Ecologist - Writer - Photographer

Ancient giant flying reptiles cared for their young

Dimorphodon macronyx, a Rhamphorhynchoid Pterosaur
The Blog below is an extended version of the article, Why we think that some extinct giant flying reptiles cared for their young, published in The Conversation. The Blog explores the implications of a scientific paper looking at limb bone growth allometry in pterosaurs. Patterns of growth indicate that giant pterosaurs experienced positive growth allometry, which in living species of bird and mammal, is associated with altricial development (inability to move independently at hatch or birth, and greater dependence on adults, often via parental care). In contrast, small pterosaurs show isometry or negative allometry in proximal limb bone growth. In extant birds and mammals this is associated with precocial development - an ability to move independently shortly after hatching or birth. So, giant pterosaurs may have been able to grow so large because of their growth pattern, which may have been facilitated by altricial development, via parental care.

You can view the scientific paper, Allometric wing growth links parental care to pterosaur giantism, by Yang et al 2023 here.

Ancient giant flying reptiles cared for their young

We all have a perspective on the world around us. And how the creatures that inhabit it look, and act. We get it from our own experiences, and knowledge learned from books, TV, the internet, and scientific papers. Ultimately, for extant animal species (that are still alive today), our understanding of behaviour comes from observations. We can see how animals are born into the world, how they grow and develop, what they look like, and how they behave.

Reimagining dead animals from the grave

But how and what do we know about animals that are extinct, species that died out hundreds of millions of years ago? The science of studying the animals of the past is palaeontology, a field of study that straddles geology (via rocks) and biology (mostly via anatomy).

Fossils are the remains of animals preserved in rocks, as rock. Very few individual animals from within a population ever fossilise (the odds on dying/decaying in a place with just the right set of narrow conditions are slim). And when they do, rarely are whole animals preserved in-situ, and often they are disarticulated, and even squished. Oh, and mostly it’s only the hard parts of the animal that survive as fossils, e.g. the bones (endoskeleton) of vertebrates.

So, at best, we can re-imagine what an animal looked like, right? Historically, even that was tricky, and many early representations of dinosaur skeletons were (with improved science, and hindsight) a bit off. But our knowledge and understanding has improved, and now palaeontologists and palaeoartists can give us pretty good (and bio-realistic) interpretations of (at least the) hard parts of a long extinct animal. Although, even then, there are still heated debates and disagreements on what dinosaurs and other extinct animals, large and small, looked like… For example, did dinosaurs have lips?

OK. But what about knowing how a long-extinct animal behaved? How can we possibly deduce how an animal moved, what it ate, when it was active, was it social, what its’ mating system was, whether or not it cared for its’ offspring?

On occasions, animals are fossilised in the act of behaviour. But this is rare. However, you may be surprised at how much information can be gleaned from just fossil bones. We can determine a lot about an animal’s ecological niche and life history from its’ anatomy. We know a lot from studying species alive today. And we can apply much of that knowledge to interpreting patterns within and between specimens of long-dead individuals preserved as fossils. Sometimes that requires applying Sherlock Holmes-ian science.

And that’s where I come in. I am a behavioural ecologist. I study living animals: mongooses, chimps, and spiders; amongst other things. But palaeontology holds a special pull over me. Because with palaeontology, we aim to re-imagine, to reconstruct, the anatomy and behaviour of individual animals within species, and ecosystems. And we can’t simply watch the animals in action.

Pterosaurs, allometric growth & parental care

In a scientific paper by Zixiao Yang and colleagues, scientists compared the growth of small pterosaurs, with giant pterosaurs. To try to understand what, if anything, was different about how giants got so big. In particular, they looked at limb bones, that are critical to locomotion: the forelimbs to flight, the hindlimbs to terrestrial movement.

Pterosaur (pteranodon) bone map

Pteranodon skeleton with bones labelled. Source: Kansas Fossil Hunter.

Pterosaurs are extinct flying reptiles that dominated the skies for over a 150 million years, from ~228 to 66 mya. They are not dinosaurs, but co-occurred within their land-bound 'cousin's in the Jurassic and Cretaceous geological periods. Whilst the earliest known pterosaurs were small, some evolved to become the largest animals to ever fly.

