Reptiles - reptile

Japanese: 爬虫類 - はちゅうるい（英語表記）reptile

A general term for cold-blooded animals that belong to the phylum Reptilia and are covered with horny body scales. This group, Reptilia, has the following history:

[Takahiro Matsui]

Lineage and evolution

Reptiles differentiated from amphibians at the end of the Paleozoic era, and developed dry skin and limbs, laying shelled amniotic eggs (eggs that produce an amniotic membrane during development) on land, and thus moved onto land. Eventually, in the Mesozoic era, they made great advances, and built a period of prosperity known as the Age of Reptiles. The first ancestor of reptiles was the gosaurids such as Limnoscelis , which appeared in the Permian period, and already had a complete reptilian skeleton. It is thought that their origin was the Seymouria , which had both amphibian and reptile characteristics. From the Jurassic to the Cretaceous periods, they differentiated into many groups, including large dinosaurs, and expanded their habitats not only to land but also to the sea and sky. However, at the end of the Mesozoic era, they suddenly declined for unknown reasons, and instead, mammals and birds, which branched off from reptiles, began to emerge. There are approximately 6,000 extant species of reptiles, including turtles, tuatara, lizards, snakes, and crocodiles, and they are distributed all over the world except Antarctica.

[Takahiro Matsui]

Morphological characteristics

The body surface is covered with scales or keratinous skin, has few secretory glands, and is dry. The scales are the keratinized surface layer of the epidermis, and many also contain bony osteoderms. Many arboreal lizards, including chameleons, have effective protective coloration that utilizes changes in body color caused by the contraction and expansion of chromatophores in the dermis. Molting is the shedding of the keratinous epidermis as the lizard grows; in snakes, the entire body is shed at once, in lizards, partially, and gradually in turtles and crocodiles. The skeleton is more rigid than that of amphibians, and the skull is articulated with the cervical vertebrae at a single occipital condyle (a depression at the back of the head), allowing the head to move quite freely. The structure of the skull, especially the number and position of the temporal fenestra (openings behind the eyes), are important factors in classification, and the whole is divided into six or seven groups (subclasses). Of these, only the following three groups contain extant species; all others are extinct fossil species.

[Takahiro Matsui]

Extant species

(1) Anapsida: This is the most primitive group, and includes the gobysaurs. The skull is made up of a few strong bones, there is no temporal fenestra, and the temporal region is completely covered. The only extant species is the Testudinida, and the ancestral type with a carapace appeared in the Triassic period. The carapace is a strong box shape made up of dermal bone plates, vertebrae, ribs, and clavicles, and the surface is covered with a horny carapace (scale plate), but in some places the carapace and carapace have degenerated secondarily. The upper and lower jaws are toothless, but the ancestral type had teeth on the palate. The anal cleft is parallel to the body axis. All species are oviparous, and even aquatic and marine species lay eggs on land. There are about 220 species known in 11 families, and they are broadly divided into the suborders Cryptocephali and Sclera based on the way the head and neck are retracted.

(2) Lepidosauria: Generally, there are two temporal fenestrae, one above and one below, and the body surface is covered with keratinized fine scales. The anal cleft is perpendicular to the body axis. The order Tuatarachothala, which differentiated and flourished in the Early Triassic Period, has only one species, the family Tuatara, surviving in New Zealand today. The order Squamata currently consists of about 3,400 species in 23 families of Lacertilia and about 2,500 species in 11 families of Serpentes, with only one temporal fenestra that is incomplete at the bottom, and the lower jaw is loosely articulated to the skull via the stapedium, allowing the mouth to open wide.

(3) Archosauria: These subclasses have two temporal fenestrae, one above the other, and anal clefts parallel to the body axis. They include representative large dinosaurs, and extant species are 23 species in two families of crocodilians, which evolved from the order Alcyonidae. They are classified into the families Alligatoridae and Crocodylidae due to differences in tooth arrangement and scales. Many of these species are large, measuring 3 to 7 meters in length, and are the most powerful of all reptiles.

Among reptiles, poisonous snakes and a few large crocodiles cause harm to humans and livestock, but on the other hand, many species are useful for catching rodents and small mammals that are pests in agriculture and forestry. Some are used for food, materials for crafts, pets, and folk medicine.

[Takahiro Matsui]

"Shogakukan's Educational Encyclopedia 36: Amphibians and Reptiles" (1982, Shogakukan)" ▽ "Gakken's Illustrated Encyclopedia: Reptiles and Amphibians" (1973, Gakken) ▽ "Nakamura Kenji and Ueno Shunichi's "The Colored Illustrated Encyclopedia of Japanese Amphibians and Reptiles" (1963, Hoikusha)"

Homeosaurus maximiliani (Reptile)
Meyer, Late Jurassic period, body length approx. 37 cm, from Eichstatt, Germany. Photo courtesy of the Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (GSJ F7664)

Homeosaurus maximiliani (Reptiles)

Aldabra Giant Tortoise
Smithsonian Institution ">

Aldabra Giant Tortoise

Caiman Lizard
Smithsonian Institution (Chris Wellner)

