","CANDELA_OUTCOMES_GUID":"107c3237-6b9d-4628-a412-ab2cce78b501, b2f97a3a-848e-4435-be48-009a3c8b5412","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-2544","chapter","type-chapter","status-publish","hentry"],"part":148,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/2544","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":10,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/2544\/revisions"}],"predecessor-version":[{"id":8470,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/2544\/revisions\/8470"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/parts\/148"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/2544\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/media?parent=2544"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapter-type?post=2544"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/contributor?post=2544"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/license?post=2544"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}