Allometric growth of limb elements, where body proportions change due to different growth rates of different parts of the body during development, has been shown in pterosaurs from numerous species.

Growth allometry of limb elements in pterosaurs - Yang et al 2023

Growth allometry of limb elements in the studied pterosaurs. Allometric growth of limbs from early juveniles of 0.3 m wingspan (right) to adults of 7 m wingspan (left); note that the adult sizes are hypothetical for all pterosaurs except Pteranodon. Skeletal reconstructions are modified from those in Wellnhofer [30], Bennett [14,32], Hone et al. [16] and Beccari et al. [42]. Allometric coefficients are indicated for the corresponding limb elements; positive and negative allometry and isometry are indicated in blue, orange and black, respectively. Figure 2 from: Allometric wing growth links parental care to pterosaur giantism. By Zixiao Yang, Baoyu Jiang, Michael J. Benton, Xing Xu, Maria E. McNamara and David W. E. Hone. Proceedings of the Royal Society B, 2023.

Yang and colleagues discovered that the large Pteranodon (represented by 2 very similar species, Pteranodon sternbergi and Pteranodon longiceps, in contrast to the smaller pterosaurs) displayed positive allometry in elements of the forelimbs and hindlimbs; as a Pteranodon matured, its’ proximal limb bones (the ones nearer the body) grew relatively faster/larger than other elements of its’ skeleton. The smaller pterosaurs, Rhamphorhynchus muensteri, Pterodactylus antiquus and Sinopterus dongi, and assorted Anurognathid species, showed negative allometry or isometry, basically the proximal limb bones got smaller (grew more slowly, or grew at same rate) relative to total body size as they aged and grew. In the study, Pteranodon had a wingspan range of 3.91–6.37m, with the smallest pterosaur, Rhamphorhynchus, coming in at just 0.19–0.74m.

Pteranodon longiceps

Pteranodon longiceps, pictured with lead discoverer, Othniel C. Marsh, for scale. Palaeoart by Mark Witton.

This pattern is associated, in birds and mammal species alive today, with developmental strategy. Extant species displaying negative allometry of limb bones tend to be precocial, meaning that they are able to move around independently when they hatch. Whilst not necessarily lacking in parental care, such species tend to be less dependent/demanding of parental care. In contrast, in living species with positive allometry of limb bones, the newborn/hatched young are not capable of independent movement, known as altricial development, and parental care is the norm – parents provision young with food during early-life development.

Altricial v Precocial birds

Altricial v Precocial birds. Source: Di (they-them)

Yang and colleagues modelled body biometrics with growth, and estimated that Pteranodon was lighter in body mass than the other pterosaur species throughout growth, with wing planform (shape) changing during development. Wing aspect ratio (wing length relative to wing area, applicable to any flying object – birds, bats, flying insects, and aeroplanes) increased with growth in Pteranodon, compared to a consistent or decreasing wing aspect ratio during growth in the smaller pterosaurs. A long narrow wing has a high wing aspect ratio, a short wide wing has a low wing aspect ratio. High wing aspect ratio is associated with a soaring lifestyle in modern birds. Low wing aspect ratio provides greater manoeuvrability. All the pterosaurs improved in flying efficiency and glide ratio (distance travelled relative to height lost) with age; whilst adults would lose height more rapidly than juveniles when gliding, they travelled faster and further.

Pterosaur wing planforms

Pterosaur wing planform & wing aspect ratio. Wing planform changes from early juveniles (0.3 m wingspan) to giant adults (7 m wingspan) using taxon-specific postures. Solid and dashed outlines indicate actual and hypothetical growth, respectively. From: Allometric wing growth links parental care to pterosaur giantism. Yang et al 2023. Supplemental Materials. Figure S1.

Pteranodon’s allometric growth divergence from smaller species of pterosaur indicates that altricial development may have provided its’ route to large adult size. Pterosaurs ultimately grew to be the largest flying animals of all time. Hatzegopteryx thambema may have been the largest, with a wingspan of up to 12m. Comparable to another famous giant, Quetzalcoatlus. In comparison, the smallest, Nemicolopterus, had a wingspan of just 25cm. But all pterosaurs started small.

Hatzegopteryx thambema - the largest pterosaur?

