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TRACE FOSSILS AND DINOSAUR PHYSIOLOGY

TRACE FOSSILS OF DINOSAURS

      There are a number of artifacts that can give information about prehistoric life other than the remains of actual individuals such as skin impressions, footprints, eggs, gizzard stones (gastroliths), and feces (coprolites).

FOOTPRINTS

--after Thulborn, 1990

Footprints

      Fossil dinosaur footprints are known from every continent except Antarctica.  Some can be matched to a specific dinosaur species by comparing to skeletons while many footprints can’t be linked to known dinosaurs (indeed, some footprints may have been made by species that have not yet been discovered).  Some footprints can be linked to a group of dinosaurs but the individual species of this group don’t differ significantly in the morphology of their feet.  Caution must be used since even in a single trackway made by one dinosaur, footprints can vary in shape.

      What can information can footprints provide about dinosaurs?

      Footprints indicate whether a dinosaur was bipedal or quadrupedal.  Sometimes a skeleton makes it obvious that a dinosaur was bipedal (as in most theropods) while other skeletons make this less obvious and from the skeleton alone no firm conclusion can be made.  Footprints of ceratopsians and armored dinosaurs indicate that their forelimbs were held upright which would not be certain from the skeletons alone.  Trackways can indicate whether the tail was dragged or not (it typically was not) and, if it was, whether this drag was continuous or occasional.  Some indicate the body position of a dinosaur while it rested.  Some trackways include signs of vegetation being dragged (Thulborn, 1990).  One sauropod trackway indicates that the animal was floating and only using its forelegs to walk (Lucas, 2004).

FOOTPRINTS 2 FOOTPRINTS 3

--after Thulborn, 1990

      Footprints can also be used to attempt to estimate the speed of a dinosaur.  A stride is the distance separating a certain point of the same foot on two separate prints.  There are a number of ways of estimating speed.  Some have calculated a relative stride as the length of stride divided by the leg length (obviously, leg length information must come from known specimens).  Living mammals and ostriches have a direct relation between their relative stride and a factor called dimensionless speed (actual speed divided by the square root of leg length times gravitational acceleration).  Why a second factor? This simply controls for the observation that large animals move faster than small ones with the same relative stride.  If we assume that this relationship which holds true for living animals holds true for dinosaurs as well, the relative stride of a dinosaur allows the calculation of dimensionless speed.  From dimensionless speed, actual speed can be calculated.  While most trackways indicate that dinosaurs were walking, there are some which indicate running.  Some theropods apparently could reach speeds of 43 km/h (25 miles an hour). 

FOOTPRINTS 4 footprints 5

     Footprints can also give information about group living.  There are many trackways in which groups of prints all move in the same direction, indicating that they were traveling in a herd.  In one sauropod trackway, the tracks of the larger individuals are on the outside and those of the smaller individuals on the inside, suggesting a strategy for the protection of smaller animals.

      Claims that human footprints have been found in dinosaur-bearing strata have, to date, been shown to be poor interpretation (for example, an eroded print of a 3-toed dinosaur taken as human) or frauds, which were carved out of stone.

 

 

Eggs

      Dinosaur eggs were first found in France in 1869; the most famous eggs belonged to Protoceratops found in Mongolia in 1923.  Dinosaur eggs are known from 4 continents ranging from Late Triassic to Late Cretaceous and are known from most dinosaur groups.  They vary in shape (including round and oblong shapes) and size (sauropods with the largest eggs at 30 cm).

      Some were laid individually, others in nests.  Some contain embryos that can be studied.  Some nests have been found with remains of vegetation (which may indicate that they were covered by the mother) and some have been found with large quantities of seeds in the vicinity (which may indicate that the parents brought food to the nest or regurgitating it).  In some nests, the eggshells aren’t cracked (which suggest the young did not stay at the nest after hatching) while in others they were (which might indicate that the young did remain at the site of the nest to receive parental care) (Fastovsky, 1996; Lucas, 2004).

      In some cases, the skeletons of young and adults individuals were found in the vicinity of the nest site, suggesting parental care.  Some sites have a number of nests which are about the body length of an adult separated from each other, which may indicate that groups of animals nested together.  Some features are shared by nests of Protoceratops, Maiasaura, Orodromeus, and nests from France share features such as closely spaced nests suggesting group nesting, indications that the same sites were used for prolonged time periods, skeletal remains of a wide range of age groups around the rookery, and eggs deliberately positioned in nests (such as in a circular formation, and even pointed in the same direction)(Weishampel, p. 40; (Lucas, 2004). 

Vegetation was used in the nests of Maiasaurua. In several dinosaur species (Protoceratops, Maiasaurua, and Orodromeus), nesting sites seem to have been used by multiple individuals over multiple years. The presence of adults and juveniles around the nests is evident in Protoceratops and possible in Maiasaurua, Orodromeus, and hypsilophodonts (Coombs, 1989; Winkler, 1989). Fossil of Psittacosaurus adult with 34 young, perhaps trapped alive when a burrow collapsed (Reaney, 2004).

