﻿272 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  192 
  9 
  

  

  The 
  Mesozoic 
  land 
  reptiles, 
  although 
  differing 
  in 
  all 
  other 
  respects 
  

   from 
  the 
  trilobites, 
  had 
  many 
  features 
  in 
  common 
  with 
  them 
  that 
  

   led 
  to 
  extinction. 
  Exaggeration 
  of 
  different 
  parts 
  of 
  the 
  bodily 
  

   structure; 
  extraordinary 
  size 
  in 
  these 
  reptiles, 
  which 
  attained 
  the 
  

   possible 
  limit 
  of 
  size 
  for 
  a 
  land 
  animal; 
  inharmonious 
  development 
  

   of 
  different 
  parts 
  of 
  the 
  body; 
  development 
  of 
  spines 
  surpassing 
  

   imagination 
  in 
  form, 
  size, 
  and 
  abundance 
  — 
  all 
  these 
  features 
  in 
  the 
  

   Mesozoic 
  land 
  reptiles, 
  as 
  in 
  the 
  trilobites, 
  point 
  to 
  the 
  senility 
  of 
  a 
  

   race 
  preceding 
  its 
  extinction. 
  And 
  in 
  the 
  reptiles, 
  as 
  in 
  the 
  trilobites, 
  

   these 
  features 
  appeared 
  as 
  special 
  devices 
  to 
  meet 
  real 
  needs, 
  but 
  all 
  

   of 
  them 
  eventually 
  became 
  so 
  much 
  exaggerated 
  as 
  to 
  become 
  handi- 
  

   caps. 
  A 
  good 
  example 
  is 
  that 
  gigantic 
  spiniferous 
  animal 
  the 
  Stego- 
  

   saurus, 
  which 
  was 
  developed 
  as 
  a 
  special 
  senile 
  side 
  branch 
  and 
  

   died 
  out 
  without 
  issue. 
  ^^ 
  Specialization 
  aiming 
  at 
  some 
  end 
  may 
  

   become 
  overspecialization. 
  As 
  now 
  used, 
  the 
  term 
  overspecializa- 
  

   tion 
  generally 
  implies 
  the 
  idea 
  of 
  a 
  tendency 
  toward 
  extinction. 
  

   Overspecialization 
  leads 
  to 
  extinction, 
  according 
  to 
  Gregory 
  ;^^ 
  

   "extreme 
  specialization 
  may 
  become 
  a 
  cause 
  of 
  extinction," 
  says 
  

   Osborn.^^ 
  Wieland, 
  writing 
  of 
  the 
  extinction 
  of 
  the 
  dinosaurs, 
  

   explains 
  it 
  by 
  saying 
  that 
  "the 
  growth 
  forces 
  and 
  the 
  responses 
  to 
  

   environment 
  were 
  no 
  longer 
  in 
  adjustment," 
  ^^ 
  a 
  condition 
  that 
  is 
  

   practically 
  equivalent 
  to 
  overspecialization. 
  

  

  We 
  thus 
  reach 
  the 
  same 
  conclusion 
  concerning 
  the 
  Mesozoic 
  

   reptiles 
  that 
  we 
  reached 
  concerning 
  the 
  trilobites 
  — 
  that 
  before 
  their 
  

   final 
  extinction 
  they 
  had 
  lost 
  their 
  vital 
  racial 
  force 
  and 
  were 
  unable 
  

   to 
  maintain 
  that 
  natural 
  balance 
  which 
  was 
  especially 
  necessary 
  to 
  

   adapt 
  them 
  to 
  a 
  change 
  in 
  environment. 
  

  

  THE 
  AMMONITES 
  

  

  The 
  most 
  mysterious 
  event 
  of 
  this 
  kind 
  is 
  the 
  extinction 
  of 
  the 
  am- 
  

   monites 
  in 
  the 
  Mesozoic 
  era. 
  They 
  had 
  been 
  declining 
  rapidly 
  during 
  

   late 
  Triassic 
  time, 
  but 
  they 
  recovered 
  in 
  Liassic 
  time 
  and 
  increased 
  in 
  

   numbers 
  and 
  in 
  varieties 
  of 
  form 
  so 
  great 
  that 
  in 
  the 
  Jurassic 
  and 
  Cre- 
  

   taceous 
  periods 
  the 
  seas 
  were 
  swarming 
  with 
  them. 
  However, 
  in 
  the 
  

   Cretaceous 
  period 
  the 
  ammonites 
  suffered 
  complete 
  extinction. 
  The 
  

   periodic 
  appearance 
  and 
  disappearance 
  of 
  the 
  ammonites 
  has 
  been 
  com- 
  

   pared 
  with 
  the 
  corresponding 
  appearance 
  and 
  disappearance 
  of 
  the 
  sea 
  

   reptiles 
  by 
  which 
  they 
  are 
  supposed 
  to 
  have 
  been 
  exterminated. 
  But 
  

   with 
  the 
  ammonites 
  as 
  with 
  the 
  trilobites, 
  such 
  an 
  extermination 
  could 
  

   not 
  have 
  gone 
  on 
  through 
  a 
  number 
  of 
  geological 
  periods, 
  a 
  time 
  long 
  

   enough 
  for 
  a 
  normal 
  vigorous 
  stock 
  to 
  establish 
  a 
  balance. 
  There 
  are 
  

  

  i« 
  R. 
  S. 
  Lull, 
  Organic 
  evolution, 
  p. 
  524. 
  

  

  " 
  W. 
  K. 
  Gregory, 
  Two 
  views 
  on 
  the 
  origin 
  of 
  man. 
  Science, 
  Vol. 
  LXV, 
  No. 
  1G95, 
  p. 
  602, 
  1927. 
  

  

  " 
  H. 
  F. 
  Osborn, 
  The 
  age 
  of 
  mammals, 
  p. 
  S4. 
  

  

  i» 
  G. 
  R. 
  Wieland, 
  Dinosaur 
  extinction. 
  American 
  Naturalist, 
  Vol. 
  LIX, 
  No. 
  665, 
  pp. 
  557-565. 
  

  

  