Caiman Lizard

Mela Chameleon
Genus Chameleon Smithsonian Institution (Jennifer Zoon) ">

Mela Chameleon

King Cobra
Smithsonian Institution ">

King Cobra

American Crocodile
Smithsonian Institution ">

American Crocodile

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

脊椎(せきつい)動物門爬虫綱に属する、角質の体鱗(たいりん)に覆われた変温動物の総称。この仲間Reptiliaは次のような歴史を有する。

［松井孝爾］

系統と進化

爬虫類は古生代末期に両生類から分化し、乾燥に強い皮膚と発達した四肢を備え、卵殻をもつ有羊膜卵（発生過程で羊膜を生ずる卵）を陸に産むことで、陸上に進出した。やがて、中生代には大発展を遂げて、爬虫類時代とよばれる繁栄を築きあげた。爬虫類の最初の祖先型は、ペルム紀（二畳紀）に出現したリムノスケリスLimnoscelisなどの杯竜類(はいりゅうるい)で、すでに爬虫類型の骨格を完成しており、その起源は、両生類と爬虫類の両方の特徴をもつシームリアSeymouria類と考えられている。ジュラ紀から白亜紀にかけて、大形恐竜類を含む多数の群に分化し、陸ばかりでなく海、空にまで生活圏を拡張していった。しかし中生代末期には不明の原因で急に衰微し、かわって爬虫類から分岐した哺乳(ほにゅう)類、鳥類が台頭するようになった。爬虫類の現生種はカメ、ムカシトカゲ、トカゲ、ヘビ、ワニの総計約6000種で、南極を除く世界の各地に分布する。

［松井孝爾］

形態的特徴

体表は鱗(うろこ)または角質の皮膚に覆われ、分泌腺(せん)が乏しくて乾燥している。体鱗は表皮の表層が角質化したもので、さらに骨質の皮骨を含むものも少なくない。カメレオンをはじめ樹上性のトカゲ類には、真皮内の色素胞の収縮・拡張による体色変化を利用した、効果的な保護色をもつものが多い。脱皮は、成長に伴って表皮の角質部が脱落するもので、ヘビでは全身が一度に、トカゲでは部分的に、カメやワニでは徐々に行われる。骨格は両生類よりもさらに硬骨化し、頭骨はただ1個に減った後頭顆(こうとうか)（後頭部にあるくぼみ）で頸椎(けいつい)と関節するため、頭部はかなり自由に動く。頭骨の構造、とくに側頭窓（目の後方にある開口部）の数や位置は分類上重要な要素であり、全体が6ないし7群（亜綱）に分類される。そのうち現生種が含まれるのは次の3群のみで、他はすべて絶滅した化石種である。

［松井孝爾］

現生種

(1)無弓亜綱Anapsida　もっとも原始的な一群で杯竜類も含まれる。頭骨は堅固な少数の骨で構成され、側頭窓がなく側頭部は完全に覆われている。現生種はカメ目だけで、甲を備えた祖先型はすでに三畳紀に出現していた。甲は、皮骨である骨板と脊椎骨、肋骨(ろっこつ)、鎖骨が結合した堅固な箱形で、表面は角質の甲板(こうばん)（鱗板）で覆われるが、一部では二次的に甲や甲板が退化している。上下のあごとも歯を欠くが、祖先型では口蓋(こうがい)に歯を残していた。肛門裂(こうもんれつ)は体軸に平行である。すべて卵生で、水生・海生種も陸で産卵する。11科約220種が知られ、頭頸部の引っ込め方で、潜頸亜目と曲頸亜目に大別される。

(2)有鱗亜綱Lepidosauria　原則として上下2個の側頭窓があり、体表は角質化した細鱗で覆われる。肛門裂は体軸に直角である。三畳紀初期に分化して栄えた喙頭目(かいとうもく)は、現在ムカシトカゲ科の1種がニュージーランドに生存するだけである。有鱗目で現存するのは、トカゲ亜目の約23科3400種とヘビ亜目の約11科2500種で、側頭窓は下方で不完全となって1個しかなく、下あごは方骨を介して頭蓋に緩く関節するため、口を大きく開くことができる。

(3)鰐形亜綱(がくけいあこう)Archosauria　上下2個の側頭窓をもち、肛門裂は体軸に平行である。代表的な大形恐竜を含み、現生種は槽歯目から分化したワニ目の2科23種である。歯並びや鱗板の違いで、アリゲーター科とクロコダイル科に分類される。全長3～7メートルの大形種が多く、爬虫類ではもっとも強力な存在である。

　爬虫類は、毒ヘビと少数の大形ワニが人畜に被害を与えるが、反面、多くの種が農林業の害獣であるネズミ類や小哺乳類をとらえて、役だっている。一部が食用、細工物の材料、ペット、民間薬用に供せられる。

［松井孝爾］

『『小学館の学習百科図鑑36　両生・はちゅう類』（1982・小学館）』▽『『学研の図鑑　爬虫・両生類』（1973・学習研究社）』▽『中村健児・上野俊一著『原色日本両生爬虫類図鑑』（1963・保育社）』

ホメオサウルス・マキシミリアニ（爬虫類）
Meyer　中生代ジュラ紀後期　体長約37cm　ドイツ　アイヒシュタット産写真提供／産業技術総合研究所地質調査総合センター（GSJ　F7664）">

ホメオサウルス・マキシミリアニ（爬虫類…

アルダブラゾウガメ
スミソニアン協会">

アルダブラゾウガメ

カイマントカゲ
スミソニアン協会（Chris Wellner）">

カイマントカゲ

メラ―カメレオン
　カメレオン属スミソニアン協会（Jennifer Zoon）">

メラ―カメレオン

キングコブラ
スミソニアン協会">