 

 

Gastroliths

gastrolith
      Many reptiles and birds swallow stones to help grind vegetation and aquatic animals may swallow stones for ballast.  Some animals even swallow stones when they are very hungry.  Fossilized gizzard stones are called gastroliths.  Polished stones associated with the abdomen of a dinosaur are certainly gastroliths while isolated stones without a skeleton are more difficult to conclusively identify as gastroliths (Lucas, 2004). The remains of grasses have been found in the coprolites of late Cretaceous sauropods from India (Prasad, 2005).

Coprolites

coprolite

      Coprolites are fossilized feces and very little attention has been given to this field of study to date.   It is very difficult to link a coprolite to any specific animal and many contain no useful information.  Some are feces of predatory animals that contain fish scales and bone fragments.  Because of bone fragments and the presence of calcium, feces of predatory dinosaurs were more likely to fossilize.  Some are feces of herbivorous animals and have been observed to contain woody materials and conifer debris (Lucas, 2004).

 

DINOSAUR PHYSIOLOGY BEHAVIOR

      Some fossils have given information about the external appearance of dinosaurs.  Fossilized skin impressions known from ornithopod, theropod, and ceratopsians, indicating that they were covered with scales, similar to some reptiles.  Recent finds indicate a number of theropods had feathers (discussed with birds).

       How did dinosaurs grow?  Modern reptiles display indeterminate growth in that they grow throughout their entire lives (although the rate decreases with age).  Birds and mammals display determinate growth in which growth stops at a certain point in adulthood.  Warm-blooded animals grow 10 times faster than cold-blooded animals.  If dinosaurs were cold blooded, it would have taken a very long time to reach their adult sizes (centuries in the case of sauropods).

      Some dinosaurs may have lived in groups.  Many possessed display structures, such as crests.  Typically, there was sexual dimorphism in these display structures, often occurring in adulthood.  There are numerous examples of mass graves although many may represent where rivers washed bones from different sites (Fastovsky, 1996; Lucas, 2004).

 

Hot Blooded Dinosaurs?

     Most reptiles (like the snake in the photo below) depend on the environment to heat their bodies.

snake

      Endotherms maintain a constant temperature regardless of the environmental conditions while ectotherms can have their temperatures vary as a result of environmental conditions.  These classifications (ectotherm vs. endotherm) are not absolute.  Ectotherms can have warmer body temperatures at a given moment than endotherms (so “cold blooded” doesn’t actually refer to the temperature of the blood).  Ectotherms try to keep relatively constant temperatures by their behavior (reptiles can sun themselves, come out only at night, and face directly into the sun, for example). Some do this so well that their temperature actually varies very little.  Some ectotherms maintain higher and more constant temperatures than others; some endotherms have trouble in doing so.

     Two additional words are sometimes used: homeotherms (animals which maintain the same temperature) and poikilotherms (animals in which the temperature varies).  Some lizards and snakes are functional homeotherms, through their behavior they maintain a constant body temperature.  Bats and hummingbirds are endothermic poikilotherms (as are bears when they hibernate).

      Endothermy has evolved separately about 13 times—it has been observed in some sharks, moths, dragonflies, beetles, bees.  Endotherms need more food (a zoo can keep 40 crocodiles or so on the food it gives one tiger).  Neither is better than the other (remember that the vast majority of vertebrates are ectothermic) and there are many environments today in which ectotherms have the advantage over endotherms (tropics, deserts).  Virtually all ectotherms are found within 45 degrees of the equator and most ectothermic species live 20 degrees from the equator.  Larger modern ectotherms tend to be aquatic (sea turtles, crocodiles) and water helps them to keep a stable temperature (May, 1995).

     In crocodiles, larger animals have higher body temperatures and smaller fluctuations in body temperature than smaller animals.  Large dinosaurs might have had stable body temperatures, even in winter, due to their size alone.  Small dinosaurs (those under 100 kg) such as coelurosaur theropods and hypsilophodont ornithopods are the most likely candidates to have evolved endothermy.  Endotherms do not maintain constant body temperatures through any structures that they alone possess; rather they do so by regulating muscle contraction, ion pump activity, and mitochondria number in cells.  They simply over-express a number of physiological mechanisms which ectotherms already possess.  Many of the feathered dinosaurs would have lived at 45-50 degrees paleolatitude and would have experienced cold temperatures (Seebacher, 2003).

     Is there any evidence for the body temperature of dinosaurs?  A number of aspects of dinosaur physiology have been examined.

      Very fast dinosaurs might have required endothermy; fast speeds may be supporting evidence for endothermy although many ectotherms can also be fast.  Small theropods and ornithopods were perhaps fast enough to require endothermy.

      Dinosaurs walked erect and this requires more energy than sprawling.  All modern animals which walk erect are endotherms.

     The fossil of an ornithischian dinosaur (a hypsilophodont) indicates that it possessed a single systemic aorta (like birds and mammals) as opposed to a pair of them (like reptiles such as turtles and crocodilians) (Fisher, 2000).