Hatzegopteryx thambema. The largest known pterosaur with a wingspan of 12m. Palaeoart by Mark Witton.

Pterosaur hatchlings were size-limited, ultimately by egg size, which was constrained by the size of the pelvic opening of female pterosaurs, and by the soft eggshell produced by pterosaurs. Compared to hard-shelled bird eggs, soft-shelled eggs are weaker and cannot support larger egg sizes. In order to grow big, pterosaurs had to do most of their growing after they hatched.

But fossil pterosaur eggs and hatchlings are in short supply. The discovery of a pterosaur egg bonanza (200 eggs and 16 embryos) in 2014 from a relatively large species, Hamipterus tianshanensis, with a wingspan of 1.5-3.5m, indicated that the species lived in groups and that young were cared for by their parents, and suggested that the young were not flight-capable at hatching. However, recent analysis of hatchling pterosaurs of two species, Sinopterus, and Pterodaustro, the latter large (with adult wingspan ~3m), indicates that they were both probably able to fly at, or shortly after, hatching.

Yang et al do not rule out the ability of young Pteranodon to fly. Wing loading would have been low (relatively light body mass to high wing area) in immature Pteranodon, and bone density was likely strong enough for them to fly. But we don’t know. Because no Pteranodon smaller than 1.76m wingspan has yet been discovered. We need to find baby Pteranodon to fully understand its’ growth and locomotion.

Fossil pterosaur eggs, Hamipterus tianshanensis

Fossil pterosaur eggs, Hamipterus tianshanensis. Source: Figure 2 in Egg accumulation with 3D embryos provides insight into the life history of a pterosaur by Wang et al 2017. Eggs preserved with pterosaur bones (IVPP V 18942). (A) Close-up of egg concentration in Fig. 1; scale bar, 100 mm; (B to F) selected eggs indicated by pink arrows b to f in Fig. 1, showing different degrees of deformation. The red and yellow arrows indicate the fissure in the egg and the mudstone pellet, respectively. Scale bar, 20 mm. Photo by Mr. Wei Gao. Reproduced with permission.

Parental care may have released large pterosaurs from growth and size constraints. An extended and parentally supported (via protection and provisioning with food) maturation period may have allowed a bending of developmental physics, with larger body size, lighter skeleton, and more robust joints, all found in the larger pterosaurs. In contrast, precocial young may have been locked into maturity at smaller sizes.

The Ecology of Large Pterosaurs

To grow to such a large body size, the giant pterosaurs needed, not only physics and growth conditions to enable achievement of large size, but also environments capable of supporting them. The ecological niche space was evidently available for them to fill. They likely needed two principal elements to their habitats: space and updrafts. Big pterosaurs would have been principally soarers. Their body plan (high wing aspect ratio, seen in living soaring birds) and the environments that they lived in, puts them in soaring morphospace and ecospace, meaning that they would soar, using updrafts to stay aloft, and economise on energy use by minimising flapping. Pteranodon glided across marine environments. The giant Quetzalcoatlus drifted over warm open environments.

Giant pterosaurs also needed a food supply to support their biomass and fuel their metabolic engine. Whilst competitors for food were likely in short supply for adults, subadults being smaller, would be more likely to have ecological niche overlap with smaller pterosaur species. However, assuming ontogenetic niche partitioning (as also postulated for the ultimate giant of theropod dinosaurs, Tyrannosaurus rex) – at least subadult giant pterosaurs would not be competing with adults of the same species.

Ontogenetic niche partitioning in giant pterosaurs

Visual summary of how basic, size-dependent flight parameters (wing loading, wingspan and aspect ratio) could have influenced pterosaur ecology throughout ontogeny. The animals shown here are giant azhdarchids, species which likely had the largest ontogenetic mass differentials of any pterosaurs (see text) and thus potentially the broadest ecological range across their various growth stages. Azhdarchids were primarily terrestrial pterosaurs75, which is reflected in this figure, though the environments and points made here are generalised: they do not expressly pertain to any azhdarchid taxon. Ontogenetic niche exploitation may have differed in other environments. Figure 8 from Powered flight in hatchling pterosaurs: evidence from wing form and bone strength by Darren Naish, Mark P. Witton & Elizabeth Martin-Silverstone, 2021.

