From fossil evidence alone the question of whether or not dinosaurs cared for their young is very difficult to answer. Because behaviors are not preserved in the fossil record, we can only make inferences from indirect evidence. Parental care can be divided into two types of behavior: prehatching (building nests and incubating eggs—for example, sitting on top of them so as to warm the eggs and encourage hatching) and posthatching (feeding the young and guarding the nests). Most of our evidence comes from alleged dinosaur rookeries (places where nests are built). Several have been excavated in eastern Montana, where a large concentration of dinosaur nests was found at a place now called Egg Mountain. Most of these probably belonged to the hadrosaur Maiasaura. Preserved in these nests are the bones of baby dinosaurs. The finds at Egg Mountain and other sites around the world document that dinosaurs laid their eggs in nests.

The nests at Egg Mountain are reported to be equally spaced, separated by a space corresponding to the length of an adult Maiasaura. From this arrangement scientists have inferred that the nests were separated in this way to allow incubation in a tightly packed nesting colony. Although this interpretation is open to challenge, the discovery of Oviraplor adults on top of Oviraplor egg clutches (as determined by embryos in some eggs), is relatively powerful evidence that at least these dinosaurs incubated their nests.

Evidence for parental care following hatching is much more controversial. Behavioral speculation based on indirect fossil evidence is dangerous because the data is not always as unambiguous as might appear. At Egg Mountain, many nests contain baby dinosaur bones. Not all the dinosaurs in the nest are the same size. Many of the small bones found in the nests are associated with jaws and teeth, teeth that show signs of wear. It seems reasonable to assume that the wear was caused by the chewing of the coarse plants that were the hatchlings’ diet. Because the young were still in the nest, this food may have been brought to the rookery by foraging adults. This line of reasoning suggests that these animals had an advanced system of parental care. A closer look at the evidence clouds this interpretation. Analysis of dinosaur embryos indicates that worn surfaces are present on the teeth of juveniles even before hatching. Just as a human baby moves inside the mother before birth, modern-day archosaurs also grind their teeth before birth, wearing the surface in some spots. Thus, the fossil evidence for an advanced parental care system in extinct dinosaurs is suggestive but inconclusive, and it is hard even to imagine the sort of paleontologic discovery that could settle this debate for good.

The strongest evidence that extinct dinosaurs had some form of advanced parental care system is based on an understanding of the phylogenetic relationships among dinosaurs and their closest living relatives. Living dinosaurs (birds), even primitive ones such as ostriches and kiwis, exhibit parental care, so some form of parental care can be inferred to have existed in the last common ancestor of all birds. Although unappreciated, crocodiles are reptiles that are also caring parents. They build nests, guard the nests, and in some cases dig their young out of the nest when they hear the chirping young ones hatching. The young even communicate with each other while still in the egg by high-frequency squeaks (as birds do). Some evidence suggests that this squeaking is a cue for the synchronization of the hatching. Since birds and crocodiles share a common ancestor, the simplest explanation for the characteristics they share (such as nest building and some form of parental care) is that they evolved only once—that these attributes were present in their common ancestor and passed on to its descendants. Because extinct dinosaurs also descended from that ancestor, the simplest and most general theory is that extinct dinosaurs also shared these characteristics, even though they cannot be directly observed, and we cannot be sure how elaborate their parental care was.

仅从化石证据就可以判断恐龙是否关心它们的幼崽，这是很难回答的。因为它们的行为没有保存在化石记录中，我们只能从间接证据中作出推论。亲代抚育可以分为两种行为：预先孵化（建巢和孵化蛋），例如，坐在它们的顶部以温暖蛋并鼓励孵化）和后孵化（喂养幼龙和守巢）。我们的大多数证据都来自恐龙化石（筑巢的地方）。在蒙大拿州东部，在现在称为蛋山的地方人们一处巨大的恐龙巢穴，其中大部分可能属于鸭嘴龙。这些巢穴中保存着的是幼年恐龙的骨头。在蛋山和世界各地的其他地方发现了恐龙在巢穴中放置蛋的情况。 据报道，蛋山的巢穴间距相等，与成年慈母龙的长度相对应。根据这种安排，科学家们已经推断，巢穴以这种方式分离，以便在一个紧密拥挤的筑巢区孵化。尽管这种解释是有挑战性的，但是在卵翼蛋的顶端发现了Oviraplor（由一些卵中的胚胎所决定），这是一个相对有力的证据，证明至少这些恐龙在它们的巢穴中孵化。 孵化后父母照顾的证据方面也更具争议性。基于间接化石证据的行为推测是危险的，因为数据并不总是像可能出现的那样明确。在蛋山，许多巢包含小恐龙骨骼，但并非巢中所有的恐龙都是相同的大小。在巢穴中发现的许多小骨头都与颌骨和牙齿相关，这些牙齿显示出磨损的迹象。假设这种磨损是由于幼体咀嚼的粗糙的植物的饮食引起的，似乎是合理的。因为年轻的龙还在窝里，所以这些食物可能已经被成年龙觅食了。这一推理表明，这些动物拥有先进的父母照护系统。仔细观察证据可以发现这种解释。人们对恐龙胚胎的分析表明，即使在孵化之前，幼体牙齿上也存在磨损的表面。正如一个人类婴儿在出生前在母亲身体内移动一样，现代的恐龙在出生前也会磨牙，在某些地方裸露着。因此，在灭绝的恐龙中，一个先进的亲代抚育系统的化石证据是有启发性的，但却没有定论，甚至很难想象那种可以解决这场争论的古生物学发现。 灭绝的恐龙拥有某种形式的先进的亲代抚育系统，这是基于对恐龙及其近亲之间的进化关系的理解。现存的恐龙（鸟类），甚至是象鸵鸟和基瓦这样的原始动物，都表现出了父母的关怀，因此某种形式的父母关怀可以被推断是存在于所有鸟类的最后共同祖先的。虽然不受赏识，但鳄鱼也是抚养父母的爬行动物。他们建筑巢穴，守卫巢穴，并在某些情况下，当他们听到唧唧喳喳的年轻人孵化时，将他们的幼鸟从窝里挖出来。年轻人甚至在高频率的吱吱声中（如鸟类）仍然在鸡蛋中相互交流。一些证据表明，这种吱吱声是孵化同步的一个提示。由于鸟类和鳄鱼有着共同的祖先，对它们共有的特征（如筑巢和某种形式的父母照顾）的最简单的解释是它们只发展一次 - 这些特征出现在它们共同的祖先中，并传递给它的共同祖先后人。由于已灭绝的恐龙也来自该祖先，所以最简单和最普遍的理论是，即使不能直接观察到已灭绝的恐龙也具有这些特征，我们无法确定他们的亲代抚育的复杂程度