      Endotherms need more energy and therefore need to process food more efficiently.  Dinosaurs had some adaptations for better food processing (such as the ability to chew in most ornithischians).

      Larger brains require more energy and today the largest-brained animals are endotherms.  Some theropods had relative brain sizes equivalent to birds and even primitive mammals and thus may have been endothermic (Hopson, 1977). 

      There are channels for blood vessels in bone.  The bone of ectotherms has few channels for blood vessels; these are abundant in endotherms.  Unfortunately, this is not an absolute relationship: large ectotherms can have them and small endotherms can possess few.  Dinosaurs have many blood vessels in their bones, suggesting they were endothermic.   Dense bone results from bone remodeling and has only been observed in living endotherms, dinosaurs, pterosaurs, some mammal-like reptiles.  Some dinosaurs have growth rings in their bones (caused by annual fluctuations in bone growth); this is typical of ectotherms today.  Were the dinosaurs, which lacked them, endothermic?  Unfortunately, growth rings in bones are not comparable to tree rings and researchers are not quite sure what to make of them.  Some hadrosaurs had differing numbers of growth rings in their arms and legs (obviously their arms and legs were not of different ages).  Growth rings do occur in some mammals like African buffalo and desert bighorn sheep (Fastovsky, 1996).

      The ratio of two oxygen forms or isotopes (18O and 16O) depends on temperature.   In endotherms, limb and body core temperatures more similar than in ectotherms; this ratio in dinosaurs falls within the bird and mammal range.

      Modern endotherms have plates of bone in their nose (turbinates) which helps limit water loss with higher breathing rates; dinosaurs have none.

     Dinosaurs (iguanodonts, hadrosaurs, ceratopsians, theropods, and basal ornithopods) have been found near the poles living with cold is difficult for ectotherms (for example, no fossil crocodiles are found north of Montana).  Although the Cretaceous was warmer than today, dinosaurs have been found within 15-5o of the South Pole and within 10o of the North Pole.  If the similarities between some North American and Asian dinosaurs indicate that there was a Bering land bridge connecting the continents, then some dinosaurs traveled within 1o of the Cretaceous North Pole.      Although some dinosaurs might have been feathered, there are naked-skinned mammals that inhabit polar regions (whales, walruses, and seals).  Could they have migrated?  For the Northern Hemisphere the migration from the Arctic circle to Montana would have been over 3,700 km—longer than any known terrestrial migration (caribou and polar bears migrate 2,500 km).  Such a migration would have required an estimated 8 months of the year raising very difficult issues with regard to feeding and the raising of young.  It is questionable whether ectotherms could have managed such a migration; it is even more unlikely that ectotherms could have been year-round residents in polar areas (although some large amphibians did survive in the southern polar regions) (Paul, 1988; Fastovsky, 1996).

     Dinosaurs gave rise to birds and birds are endothermic although it is not known at what point of bird evolution they became endothermic.  Some theropods seem to have been covered with downy feathers which birds use for insulation and to help regulate body temperature.  Some pterosaurs were covered with fine hairs.

      Endotherms have fewer predators per unit of prey animals than ectotherms.  Why? Endotherms need more food, so a certain set of prey animals can support fewer endothermic predators than ectothermic predators.  The predator-prey ratios in dinosaurs support endothermy but could result from some aspect of fossilization (i.e. perhaps theropods fossilize less well).

     The largest sauropods probably weren’t endotherms.  Big animals lose heat more slowly and endothermic animals of this size would have overheated (Fastovsky, 1996).

     Dinosaur lungs:  The thoracic and abdominal cavities of one theropod (Sinosauropteryx, a compsognath) were apparently separate; in mammals this separation is due to a diaphragm muscle whose role in breathing is critical in the higher metabolism of mammals.  There is a remarkable find of another theropod (Scipionyx, a coelurosaur) which actually gives a great deal of information about soft, internal organs.  Its breathing mechanism was much more like that of crocodiles than birds: the avian style of lung is absent and diaphragm muscles which connect the gastralia and pubis to the liver (allowing the hepatic piston diaphragm assisted breathing found in crocodiles).  This implies that although Scipionyx breathed primarily with its ribs (as do modern ectotherms), a much greater ability to exchange gases was possible, perhaps allowing brief periods of high metabolism, even within mammal range.  (Unfortunately, the full potential of this type of breathing is hard to estimate from modern crocodiles which long ago abandoned running on land as the primary form of locomotion.) (Ruben, 1999; Dal Sasso, 1998).

 

Conclusion: It is not certain whether dinosaurs were endothermic or ectothermic and there is evidence which could support each alternative.  The possibility that dinosaurs were endothermic must be taken seriously however.  The ultimate answer may even be more complicated than this: dinosaurs may have varied in metabolism with some being endothermic while others weren’t.  Other dinosaurs may have varied their metabolic rates during the course of their lives, being endothermic when they were young.