To be successful (i.e. to sustain), individuals of every species need to do two things, to eat (to survive) and to avoid being eaten by something else. At such large adult body size, predation (and predators) was presumably limited. What dinosaurs, or other prehistoric beasts, would be big and hard enough to take on such an imposingly sharp-beaked monster?

Dearc sgiathanach and Tyrannosaur - palaeoart by Natalia Jagielska

Dearc sgiathanach, the largest known Jurassic pterosaur (wingspan 2.5-3m), toys with a large Tyrannosaur theropod. Discovered on the Isle of Skye. Palaeoart by Natalia Jagielska. For more on Dearc sgiathanach, see my Blog, THE GIANT PTEROSAUR THAT PTERRORISED SKIES OVER SKYE. For the paper: A skeleton from the Middle Jurassic of Scotland illuminates an earlier origin of large pterosaurs, by Jagielska et al 2022.

Parental care

We don’t tend to think of reptiles as good parents. Most living reptiles do not provide parental care for their young; they lay their eggs and they abandon them to hatch unattended, with the hatchlings having to make their own way in the world without further assistance. Think lizards, snakes, turtles. But some crocodilians do provide post-hatching care (by way of protection) for their offspring. And parental care is the norm in birds, likely inherited from their theropod ancestors.

There is no reason to think that animals in ancient times followed different anatomical and physiological rules from animals alive today. So, it appears that the evolution of gigantism in pterosaurs may have been facilitated by parental care, enabling the elongation of forelimbs to support large body size in flight. The thought that these giant predatory reptiles were good parents (and why not?) is a dramatic visual image. And one brought to life in the recent Prehistoric Planet documentary series.

Pterosaur parenting - Tethydraco

Pterosaurs parenting - Tethydraco colony (Prehistoric Planet, Episode 1: Coasts). From Max's Blog.

So, incredible as it seems, we can infer parental care (and lack of) in long dead species of flying reptile. The odds of a fossil specimen of Pteranodon or other giant pterosaur being preserved in the act of unambiguous parental care are incredibly slim. So, evidence from bones, and understanding patterns from contemporary species, are critical to advancing our understanding of past species and ecosystems. Hopefully, at some point, someone will find juvenile Pteranodon or other giant pterosaur species, and perhaps even hatchlings, or eggs with embryos, to enable confirmation of the anatomy of the species during early development.

Big Mama - Oviraptor dinosaur

Big Mama - Oviraptor (Citipati osmolskae). An incredibly rare fossil - preserving parental care (incubation) in action (in a dinosaur). Figure 3 from Mark A. Norell, Amy M. Balanoff, Daniel E. Barta, Gregory M. Erickson (2018) A Second Specimen of Citipati osmolskae Associated With a Nest of Eggs from Ukhaa Tolgod, Omnogov Aimag, Mongolia. American Museum Novitates, no. 3899. Photo by Daniel Barta.

Otherwise, many questions remain regarding the period of parental care. What was the nature of the parental care: thermoregulatory, protective, provisioning? What was the clutch size? Was nesting communal? Where did they nest? Was care biparental or were females, or males more inclined to do the work? How long did the period of care last? And, may there have been alloparental care (where individuals care for young for which they are not the parent)? To answer these questions, and more fully re-imagine the early life, and parental behaviour, of giant pterosaurs, we need more fossils. Please go find them.

Pteranodon feeding young

Pteranodon feeding young by Zdenek Burian 1960. Source: Zdenek Burian

Keywords: allometry, altricial, anatomy, biomechanics, comparative anatomy, fossils, palaeontology, paleontology, parental care, precocial, Pteranodon, pterosaur, pterosaurs.

To read my Blog about Dearc sgiathanach, the largest known Jurassic pterosaur, see THE GIANT PTEROSAUR THAT PTERRORISED SKIES OVER SKYE.

To read my article in The Conversation on dinosaur egg bonanza, see Dinosaur egg bonanza gives vital clues about prehistoric parenting.

To read my extended Blog version of the dinosaur egg bonanza article, see DINOSAUR EGG DISCOVERY REVEALS SOCIAL LIVES OF DINOSAURS.

All of my palaeontology articles and Blogs can be viewed here.