Data from: Re-evaluation of the ontogeny and reproductive biology of the Triassic fish Saurichthys (Actinopterygii, Saurichthyidae)

Viviparity has evolved independently at least 12 times in ray-finned
fishes. However, the fossil record of actinopterygian viviparity is poor,
with only two documented occurrences. Both of these are from the
non-teleost actinopterygian Saurichthys, and include S. curionii and S.
macrocephalus from the Middle Triassic Meride Limestone (Monte San
Giorgio, Switzerland). Here, we present new data on the reproductive
biology of these species, giving unprecedented insights into their
life-history. Based on positional and preservational criteria, six
specimens were identified as unambiguously gravid. Embryos were positioned
dorsal to the gastrointestinal tract, parallel to the axial skeleton and
to each other, in the posterior two-thirds of the abdominal region. A
minimum of 16 embryos are preserved in the most fecund females and, based
on the largest preserved embryos and smallest preserved neonates, birth
must have occurred at 7–12% of maternal fork length. Embryonic crania and
teeth are relatively well-ossified, however ossification of the parietal
region is delayed. In the postcranium, the median scale rows and
lepidotrichia are ossified, but not the lateral scale rows. Ossified
squamation and gradual allometric growth suggests that neonates did not
undergo metamorphosis and were relatively precocial. When considered in a
phylogenetic context, neither live birth nor internal fertilization
appears to represent the primitive state for saurichthyid fishes.

胎生现象在辐鳍鱼中至少独立演化了12次。然而，辐鳍鱼类胎生的化石记录十分匮乏，仅记载有两例。这两例均来自非硬骨辐鳍鱼类的龙鱼属（Saurichthys），包括中三叠世梅里德石灰岩（瑞士圣乔治山）中的库里奥龙鱼（S. curionii）和大头龙鱼（S. macrocephalus）。本文报道了这些物种繁殖生物学的新数据，为其生活史提供了前所未有的见解。基于位置和保存特征，6件标本被明确鉴定为妊娠个体。胚胎位于胃肠道背侧，与轴骨及彼此平行，分布于腹部区域的后三分之二处。繁殖力最强的雌性个体中至少保存了16个胚胎；根据最大的保存胚胎和最小的幼体推算，出生时幼体的叉长约为母体叉长的7%至12%。胚胎的颅骨和牙齿骨化程度较高，但顶骨区域的骨化延迟。颅后骨骼中，中侧鳞列和鳍条已骨化，而侧鳞列尚未骨化。骨化的鳞被和渐进的异速生长表明，幼体未经历变态过程，属于相对早成的类型。从系统发育背景来看，胎生和体内受精均不代表龙鱼科鱼类的原始状态。