**키노돈트**(Cynodont), 또는 **키노돈아목**(Cynodontia)은 페름기에 처음 나타난 수궁류 분류군이며 여기에는 현대 포유류와 포유류의 멸종된 공통 조상과 그 친척들이 속한다. 비포유류형 키노돈트는 남쪽 곤드와나 대륙 전체에 퍼져 있었으며 남아공, 아프리카, 인도, 남극 등지에서 화석이 발견되고 북부 대륙에서는 북아메리카, 벨기에, 프랑스 북서부 지방에서 발견된다. 키노돈아목은 수궁류 중에 가장 다양성 있다고 여겨지는 분류궁 중 하나이다.

분류
--

프로키노수쿠스 엑사에레토돈디아데모돈트리틸로돈 올리고키푸스

  

  

리처드 오웬은 1861년에 **키노돈아목**(Cynodontia)라는 학명를 명명하고 키노돈트를 **아노모돈아목**에 분류했었다. 그러나 **에버렛 올슨** (Everett Olson)은 키노돈트를 **테리오돈트류**(Theriodontia)로, **부루스 루비지**(Buruce Rubidgi)와 **크리스티안 시더**(Cristian Sidor)는 키노돈트를 **진테리오돈트류**(Eutheriodontia)로 분류했었다. 윌리엄 킹 그레고리(1910), 로버트 브룸(1913), 캐럴(1988), 고티에(1989), 제임스 홉슨과 키칭(2001), 보타(2007) 모두 키노돈아목을 수궁목 바로 아래에 분류했다. 브링크(2007)는 오웬(1861)의 분류법을 따랐지만 분류 계급을 지정하지 않은 것으로 보인다.

다음은 키노돈트의 분류에 대한 연구 결과를 참고해서 만든 분지도이다.


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특징
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### 골격

프로키노수쿠스의 골격
초기 키노돈트는 포유류의 골격적 특징을 많이 가지고 있었다. 치아가 완전히 분화되어 있었고 뇌관이 불룩하게 변했고 일부 왕관군 포유류(를 제외한 모든 키노돈트는 알을 낳았었을 것이다. 특히 측두창은 다른 수궁류보다 컸고, 포유류의 두개골의 광대뼈 확대는 비교적 강한 턱 근육을 가지게 했을 것이다. 그리고 테로세팔루스류를 제외한 다른 원시 수궁류에게는 없었던 2차 구개를 가지고 있었다.

### 털

키노돈트는 항온 동물로 추정되는데 이로 인해 키노돈트가 털을 가진 것으로 추측하기도 한다. 하지만 이들의 털에 대한 화석 증거는 찾기 어려울 뿐더러 현대 포유류는 털을 덮기 위해 지방질을 분비하는 **하르더샘**을 가지고 있지만, 하르더샘은 원시 포유류 **모르가누코돈**과 그 이후의 후손에게서만 발견된다. 그런데 페름기 단궁류의 대변에 대한 최근 연구는 많은 초기 수궁류가 털을 가졌을 수 있다는 사실이 밝혀지게 했으며 카스토로카우다(Castorocauda)와 메가코누스(Megaconus) 같은 초기 포유류에게 이미 털이 존재했음을 보여준다.

진화
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### 비포유류형 키노돈트

#### 등장

키노돈트는 약 2억 6500만 년 전에 처음 등장했다. 이들이 처음 등장했을 때는 여타 수궁류의 특징을 많이 가지고 있었으나 포유류의 특징(2차 구개 등등)도 어느 정도 가지고 있었다. 이는 키노돈트와 가까운 **테로세팔루스류**(Therocephalia)에게도 있는 특징이다.

#### 초기

초기 키노돈트 중 하나인 트리낙소돈
가장 초기의 키노돈트는 차라소그나투스과에 속하는데 이들은 주로 남아공의 카루 누층군의 퇴적물에서 발견된다. 페름기 후기 키노돈트의 화석은 남아프리카 이외의 지역에서는 비교적 드물게 발견된다. 이 중 가장 널리 분포하는 프로키노수쿠스는 남아공, 독일, 탄자니아, 잠비아, 러시아 등 다양한 곳에서 발견된다.

#### 번성

키노돈트는 페름기 대멸종 이후 급속도로 다양해졌다. 특히 트라이아스기 초기에는 비교적 진보된 진키노돈트류(Eucynodontia)가 지배했는데 여기에는 주로 초식성이었던 키노그나투스류(Cynognathia)와 주로 육식성이었던 프로바이노그나투스류(Prbainognathia)가 주요 하위 분류군으로 트라이아스기 육상 생태계를 지배하고 있었다. 트라이아스기 초-중기에는 키노그나투스류가 다양했었는데 반대로 프로바이노그나투스류는 트라이아스기 후기까지는 크게 번성하지 못했다. 거의 모든 트라이아스기의 키노돈트는 곤드와나 대륙에 널리 퍼져있었는데 당시 북반구(로라시아 대륙)에서는 단 하나의 속(나노곰포돈,Nanogomphodon)만이 발견된다. 트라이아스기 중-후기에 눈에 띄게 번성했던 키노돈트는 당시 곤드와나에 주로 살았던 초식성 키노돈트 트레버소돈과(Traversodontidae)에 속했었다.

### 포유류

#### 포유류의 등장

포유류는 트라이아스기 후기에 프로바이노그나투스류에서 진화한 곤충을 먹는 작은 동물이었다.

#### 최후의 비포유류형 키노돈트

한편으로 트라이아스기 말 이후에는 두 형태의 비포유류 키노돈트만이 존재했으며, 둘 다 프로바이노그나투스류에 속했다. 첫 번째는 식충성이었던 트리틸로돈으로 쥐라기 초기에 멸종했다. 두 번째는 중국의 포시오마누스로, 약 1억 2천만년전, 즉 백악기 전기까지 번성했다. 이는 쥐라기 후기에 처음 등장한 초식성 키노돈트이다.

#### 턱의 변화

비포유류형 키노돈트가 진화하는 동안 포유류의 턱 뼈 수는 줄어들고 있었다. 하악골이 안쪽을 향해 이동했고 턱에 있는 다른 뼈들의 관절과 각이 두개골로 이동할 수 있는 길을 열었다. 이렇게 등장한 새로운 형태의 두개골은 포유류의 청각 시스템의 일부라고 할 수 있다.

또 입천장에 2차 구개가 생겼고 이는 콧구멍에서 나오는 공기 흐름이 직접적으로 통하는 대신 입 뒤쪽의 위치로 이동하게 되어 음식을 씹으면서 숨을 쉴 수 있게 되었다. 이는 지구상의 모든 포유류들이 가지고 있는 형질이다.