The ontogenetic trajectories of septal spacing between succeeding chambers of two phylloceratid ammonoids, Hypophylloceras subramosum and Phyllopachyceras ezoense, from the Haboro and Kotanbetsu areas, north‐western Hokkaido, Japan, were analysed. The ontogenetic trajectories of septal spacing of H. subramosum demonstrate a general trend with large intraspecific variation: two cycles of increasing to decreasing spacing followed by almost constant spacing. The large intraspecific variation can be subdivided into three types, making this species polymorphic. The ontogenetic trajectories of septal spacing of P. ezoense also show intraspecific variation, but with a different trend: one cycle of increasing to decreasing spacing and an increasing trend after that. The intraspecific variation can be subdivided into two types, which suggests that this species is dimorphic, possibly sexually dimorphic. Dimorphism is supported by two observations: (1) the difference in the ontogenetic trajectories of septal spacing is only seen in the later ontogenetic stages; and (2) the two types co‐occur with comparable abundance throughout all stratigraphic horizons. Detailed analyses of ontogenetic trajectories of septal spacing may reveal polymorphism in other ammonoid clades.

中文翻译：

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晚白垩世叶状体铵类动物的多态性：间隔间隔的发生轨迹的证据。

分析了来自日本北海道西北部Haboro和Kotanbetsu地区的两个叶状菊科动物类氨化物次小亚种（Hypophylloceras subramosum）和Phyllopachyceras ezoense的后房间隔的发生轨迹。次生间隔壁的个体发育轨迹显示出种内变异大的总体趋势：间隔从增大到减小的两个周期，随后是几乎恒定的间隔。种内变异较大，可分为三种类型，使该种具有多态性。E. ezoense间隔的发育轨迹。也显示种内变异，但趋势不同：一个周期从增大到减小的间隔，然后是增大的趋势。种内变异可以分为两种类型，这表明该物种是双态的，可能是有性的。二态性得到两个观察结果的支持：（1）间隔间隔的个体发育轨迹的差异仅在个体发育的后期才可见；（2）两种类型在所有地层范围内都以相当的丰度同时出现。间隔间隔的发生轨迹的详细分析可能揭示其他氨类进化枝的多态性。