일반명 : 직선형앵무조개(straight nautiloids)

학명 : Orthoceras sp.

지질시대 : 고생대데본기(Devonian,395-345 million years ago)

크기 : 117 mm in length on 126 mm by 35 mm matrix (visible)
원산지 : Atlas Mountain Range, South Morocco (Northern Sahara Desert)

직선형 두족류는 화석에서는 두가지를 의미한다. 직선형암모나이트(straight ammonoids)를 바큘라이트(Baculites)라 부르고, 직선형앵무조개(straight nautiloids)는 오소세라스(Orthoceras), 초아강(Coleoidea)두족류를 벨렘나이트라 한다.

두족류는 후기캄브리아기부터 진화하기 시작했다. 이들의 몸들은 원뿔의 조개 형태로 존재했다. 이들 중 일부는 반쯤감긴(semi-coiled)모습과 감긴모습(coiled)의 암모나이트와 앵무조개로 진화했습니다.

The term 'straight' cephalopods is used here to describe fossil examples of straight ammonoids called Baculites, straight nautiloids called Orthoceras and Belemnites.  

Cephalopods evolved during the Late Cambrian Period.  Their bodies were predominantly elongate with conical shells.  Some of these creatures evolved into semi-coiled forms eventually giving rise to coiled cephalopods like ammonites and nautili. 

Another branch of straight-shelled cephalopods continued to coexist with the coiled forms on into the Pennsylvanian Period.  These straight forms were much less diverse and abundant than the coiled cephalopods, as a whole.  

Straight cephalopods were among the most advanced invertebrates of their time having eyes, jaws, and a sophisticated nervous system.  These creatures were predators that swam freely using a jet propulsion system by squirting water from their bodies.  They had tentacles and ink sacs also much like the present-day squid.

Except for belemnites, cephalopods had external shells with hollow internal chambers separated by walls called septa.  A tube called the siphuncle, connected the body with the chambers allowing the animal to fill them with water or air, changing its buoyancy in order to rise or drop in the ocean.  Only the last and largest chamber was occupied by the living animal.  Belemnites were different in that they had internal shells called 'guards' which were covered with the soft, muscular tissues of their bodies.  These shells were also chambered but much less complex than the straight varieties of nautiloids and ammonoids.